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1 <html>
2 <head>
3 <title>JSLint: The JavaScript Code Quality Tool</title>
4 <link rel="icon" type="image/gif" href="">
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30 </head>
31 <body bgcolor="gainsboro">
32 <table id="top" border="0">
33 <tr>
34 <td><img src="jslint.gif" width="383" height="120" alt="JSLint"> </td>
35 <td>
36 <p><big><code>JSLint</code>: The
37 <a href="">JavaScript</a> Code Quality Tool</big></p>
38 <p><a href="" target="_top">&copy;2002 Douglas Crockford</a></p>
39 </td>
40 </tr>
41 </table>
42 <br clear="all">
43 <h2 id=warning>Warning!</h2>
44 <p><a href="" target="_blank"><code>JSLint</code></a>
45 will hurt your feelings.</p>
46 <h2 id=what>What is <code>JSLint</code>?</h2>
48 <p><a href="" target="_blank"><code>JSLint</code></a>
49 is a JavaScript program that looks for problems in JavaScript programs.
50 It is a code quality tool.</p>
52 <p>When <a href="">C</a>
53 was a <a href="">young</a>
54 programming language, there were several common programming errors that
55 were not caught by the primitive compilers, so an accessory program called
56 <code><a href="">lint</a></code>
57 was developed that would scan a source file, looking for problems.</p>
59 <p>As the language matured, the definition of the language was
60 strengthened to eliminate some insecurities, and compilers got better
61 at issuing warnings. <code>lint</code> is no longer needed.</p>
63 <p><a href="">JavaScript</a> is a young-for-its-age
64 language. It was originally intended to do small tasks in webpages, tasks
65 for which Java was too heavy and clumsy. But JavaScript is a very capable
66 language, and it is now being used in larger projects. Many of the features
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67 that were intended to make the language easy to use are troublesome when projects become complicated. A <code>lint</code> for JavaScript is needed: <a href=""><code>JSLint</code></a>,
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68 a JavaScript syntax checker and validator.</p>
70 <p><code>JSLint</code> takes a JavaScript source and scans it. If it finds
71 a problem, it returns a message describing the problem and an approximate
72 location within the source. The problem is not necessarily a syntax error,
73 although it often is. <code>JSLint</code> looks at some style conventions
74 as well as structural problems. It does not prove that your program is
75 correct. It just provides another set of eyes to help spot problems.</p>
77 <p><code>JSLint</code> defines a professional subset of JavaScript, a stricter
78 language than that defined by <a href="" target="ecma">Third
79 Edition of the <i>ECMAScript Programming Language Standard</i></a>. The
80 subset is related to recommendations found in <a href="" target="sun"><i>Code
81 Conventions for the JavaScript Programming Language</i></a>. </p>
82 <p>JavaScript is a sloppy language, but inside it there is an elegant, better
83 language. <code>JSLint</code> helps you to program in that better language
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84 and to avoid most of the slop. JSLint will reject programs that browsers will accept because JSLint is concerned with the quality of your code and browsers are not. You should accept all of JSLint's advice.</p>
85 <p><code>JSLint</code> can operate on JavaScript source, HTML source, CSS source, or <a href="">JSON</a>
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86 text.</p>
87 <h2 id=global>Global Variables</h2>
88 <p>JavaScript's <a href="">biggest
89 problem</a> is its dependence on global variables, particularly implied
90 global variables. If a variable is not explicitly declared (usually with
91 the <code>var</code> statement), then JavaScript assumes that the variable
92 was global. This can mask misspelled names and other problems.</p>
93 <p><code>JSLint</code> expects that all variables and functions are declared
94 before they are used or invoked. This allows it to detect implied global
95 variables. It is also good practice because it makes programs easier to
96 read.</p>
97 <p>Sometimes a file is dependent on global variables and functions that
98 are defined elsewhere. You can identify these to <code>JSLint</code> with a <code>var</code> statement that lists the global functions and objects
99 that your program depends on. </p>
100 <p>A global declaration can look like this:</p>
101 <pre>var getElementByAttribute, breakCycles, hanoi;</pre>
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102 <p>The declaration should appear near the top of the file. It must appear before the use of the variables
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103 it declares. </p>
104 <p>It is necessary to use a <code>var</code> statement to declare a variable before that variable is assigned to. </p>
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105 <p><code>JSLint</code> also recognizes a <code>/*global */</code> directive that can indicate to <code>JSLint</code> that variables used in this file were defined in other files. The comment can contain a comma separated list of names. Each name can optionally be followed by a colon and either <code>true</code> or <code>false</code>, <code>true</code> indicating that the variable may be assigned to by this file, and <code>false</code> indicating that assignment is not allowed (which is the default). The directive respects function scope.</p>
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106 <p id=browser>Some globals can be predefined for you. Select the <i>Assume
107 a browser</i> (<code>browser</code>) <a href="#options">option</a> to
108 predefine the standard global properties that are supplied by web browsers,
109 such as <code>document</code> and <code>addEventListener</code>. It has the same
110 effect as this comment:</p>
111 <blockquote>
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112 <code>/*global
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113 clearInterval: false, clearTimeout: false, document: false, event: false, frames: false, history: false, Image: false, location: false, name: false, navigator: false, Option: false, parent: false, screen: false, setInterval: false, setTimeout: false, window: false, XMLHttpRequest: false
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114 */</code></blockquote>
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115 <p>Select the
116 <em>Assume console, alert, ...</em>
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117 (<code>devel</code>) <a href="#options">option</a> to predefine globals that are useful in development but that should be avoided in production, such as <code>console</code> and <code>alert</code>. It has the same
118 effect as this comment:</p>
119 <pre>/*global alert: false, confirm: false, console: false, Debug: false, opera: false, prompt: false */</pre>
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120 <p id=node>Select the
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121 <em>Assume Node.js</em>
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122 (<code>node</code>) <a href="#options">option</a> to predefine globals that are used in the Node.js environment<code></code>. It has the same
123 effect as this comment:</p>
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124 <pre>/*global Buffer: false, clearInterval: false, clearTimeout: false, console: false, global: false, module: false, process: false, querystring: false, require: false, setInterval: false, setTimeout: false, util: false, __filename: false, __dirname: false */</pre>
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125 <p id=rhino>Select the <i>Assume Rhino</i> (<code>rhino</code>) <a href="#options">option</a>
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126 to predefine the global properties provided by the Rhino environment.
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127 It has the same effect as this statement:</p>
128 <blockquote>
129 <code>/*global defineClass: false, deserialize: false, gc: false, help: false, load: false, loadClass: false, print: false, quit: false, readFile: false, readUrl: false, runCommand: false, seal: false, serialize: false, spawn: false, sync: false, toint32: false, version: false */ </code>
130 </blockquote>
131 <p id=widget>Select the <i>Assume a Yahoo Widget</i> (<code>widget</code>)
132 <a href="#options">option</a> to predefine the global properties provided
133 by the Yahoo! Widgets environment. It has the same effect as this statement:</p>
134 <blockquote>
135 <code>/*global alert: true, animator: true, appleScript: true, beep: true, bytesToUIString: true, Canvas: true, chooseColor: true, chooseFile: true, chooseFolder: true, closeWidget: true, COM: true, convertPathToHFS: true, convertPathToPlatform: true, CustomAnimation: true, escape: true, FadeAnimation: true, filesystem: true, Flash: true, focusWidget: true, form: true, FormField: true, Frame: true, HotKey: true, Image: true, include: true, isApplicationRunning: true, iTunes: true, konfabulatorVersion: true, log: true, md5: true, MenuItem: true, MoveAnimation: true, openURL: true, play: true, Point: true, popupMenu: true, preferenceGroups: true, preferences: true, print: true, prompt: true, random: true, Rectangle: true, reloadWidget: true, ResizeAnimation: true, resolvePath: true, resumeUpdates: true, RotateAnimation: true, runCommand: true, runCommandInBg: true, saveAs: true, savePreferences: true, screen: true, ScrollBar: true, showWidgetPreferences: true, sleep: true, speak: true, Style: true, suppressUpdates: true, system: true, tellWidget: true, Text: true, TextArea: true, Timer: true, unescape: true, updateNow: true, URL: true, Web: true, widget: true, Window: true, XMLDOM: true, XMLHttpRequest: true, yahooCheckLogin: true, yahooLogin: true, yahooLogout: true */</code>
136 </blockquote>
137 <p id=windows>Select the <i>Assume Windows</i> (<code>windows</code>)
138 <a href="#options">option</a> to predefine the global properties provided by Microsoft Windows. It has the same effect as this statement:</p>
139 <blockquote>
140 <p><code>/*global ActiveXObject: false, CScript: false, Debug: false, Enumerator: false, System: false, VBArray: false, WScript: false */</code></p>
141 </blockquote>
142 <h2 id=semicolon>Semicolon</h2>
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143 <p>JavaScript uses a C-like syntax which requires the use of semicolons to delimit certain
144 statements. JavaScript attempts to make those semicolons optional with a semicolon
145 insertion mechanism. This is dangerous because it can mask errors.</p>
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146 <p>Like C, JavaScript has <code>++</code> and <code>--</code> and <code>(</code> operators
147 which can be prefixes or suffixes. The disambiguation is done by the semicolon.</p>
148 <p>In JavaScript, a linefeed can be whitespace or it can act as a semicolon.
149 This replaces one ambiguity with another. </p>
150 <p><code>JSLint</code> expects that every statement be followed by <code>;</code> except
151 for <code>for</code>, <code>function</code>, <code>if</code>, <code>switch</code>, <code>try</code>, and
152 <code>while</code>. <code>JSLint</code> does not expect to see unnecessary semicolons or the
153 empty statement.</p>
154 <h2 id=comma>Comma</h2>
155 <p>The comma operator can lead to excessively tricky expressions. It can also
156 mask some programming errors.</p>
157 <p><code>JSLint</code> expects to see the comma used as a separator, but not as an
158 operator (except in the initialization and incrementation parts of the <code>for</code>
159 statement). It does not expect to see elided elements in array literals. Extra
160 commas should not be used. A comma should not appear after the last element
161 of an array literal or object literal because it can be misinterpreted by some
162 browsers. </p>
163 <h2 id=scope>Scope</h2>
165 <p>In many languages, a block introduces a scope. Variables introduced in
166 a block are not visible outside of the block.</p>
168 <p>In JavaScript, blocks do not introduce a scope. There is only function-scope.
169 A variable introduced anywhere in a function is visible everywhere in
170 the function. JavaScript's blocks confuse experienced programmers and
171 lead to errors because the familiar syntax makes a false promise.</p>
173 <p><code>JSLint</code> expects blocks with <code>function</code>, <code>if</code>,
174 <code>switch</code>, <code>while</code>, <code>for</code>, <code>do</code>,
175 and <code>try</code> statements and nowhere else. </p>
176 <p>In languages with block scope, it is usually recommended that variables
177 be declared at the site of first use. But because JavaScript does not
178 have block scope, it is wiser to declare all of a function's variables
179 at the top of the function. It is recommended that a single <code>var</code>
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180 statement be used per function. This can be declined with the <code>vars</code>
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181 <a href="#options">option</a>.</p>
183 <h2 id=required>Required Blocks</h2>
185 <p><code>JSLint</code> expects that <code>if</code>, <code>while</code>,
186 <code>do</code> and <code>for</code> statements will be made with blocks
187 <code>{</code>that is, with statements enclosed in braces<code>}</code>.</p>
189 <p>JavaScript allows an <code>if</code> to be written like this:</p>
191 <pre>if (<i>condition</i><code>)
192 </code><i>statement</i>;</pre>
194 <p>That form is known to contribute to mistakes in projects where many programmers
195 are working on the same code. That is why <code>JSLint</code> expects the use of
196 a block:</p>
198 <pre>if (<i>condition</i>) {
199 <i>statements</i>;
200 }</pre>
202 <p>Experience shows that this form is more resilient.</p>
204 <h2 id=expression>Expression Statements</h2>
205 <p>An expression statement is expected to be an assignment or a function/method
206 call or <code>delete</code>. All other expression statements are considered
207 to be errors.</p>
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208 <h2 id="confusion">Type Confusion</h2>
209 <p>JSLint can do type inference. It can report cases were variables and properties are
210 used to house multiple types. The warning is <code>Type confusion:</code> {a} <code>and</code> {b}<code>.</code> where the {a} and {b} will be
211 replaced with the names of types.</p>
212 <p> It is usually easy to see what caused the
213 warning. In some cases, it can be very puzzling. In the puzzling cases, try
214 initializing your vars with typed values. For example, if you expect that <code>n</code> will
215 contain numbers, then write</p>
216 <pre>var n = 0;</pre>
217 <p>That should produce clearer warnings. </p>
218 <p>Type confusion is not necessarily an error, particularly in a language that
219 provides as much type freedom as this one does. But some inconsistencies are
220 errors, so type discipline might be something to consider adding to your
221 programming style. Also, the fastest JavaScript engines will slow down in the
222 presence of type confusion.
224 To turn off these warnings, turn on the <i>Tolerate type confusion</i> <a href="#options">option</a>. </p>
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225 <h2 id=forin><code>for</code> <code>in</code></h2>
226 <p>The <code>for</code> <code>in</code> statement allows for looping through
227 the names of all of the properties of an object. <a href="">Unfortunately,
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228 it also loops through all of the properties that were inherited through
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229 the prototype chain.</a> This has the bad side effect of serving up method
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230 functions when the interest is in data properties. If a program is written without awareness of this situation, then it can fail.</p>
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231 <p>The body of every <code>for</code> <code>in</code> statement should be
232 wrapped in an <code>if</code> statement that does filtering. It can select
233 for a particular type or range of values, or it can exclude functions,
234 or it can exclude properties from the prototype. For example,</p>
235 <pre>for (name in object) {
236 if (object.hasOwnProperty(name)) {
237 ....
238 }
240 }</pre>
242 <h2 id=switch><code>switch</code></h2>
243 <p>A <a href="">common
244 error</a> in <code>switch</code> statements is to forget to place a <code>break</code>
245 statement after each case, resulting in unintended fall-through. <code>JSLint</code>
246 expects that the statement before the next <code>case</code> or <code>default</code>
247 is one of these: <code>break</code>, <code>return</code>, or <code>throw</code>.
248 </p>
249 <h2 id=var><code>var</code></h2>
251 <p>JavaScript allows <code>var</code> definitions to occur anywhere
252 within a function. <code>JSLint</code> is more strict.</p>
254 <p><code>JSLint</code> expects that a <code>var</code> will be declared
255 only once, and that it will be declared before it is used.</p>
256 <p><code></code><code>JSLint</code> expects that a <code>function</code>
257 will be declared before it is used.</p>
258 <p><code>JSLint</code> expects that parameters will not also be declared
259 as vars. </p>
261 <p><code>JSLint</code> does not expect the <code>arguments</code> array to be declared
262 as a <code>var</code>.</p>
263 <p><code>JSLint</code> does not expect that a var will be defined in a block.
264 This is because JavaScript blocks do not have block scope. This can have
265 unexpected consequences. Define all variables at the top of the function.</p>
267 <h2 id=with><code>with</code></h2>
269 <p>The <code>with</code> statement was intended to provide a shorthand in accessing
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270 properties in deeply nested objects. Unfortunately, it behaves <a href="">very
271 badly</a> when setting new properties. Never use the <code>with</code> statement. Use
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272 a <code>var</code> instead.</p>
274 <p><code>JSLint</code> does not expect to see a <code>with</code> statement.</p>
276 <h2 id=assignment>=</h2>
277 <p><code>JSLint</code> does not expect to see an assignment statement in
278 the condition part of an <code>if</code> or <code>for</code> or <code>while</code>
279 <code></code> or <code>do</code> statement. This is because it is more
280 likely that </p>
281 <pre>if (a = b) {
282 ...
283 }</pre>
284 <p>was intended to be </p>
285 <pre>if (a == b) {
286 ...
287 }</pre>
288 <p>It is difficult to write correct programs while using idioms that are
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289 hard to distinguish from obvious errors.</p>
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290 <h2 id=eqeq>== and !=</h2>
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291 <p>The <code>==</code> and <code>!=</code> operators do type coercion before
292 comparing. This is bad because it causes <code>' \t\r\n' == 0</code> to
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293 be <code>true</code>. This can mask type errors. JSLint cannot reliably determine if == is being used correctly, so it is best to not use <code>==</code> and != and to always use the more reliable <code>===</code> and <code>!==</code> operators instead. </p>
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294 <p align="left">If you only care that a value is <i>truthy</i> or <i>falsy</i>,
295 then use the short form. Instead of </p>
296 <pre align="left">(foo != 0)</pre>
297 <p align="left">just say </p>
298 <pre align="left">(foo)</pre>
299 <p align="left">and instead of</p>
300 <pre align="left">(foo == 0)</pre>
301 <p align="left"> say</p>
302 <pre align="left">(!foo)</pre>
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303 <p>There is an <code>eqeq</code> <a href="#options">option</a> that allows the use of <code>==</code> and <code>!=</code>.</p>
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304 <h2 id=labels>Labels</h2>
305 <p>JavaScript allows any statement to have a label, and labels have a
306 separate name space. <code>JSLint</code> is more strict.</p>
308 <p><code>JSLint</code> expects labels only on statements that interact
309 with <code>break</code>: <code>switch</code>, <code>while</code>,
310 <code>do</code>, and <code>for</code>. <code>JSLint</code> expects that labels
311 will be distinct from vars and parameters.</p>
313 <h2 id=unreachable>Unreachable Code</h2>
314 <p><code>JSLint</code> expects that
315 a <code>return</code>, <code>break</code>, <code>continue</code>,
316 or <code>throw</code> statement will be followed by
317 a <code>}</code> or <code>case</code> or <code>default</code>.</p>
319 <h2 id=pluses>Confusing Pluses and Minuses</h2>
321 <p><code>JSLint</code> expects that <code>+</code> will not be followed by
322 <code>+</code> or <code>++</code>, and that <code>-</code> will not be followed
323 by <code>-</code> or <code>--</code>. A misplaced space can turn <code>+ +</code> into <code>++</code>, an error that is difficult to see. Use parens to avoid confusion..</p>
324 <h2 id=inc><code>++</code> and <code>--</code></h2>
325 <p>The <code>++</code> <small>(increment)</small> and <code>--</code> <small>(decrement)</small>
326 operators have been known to contribute to bad code by encouraging excessive
327 trickiness. They are second only to faulty architecture in enabling to
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328 viruses and other security menaces. Also, preincrement/postincrement confusion can produce off-by-one errors that are extremely difficult to diagnose. There is a <code>plusplus</code> <a href="#options">option</a>
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329 that allows the use of these operators.</p>
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330 <h2 id=bitwise>Bitwise Operators</h2>
331 <p>JavaScript does not have an integer type, but it does have bitwise operators.
332 The bitwise operators convert their operands from floating point to integers
333 and back, so they are not as efficient as in C or other languages. They
334 are rarely useful in browser applications. The similarity to the logical
335 operators can mask some programming errors. The <code>bitwise</code> <a href="#options">option</a>
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336 allows the use of these operators: <code>&lt;&lt; &gt;&gt; &gt;&gt;&gt;
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337 ~ &amp; |</code>.</p>
338 <h2 id=evil><code>eval</code> is evil</h2>
339 <p>The <code>eval</code> function (and its relatives, <code>Function</code>,
340 <code>setTimeout</code>, and <code>setInterval</code>) provide access
341 to the JavaScript compiler. This is sometimes necessary, but in most cases
342 it indicates the presence of extremely bad coding. The <code>eval</code>
343 function is the most misused feature of JavaScript.</p>
345 <h2 id=void><code>void</code></h2>
346 <p>In most C-like languages, <code>void</code> is a type. In
347 JavaScript, <code>void</code> is a prefix operator that always
348 returns <code>undefined</code>. <code>JSLint</code> does not expect to
349 see <code>void</code> because it is confusing and not very useful.</p>
351 <h2 id=regexp>Regular Expressions</h2>
352 <p>Regular expressions are written in a terse and cryptic notation. <code>JSLint</code>
353 looks for problems that may cause portability problems. It also attempts
354 to resolve visual ambiguities by recommending explicit escapement.</p>
355 <p>JavaScript's syntax for regular expression literals overloads the <code>/</code>
356 character. To avoid ambiguity, <code>JSLint</code> expects that the character
357 preceding a regular expression literal is a <code>(</code> or <code>=</code>
358 or <code>:</code> or <code>,</code> character. </p>
359 <h2 id=new>Constructors and <code>new</code></h2>
360 <p>Constructors are functions that are designed to be used with the <code>new</code>
361 prefix. The <code>new</code> prefix creates a new object based on the
362 function's <code>prototype</code>, and binds that object to the function's
363 implied <code>this</code> parameter. If you neglect to use the <code>new</code>
364 prefix, no new object will be made and <code>this</code> will be bound
365 to the global object. This is a <a href="">serious
366 mistake</a>.</p>
367 <p><code>JSLint</code> enforces the convention that constructor functions
368 be given names with initial uppercase. <code>JSLint</code> does not expect
369 to see a function invocation with an initial uppercase name unless it
370 has the <code>new</code> prefix. <code>JSLint</code> does not expect to
371 see the <code>new</code> prefix used with functions whose names do not
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372 start with initial uppercase. This can be disabled with the <code>newcap</code>
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373 <a href="#options">option</a>.</p>
374 <p><code>JSLint</code> does not expect to see the wrapper forms <code>new Number</code>,
375 <code>new String</code>, <code>new Boolean</code>. </p>
376 <p><code>JSLint</code> does not expect to see <code>new Object</code> (use <code>{}</code>
377 instead). </p>
378 <p><code>JSLint</code> does not expect to see <code>new Array</code> (use <code>[]</code>
379 instead).</p>
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380 <h2 id=type>Type Inference</h2>
381 <p>Type inference is being added to JSLint. The goal is to ultimately make JSLint more helpful in spotting type inconsistencies and confusions. If you do not want this service, then select the <code>confusion</code> <a href="#options">option</a>.</p>
383 <h2 id=properties>Properties</h2>
384 <p>Since JavaScript is a loosely-typed, dynamic-object language, it is not
385 possible to determine at compile time if property names are spelled correctly.
386 <code>JSLint</code> provides some assistance with this.</p>
387 <p>At the bottom of its report, <code>JSLint</code> displays a <code>/*properties*/</code>
388 comment. It contains all of the names and string literals that were used
389 with dot notation, subscript notation, and object literals to name the
390 properties of objects. You can look through the list for misspellings. Property
391 names that were only used once are shown in italics. This is to make misspellings
392 easier to spot.</p>
393 <p>You can copy the <code>/*properties*/</code> comment into your script file.
394 <code>JSLint</code> will check the spelling of all property names against
395 the list. That way, you can have <code>JSLint</code> look for misspellings
396 for you. The directive respects function scope.</p>
397 <p>JSLint allows the property names to be annotated with types: <code>array</code>, <code>boolean</code>, <code>function</code>, <code>number</code>, <code>object</code>, <code>regexp</code>, <code>string</code>, or <code>*</code> (a wildcard allowing any type). A function type can be followed by another type, indicating a function's return type.</p>
398 <p>For example,</p>
399 <pre>/*properties
400 charAt: function string, slice: function *
401 */</pre>
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403 <h2 id=unsafe>Unsafe Characters</h2>
404 <p> There are characters that are handled inconsistently in browsers, and
405 so must be escaped when placed in strings. </p>
406 <pre>\u0000-\u001f
407 \u007f-\u009f
408 \u00ad
409 \u0600-\u0604
410 \u070f
411 \u17b4
412 \u17b5
413 \u200c-\u200f
414 \u2028-\u202f
415 \u2060-\u206f
416 \ufeff
417 \ufff0-\uffff</pre>
418 <h2 id=not>Not Looked For</h2>
420 <p><code>JSLint</code> does not do flow analysis to determine that variables are assigned
421 values before used. This is because variables are given a value (<code>undefined</code>)
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422 that is a reasonable default for many applications.</p>
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424 <p><code>JSLint</code> does not do any kind of global analysis. It does
425 not attempt to determine that functions used with <code>new</code> are
426 really constructors (<a href="#new">except by enforcing capitalization
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427 conventions</a>), or that property names are spelled correctly (<a href="#properties">except
428 for matching against the <code>/*properties */</code> comment</a>).</p>
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429 <h2 id=html>HTML</h2>
430 <p><code>JSLint</code> is able to handle HTML text. It can inspect the JavaScript content
431 contained within <code>&lt;script&gt;</code>...<code>&lt;/script&gt;</code> tags. It
432 also inspects the HTML content, looking for problems that are known to interfere
433 with JavaScript:</p>
434 <ul>
435 <li>All tag names must be in lower case.</li>
436 <li>All tags that can take a close tag (such as <code>&lt;/p&gt;</code>)
437 must have a close tag.</li>
438 <li>All tags are correctly nested.</li>
439 <li>The entity <code>&amp;lt;</code> must be used for literal <code>'&lt;'</code>.</li>
440 </ul>
441 <p><code>JSLint</code> is less anal than the sycophantic conformity demanded
442 by XHTML, but more strict than the popular browsers. </p>
443 <p><code>JSLint</code> also checks for the occurrence of<code> '&lt;/' </code>in
444 string literals. You should always write<code> '&lt;\/' </code>instead.
445 The extra backslash is ignored by the JavaScript compiler but not by the
446 HTML parser. Tricks like this should not be necessary, and yet they are.</p>
447 <p>There is a <code>cap</code> <a href="#options">option</a> that allows
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448 use of uppercase tag names. There is also an <code>on</code> <a href="#options">option</a>
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449 that allows the use of inline HTML event handlers.</p>
450 <p>There is a <code>fragment</code> <a href="#options">option</a> that can
451 inspect a well formed HTML fragment. If the <code>adsafe</code> <a href="#options">option</a>
452 is also used, then the fragment must be a <code>&lt;div&gt;</code> that
453 conforms to the <a href="">ADsafe</a> widget rules.</p>
454 <h2 id=css>CSS</h2>
455 <p><code>JSLint</code> can inspect CSS files. It expects the first line
456 of a CSS file to be </p>
457 <pre>@charset &quot;UTF-8&quot;;</pre>
458 <p>This feature is experimental. Please report any problems or limitations.
459 There is a <code>css</code> <a href="#options">option</a> that will tolerate
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460 some of the non-standard-but-customary workarounds. </p>
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462 <h2 id=options>Options</h2>
463 <p><code>JSLint</code> provides several options that control its operation and
464 its sensitivity. In the <a href="">web edition</a>, the
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465 options are selected with several checkboxes and two fields. </p>
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466 <p>It also provides assistance in constructing <code>/*jslint*/</code>
468 </p>
469 <p>When <code>JSLINT</code> is called as a function, it accepts an <code>option</code> object
470 parameter that allows you to determine the subset of JavaScript that is
471 acceptable to you. The web page version of <code>JSLint</code> at <a href=""></a>
472 does this for you. </p>
473 <p>Options can also be specified within a script with a <code>/*jslint */</code>
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474 directive:</p>
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475 <pre>/*jslint nomen: true, debug: true,
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476 evil: false, vars: true */</pre>
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477 <p>An option specification starts with <code>/*jslint</code>. Notice that
478 there is no space before the <code>j</code>. The specification contains
479 a sequence of name value pairs, where the names are <code>JSLint</code>
480 options, and the values are <code>true</code> or <code>false</code>. The
481 <code>indent</code> <a href="#options">option</a> can take a number. A <code>/*jslint */</code>
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482 comment takes precedence over the <code>option</code> object. The directive respects function scope.</p>
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483 <table>
484 <tbody>
485 <tr>
486 <th>Description</th>
487 <th><code>option</code></th>
488 <th>Meaning</th>
489 </tr>
490 <tr>
491 <td>ADsafe</td>
492 <td><code>adsafe</code></td>
493 <td><code>true</code> if <a href="">AD<span style="color: blue;">safe</span></a>
494 rules should be enforced. See <a href=""></a>.</td>
495 </tr>
496 <tr>
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497 <td>Tolerate bitwise operators </td>
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498 <td><code>bitwise</code></td>
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499 <td><code>true</code> if bitwise operators should be allowed. <a href="#bitwise"><small>(more)</small></a></td>
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500 </tr>
501 <tr>
502 <td>Assume a browser </td>
503 <td><code>browser</code></td>
504 <td><code>true</code> if the standard browser globals should be predefined.
505 <a href="#browser"><small>(more)</small></a> </td>
506 </tr>
507 <tr>
508 <td>Tolerate HTML case </td>
509 <td><code>cap</code></td>
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510 <td><code>true</code> if uppercase HTML should be allowed.</td>
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511 </tr>
512 <tr>
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513 <td>Tolerate type confusion<br>
514 </td>
515 <td><code>confusion</code></td>
516 <td><code>true</code> if variables and properties are allowed to contain more than one type of value.</td>
517 </tr>
518 <tr>
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519 <td>Tolerate <code>continue</code></td>
520 <td><code>continue</code></td>
521 <td><code>true</code> if the <code>continue</code> statement should be allowed.</td>
522 </tr>
523 <tr>
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524 <td>Tolerate CSS workarounds</td>
525 <td><code>css</code></td>
526 <td><code>true</code> if CSS workarounds should be tolerated. <a href="#css"><small>(more)</small></a></td>
527 </tr>
528 <tr>
529 <td>Tolerate debugger statements</td>
530 <td><code>debug</code></td>
531 <td><code>true</code> if <code>debugger</code> statements should be
532 allowed. Set this option to <code>false</code> before going into production.</td>
533 </tr>
534 <tr>
535 <td>Assume <code>console</code>, <code>alert</code>, ...</td>
536 <td><code>devel</code></td>
537 <td><code>true</code> if browser globals that are useful in development should be
538 predefined. (<a href="#devel">more</a>)</td>
539 </tr>
540 <tr>
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541 <td>Tolerate <code>==</code> and <code>!=</code></td>
542 <td><code>eqeq</code></td>
543 <td><code>true</code> if the <code>==</code> and <code>!=</code> operators should be tolerated. (<a href="#eqeq">more</a>).</td>
544 <tr>
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545 <td>Tolerate ES5 syntax</td>
546 <td><code>es5</code></td>
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547 <td><code>true</code> if ES5 syntax should be allowed.</a> It is likely that programs using this option will produce syntax errors on ES3 systems.</td>
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548 </tr>
549 <tr>
550 <td>Tolerate <code>eval</code> </td>
551 <td><code>evil</code></td>
552 <td><code>true</code> if <code>eval</code> should be allowed. <a href="#evil"><small>(more)</small></a></td>
553 </tr>
554 <tr>
555 <td>Tolerate unfiltered for in </td>
556 <td><code>forin</code></td>
557 <td><code>true</code> if unfiltered <code>for</code> <code>in</code>
558 statements should be allowed. <a href="#forin"><small>(more)</small></a></td>
559 </tr>
560 <tr>
561 <td>Tolerate HTML fragments </td>
562 <td><code>fragment</code></td>
563 <td><code>true</code> if HTML fragments should be allowed. <a href="#html"><small>(more)</small></a></td>
564 </tr>
565 <tr>
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566 <td>Strict white space indentation</td>
567 <td><code>indent</code></td>
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568 <td>The number of spaces used for indentation (default is 4). If 0, then no indentation checking takes place.</td>
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569 </tr>
570 <tr>
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571 <td>Maximum number of errors</td>
572 <td><code>maxerr</code></td>
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573 <td>The maximum number of warnings reported. (default is 50)</td>
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574 </tr>
575 <tr>
576 <td>Maximum line length</td>
577 <td><code>maxlen</code></td>
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578 <td>The maximum number of characters in a line.</td>
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579 </tr>
580 <tr>
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581 <td>Tolerate uncapitalized constructors</td>
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582 <td><code>newcap</code></td>
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583 <td><code>true</code> if Initial Caps with constructor
584 functions is optional. <a href="#new"><small>(more)</small></a></td>
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585 </tr>
586 <tr>
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587 <td>Assume Node.js</td>
588 <td><code>node</code></td>
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589 <td><code>true</code> if Node.js globals should be predefined. <a href="#node"><small>(more)</small></a></td>
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590 </tr>
591 <tr>
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592 <td>Tolerate dangling _ in identifiers </td>
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593 <td><code>nomen</code></td>
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594 <td><code>true</code> if names should not be checked for initial or trailing underbars.</td>
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595 </tr>
596 <tr>
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597 <td>Tolerate HTML event handlers </td>
598 <td><code>on</code></td>
599 <td><code>true</code> if HTML event handlers should be allowed. <a href="#html"><small>(more)</small></a></td>
600 </tr>
601 <tr>
602 <td>Stop on first error </td>
603 <td><code>passfail</code></td>
604 <td><code>true</code> if the scan should stop on first error.</td>
605 </tr>
606 <tr>
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607 <td>Tolerate <code>++</code> and <code>--</code> </td>
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608 <td><code>plusplus</code></td>
609 <td><code>true</code> if <code>++</code> and <code>--</code> should
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610 be allowed. <a href="#inc"><small>(more)</small></a></td>
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611 </tr>
612 <tr>
613 <td>Predefined <small>( , separated)</small></td>
614 <td><code>predef</code></td>
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615 <td>An array of strings, the names of predefined global variables, or an object whose keys are global variable names, and whose values are booleans that determine if each variable is assignable (also see <a href="#global">global</a>). <code>predef</code> is used with the <code>option</code> object, but not
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616 with the <code>/*jslint */</code> comment. You can also use the <code>var</code>
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617 statement to declare global variables in a script file.</td>
618 </tr>
619 <tr>
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620 <td>Tolerate <code>.</code> and <code>[^</code>...<code>]</code>. in /RegExp/ </td>
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621 <td><code>regexp</code></td>
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622 <td><code>true</code> if <code>.</code> and <code>[^</code>...<code>]</code> should be allowed in RegExp
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623 literals. They match more material than might be expected, allowing attackers to confuse applications. These forms should not be used when validating in secure applications. </td>
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624 </tr>
625 <tr>
626 <td>Assume Rhino </td>
627 <td><code>rhino</code></td>
628 <td><code>true</code> if the <a href="">Rhino</a>
629 environment globals should be predefined. <a href="#rhino"><small>(more)</small></a></td>
630 </tr>
631 <tr>
632 <td>Safe Subset </td>
633 <td><code>safe</code></td>
634 <td><code>true</code> if the safe subset rules are enforced. These rules
635 are used by <a href="">ADsafe</a>. It enforces
636 the safe subset rules but not the widget structure rules.</td>
637 </tr>
638 <tr>
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639 <td> Tolerate missing&nbsp;<code>'use strict'</code>&nbsp;pragma </td>
640 <td><code>sloppy</code></td>
641 <td><code>true</code> if the ES5 <code><a href="">'use strict';</a></code> pragma
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642 is not required. Do not use this pragma unless you know what you are doing.</td>
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643 </tr>
644 <tr>
645 <td>Tolerate inefficient subscripting<br>
646 </td>
647 <td><code>sub</code></td>
648 <td><code>true</code> if subscript notation may be used for expressions
649 better expressed in dot notation.</td>
650 </tr>
651 <tr>
a27d17c @douglascrockford Tolerate
652 <td> Tolerate misordered definitions </td>
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653 <td><code>undef</code></td>
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654 <td><code>true</code> if variables and functions need not be declared before used. <a href="#undefined"><small>(more)</small></a></td>
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655 </tr>
656 <tr>
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657 <td> Tolerate unused parameters</td>
658 <td><code>unparam</code></td>
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659 <td><code>true</code> if warnings should not be given for unused parameters.</td>
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660 </tr>
661 <tr>
662 <td>Tolerate many&nbsp;<tt>var</tt>&nbsp;statements per function</td>
663 <td><code>vars</code></td>
664 <td><code>true</code> if multiple <code>var</code> statement per function
665 should be allowed. <a href="#scope"><small>(more)</small></a></td>
666 </tr>
667 <tr>
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668 <td> Tolerate messy white space</td>
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669 <td><code>white</code></td>
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670 <td><code>true</code> if strict whitespace rules should be ignored.</td>
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671 </tr>
672 <tr>
673 <td>Assume a Yahoo Widget </td>
674 <td><code>widget</code></td>
675 <td><code>true</code> if the <a href="">Yahoo
676 Widgets</a> globals should be predefined. <a href="#widget"><small>(more)</small></a></td>
677 </tr>
678 <tr>
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679 <td>Assume Windows</td>
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680 <td><code>windows</code></td>
681 <td><code>true</code> if the Windows</a> globals should be predefined. <a href="#windows"><small>(more)</small></a></td>
682 </tr>
683 </tbody>
684 </table>
685 <h2 id=report>Report</h2>
687 <p>If <code>JSLint</code> is able to complete its scan, it generates a function
688 report. It lists for each function:</p>
690 <ul>
691 <li>The line number on which it starts.</li>
692 <li>Its name. In the case of anonymous functions, <code>JSLint</code>
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693 will 'guess' the name.</li>
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694 <li>The parameters.</li>
695 <li><i>Closure</i>: The variables and parameters that are declared in
696 the function that are used by its inner functions.</li>
697 <li><i>Variables</i>: The variables that are declared in the function
698 that are used only by the function.</li>
699 <li><i>Exceptions</i>: The variables that are declared by try statements.</li>
700 <li><i>Unused</i>: The variables that are declared in the function that
701 are not used. This may be an indication of an error.</li>
702 <li><i>Outer</i>: Variables used by this function that are declared in
703 another function.</li>
704 <li><i>Global</i>: Global variables that are used by this function. Keep
705 these to a minimum.</li>
706 <li><i>Label</i>: Statement labels that are used by this function.</li>
707 </ul>
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708 <p>The report will also include a list of all of the <a href="#properties">property
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709 names</a> that were used. There is a <a href="msgs.html">list of <code>JSLint</code>
710 messages</a>.</p>
711 <h2 id=feedback>Feedback</h2>
712 <p>Please let me know if <code>JSLint</code> is useful for you. Is it too
713 strict? Is there a check or a report that could help you to improve the
714 quality of your programs? <a href=""></a></p>
716 <p>I intend to continue to adapt <code>JSLint</code> based on your comments.
717 Keep watching for improvements. Updates are announced at <a href=""></a>.</p>
719 <h2 id=try>Try it</h2>
721 <p><a href="" target="_blank">Try it.</a> Paste your script
722 into the window and click the
723 <a href="" target=jslint><input type="button" value="JSLint"></a>
724 button. The analysis is done by a script running on your machine.
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725 Your script is not sent over the network. You can set the <a href="#options">options</a> used. </p>
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726 <p>
727 JSLint is written entirely in JavaScript, so it can run anywhere that JavaScript can run. See for example <a href="">;tbl=1</a>.</p>
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728 <h2 id=implementation>Implementation</h2>
729 <p><code>JSLint</code> uses a <a href="">Pratt
730 Parser (Top Down Operator Precedence)</a>. It is written in JavaScript.
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731 The full source code is available: <a href=""></a>.</p>
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732 <a href=""><img src="jslintpill.gif" width="36" height="17" border="0"></a>
733 <a href=""><img src="adsafepill.gif" width="36" height="17" border="0"></a>
734 <a href=""><img src="jsonpill.gif" width="36" height="17" border="0"></a>
735 </body>
736 </html>
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