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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="http://www.JSLint.com/favicon.gif">
5 <style>
6 body {
7 background-color: linen;
8 margin-left: 8%;
9 margin-right: 8%;
10 }
11 pre {margin-left: 40px;}
12 table {margin: 10px; border: 0px;}
13 th, td {border: black solid 1pt; padding-left: 10px;
14 padding-right: 10px; vertical-align: top;}
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16 background-color: thistle;
17 }
18 td {
19 background-color: white;
20 }
21 #top table {margin: 0px;}
22 #top td {background-color: linen; border: 0pt; vertical-align: middle;}
23 ul {list-style-type: square;}
24 input[type="button"] {border: 2px solid black;}
25 a:link {color: darkblue;}
26 a:visited {color: purple;}
27 a:hover {color: blue; text-decoration: underline;}
28 a:active {color: red;}
29 </style>
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="http://javascript.crockford.com/">JavaScript</a> Code Quality Tool</big></p>
38 <p><a href="http://www.crockford.com/" 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="http://www.JSLint.com" target="_blank"><code>JSLint</code></a>
45 will hurt your feelings.</p>
46 <h2 id=what>What is <code>JSLint</code>?</h2>
47
48 <p><a href="http://www.JSLint.com" 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>
51
52 <p>When <a href="http://en.wikipedia.org/wiki/C_programming_language">C</a>
53 was a <a href="http://cm.bell-labs.com/cm/cs/who/dmr/chist.html">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="http://en.wikipedia.org/wiki/Lint_programming_tool">lint</a></code>
57 was developed that would scan a source file, looking for problems.</p>
58
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>
62
63 <p><a href="http://javascript.crockford.com/">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="http://www.JSLint.com/"><code>JSLint</code></a>,
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68 a JavaScript syntax checker and validator.</p>
69
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>
76
77 <p><code>JSLint</code> defines a professional subset of JavaScript, a stricter
78 language than that defined by <a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm" 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="http://javascript.crockford.com/code.html" 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="http://www.JSON.org/">JSON</a>
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86 text.</p>
87 <h2 id=global>Global Variables</h2>
88 <p>JavaScript's <a href="http://yuiblog.com/blog/2006/06/01/global-domination/">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>
102 <p>The declaration should appears near the top of the file. It must appear before the use of the variables
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> comment 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> indicated that the variable may be assigned to by this file, and <code>false</code> indicating that assignment is not allowed (which is the default).</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>
112 <code>/*global addEventListener: false, blur: false, clearInterval: false, clearTimeout: false, close: false, closed: false, defaultStatus: false, document: false, event: false, focus: false, frames: false, getComputedStyle: false, history: false, Image: false, length: false, location: false, moveBy: false, moveTo: false, name: false, navigator: false, onblur: true, onerror: true, onfocus: true, onload: true, onresize: true, onunload: true, open: false, opener: false, Option: false, parent: false, print: false, resizeBy: false, resizeTo: false, screen: false, scroll: false, scrollBy: false, scrollTo: false, setInterval: false, setTimeout: false, status: false, top: false, XMLHttpRequest: false */</code></blockquote>
113 <p> The <code>browser</code> <a href="#options">option</a> does
114 not include the aliases of the global object, <code>window</code> and
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115 <code>self</code> because the potential for misuse is so great.</p>
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116 <p id=devel>Select the
117 <label for="JSLINT_DEVEL" title="devel"><em>Assume console, alert, ...</em></label>
118 (<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
119 effect as this comment:</p>
120 <pre>/*global alert: false, confirm: false, console: false, Debug: false, opera: false, prompt: false */</pre>
121 <p id=rhino>Select the <i>Assume Rhino</i> (<code>rhino</code>) <a href="#options">option</a>
122 to predefine the global properties provided by the Rhino environment.
123 It has the same effect as this statement:</p>
124 <blockquote>
125 <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>
126 </blockquote>
127 <p id=widget>Select the <i>Assume a Yahoo Widget</i> (<code>widget</code>)
128 <a href="#options">option</a> to predefine the global properties provided
129 by the Yahoo! Widgets environment. It has the same effect as this statement:</p>
130 <blockquote>
131 <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>
132 </blockquote>
133 <p id=windows>Select the <i>Assume Windows</i> (<code>windows</code>)
134 <a href="#options">option</a> to predefine the global properties provided by Microsoft Windows. It has the same effect as this statement:</p>
135 <blockquote>
136 <p><code>/*global ActiveXObject: false, CScript: false, Debug: false, Enumerator: false, System: false, VBArray: false, WScript: false */</code></p>
137 </blockquote>
138 <h2 id=semicolon>Semicolon</h2>
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139 <p>JavaScript uses a C-like syntax which requires the use of semicolons to delimit certain
140 statements. JavaScript attempts to make those semicolons optional with a semicolon
141 insertion mechanism. This is dangerous because it can mask errors.</p>
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142 <p>Like C, JavaScript has <code>++</code> and <code>--</code> and <code>(</code> operators
143 which can be prefixes or suffixes. The disambiguation is done by the semicolon.</p>
144 <p>In JavaScript, a linefeed can be whitespace or it can act as a semicolon.
145 This replaces one ambiguity with another. </p>
146 <p><code>JSLint</code> expects that every statement be followed by <code>;</code> except
147 for <code>for</code>, <code>function</code>, <code>if</code>, <code>switch</code>, <code>try</code>, and
148 <code>while</code>. <code>JSLint</code> does not expect to see unnecessary semicolons or the
149 empty statement.</p>
150 <h2 id=breaking>Line Breaking</h2>
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151 <p>As a further defense against the semicolon insertion mechanism and editing mishaps, <code>JSLint</code>
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152 expects long statements to be broken only after one of these punctuation
153 characters or operators:</p>
154 <p align="center"><code>, ; : { } ( [ = &lt; &gt; ? ! + - * / % ~ ^ | &amp; </code></p>
155 <p align="center"><code>== != &lt;= &gt;= += -= *= /= %= ^= |= &amp;= &lt;&lt; &gt;&gt;
156 || &amp;&amp; </code></p>
157 <p align="center"><code>=== !== &lt;&lt;= &gt;&gt;= &gt;&gt;&gt; &gt;&gt;&gt;= </code></p>
158 <p><code>JSLint</code> does not expect to see a long statement broken after
159 an identifier, a string, a number, closer, or a suffix operator:</p>
160 <p align="center"><code>. ) ] ++ --</code></p>
161
162 <p><code>JSLint</code> allows you to turn on the <i>Tolerate sloppy line
163 breaking</i> (<code>laxbreak</code>) <a href="#options">option</a>. </p>
164 <p>Semicolon insertion can mask copy/paste errors. If you always break lines
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165 after infix operators, then <code>JSLint</code> can do better at finding other errors.</p>
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166 <h2 id=comma>Comma</h2>
167 <p>The comma operator can lead to excessively tricky expressions. It can also
168 mask some programming errors.</p>
169 <p><code>JSLint</code> expects to see the comma used as a separator, but not as an
170 operator (except in the initialization and incrementation parts of the <code>for</code>
171 statement). It does not expect to see elided elements in array literals. Extra
172 commas should not be used. A comma should not appear after the last element
173 of an array literal or object literal because it can be misinterpreted by some
174 browsers. </p>
175 <h2 id=scope>Scope</h2>
176
177 <p>In many languages, a block introduces a scope. Variables introduced in
178 a block are not visible outside of the block.</p>
179
180 <p>In JavaScript, blocks do not introduce a scope. There is only function-scope.
181 A variable introduced anywhere in a function is visible everywhere in
182 the function. JavaScript's blocks confuse experienced programmers and
183 lead to errors because the familiar syntax makes a false promise.</p>
184
185 <p><code>JSLint</code> expects blocks with <code>function</code>, <code>if</code>,
186 <code>switch</code>, <code>while</code>, <code>for</code>, <code>do</code>,
187 and <code>try</code> statements and nowhere else. </p>
188 <p>In languages with block scope, it is usually recommended that variables
189 be declared at the site of first use. But because JavaScript does not
190 have block scope, it is wiser to declare all of a function's variables
191 at the top of the function. It is recommended that a single <code>var</code>
192 statement be used per function. This can be enforced with the <code>onevar</code>
193 <a href="#options">option</a>.</p>
194
195 <h2 id=required>Required Blocks</h2>
196
197 <p><code>JSLint</code> expects that <code>if</code>, <code>while</code>,
198 <code>do</code> and <code>for</code> statements will be made with blocks
199 <code>{</code>that is, with statements enclosed in braces<code>}</code>.</p>
200
201 <p>JavaScript allows an <code>if</code> to be written like this:</p>
202
203 <pre>if (<i>condition</i><code>)
204 </code><i>statement</i>;</pre>
205
206 <p>That form is known to contribute to mistakes in projects where many programmers
207 are working on the same code. That is why <code>JSLint</code> expects the use of
208 a block:</p>
209
210 <pre>if (<i>condition</i>) {
211 <i>statements</i>;
212 }</pre>
213
214 <p>Experience shows that this form is more resilient.</p>
215
216 <h2 id=expression>Expression Statements</h2>
217 <p>An expression statement is expected to be an assignment or a function/method
218 call or <code>delete</code>. All other expression statements are considered
219 to be errors.</p>
220 <h2 id=forin><code>for</code> <code>in</code></h2>
221 <p>The <code>for</code> <code>in</code> statement allows for looping through
222 the names of all of the properties of an object. <a href="http://yuiblog.com/blog/2006/09/26/for-in-intrigue/">Unfortunately,
223 it also loops through all of the members which were inherited through
224 the prototype chain.</a> This has the bad side effect of serving up method
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225 functions when the interest is in data members. If a program is written without awareness of this situation, then it can fail.</p>
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226 <p>The body of every <code>for</code> <code>in</code> statement should be
227 wrapped in an <code>if</code> statement that does filtering. It can select
228 for a particular type or range of values, or it can exclude functions,
229 or it can exclude properties from the prototype. For example,</p>
230 <pre>for (name in object) {
231 if (object.hasOwnProperty(name)) {
232 ....
233 }
234
235 }</pre>
236
237 <h2 id=switch><code>switch</code></h2>
238 <p>A <a href="http://yuiblog.com/blog/2007/04/25/id-rather-switch-than-fight/">common
239 error</a> in <code>switch</code> statements is to forget to place a <code>break</code>
240 statement after each case, resulting in unintended fall-through. <code>JSLint</code>
241 expects that the statement before the next <code>case</code> or <code>default</code>
242 is one of these: <code>break</code>, <code>return</code>, or <code>throw</code>.
243 </p>
244 <h2 id=var><code>var</code></h2>
245
246 <p>JavaScript allows <code>var</code> definitions to occur anywhere
247 within a function. <code>JSLint</code> is more strict.</p>
248
249 <p><code>JSLint</code> expects that a <code>var</code> will be declared
250 only once, and that it will be declared before it is used.</p>
251 <p><code></code><code>JSLint</code> expects that a <code>function</code>
252 will be declared before it is used.</p>
253 <p><code>JSLint</code> expects that parameters will not also be declared
254 as vars. </p>
255
256 <p><code>JSLint</code> does not expect the <code>arguments</code> array to be declared
257 as a <code>var</code>.</p>
258 <p><code>JSLint</code> does not expect that a var will be defined in a block.
259 This is because JavaScript blocks do not have block scope. This can have
260 unexpected consequences. Define all variables at the top of the function.</p>
261
262 <h2 id=with><code>with</code></h2>
263
264 <p>The <code>with</code> statement was intended to provide a shorthand in accessing
265 members in deeply nested objects. Unfortunately, it behaves <a href="http://yuiblog.com/blog/2006/04/11/with-statement-considered-harmful/">very
266 badly</a> when setting new members. Never use the <code>with</code> statement. Use
267 a <code>var</code> instead.</p>
268
269 <p><code>JSLint</code> does not expect to see a <code>with</code> statement.</p>
270
271 <h2 id=assignment>=</h2>
272 <p><code>JSLint</code> does not expect to see an assignment statement in
273 the condition part of an <code>if</code> or <code>for</code> or <code>while</code>
274 <code></code> or <code>do</code> statement. This is because it is more
275 likely that </p>
276 <pre>if (a = b) {
277 ...
278 }</pre>
279 <p>was intended to be </p>
280 <pre>if (a == b) {
281 ...
282 }</pre>
283 <p>It is difficult to write correct programs while using idioms that are
284 hard to distinguish from obvious errors. If you really intend an assignment,
285 wrap it in another set of parens:</p>
286 <pre>if ((a = b)) {
287 ...
288 }</pre>
289
290 <h2 id=equal>== and !=</h2>
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
293 be <code>true</code>. This can mask type errors.</p>
294 <p>When comparing to any of the following values, use the <code>===</code>
295 or <code>!==</code> operators (which do not do type coercion): <code>0
296 '' undefined null false true</code></p>
297 <p align="left">If you only care that a value is <i>truthy</i> or <i>falsy</i>,
298 then use the short form. Instead of </p>
299 <pre align="left">(foo != 0)</pre>
300 <p align="left">just say </p>
301 <pre align="left">(foo)</pre>
302 <p align="left">and instead of</p>
303 <pre align="left">(foo == 0)</pre>
304 <p align="left"> say</p>
305 <pre align="left">(!foo)</pre>
306 <p>The <code>===</code> and <code>!==</code> operators are preferred. There
307 is an <code>eqeqeq</code> <a href="#options">option</a> that requires
308 the use of <code>===</code> and <code>!==</code> in all cases.</p>
309 <h2 id=labels>Labels</h2>
310 <p>JavaScript allows any statement to have a label, and labels have a
311 separate name space. <code>JSLint</code> is more strict.</p>
312
313 <p><code>JSLint</code> expects labels only on statements that interact
314 with <code>break</code>: <code>switch</code>, <code>while</code>,
315 <code>do</code>, and <code>for</code>. <code>JSLint</code> expects that labels
316 will be distinct from vars and parameters.</p>
317
318 <h2 id=unreachable>Unreachable Code</h2>
319 <p><code>JSLint</code> expects that
320 a <code>return</code>, <code>break</code>, <code>continue</code>,
321 or <code>throw</code> statement will be followed by
322 a <code>}</code> or <code>case</code> or <code>default</code>.</p>
323
324 <h2 id=pluses>Confusing Pluses and Minuses</h2>
325
326 <p><code>JSLint</code> expects that <code>+</code> will not be followed by
327 <code>+</code> or <code>++</code>, and that <code>-</code> will not be followed
328 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>
329 <h2 id=inc><code>++</code> and <code>--</code></h2>
330 <p>The <code>++</code> <small>(increment)</small> and <code>--</code> <small>(decrement)</small>
331 operators have been known to contribute to bad code by encouraging excessive
332 trickiness. They are second only to faulty architecture in enabling to
333 viruses and other security menaces. There is a <code>plusplus</code> <a href="#options">option</a>
334 that prohibits the use of these operators.</p>
335 <h2 id=bitwise>Bitwise Operators</h2>
336 <p>JavaScript does not have an integer type, but it does have bitwise operators.
337 The bitwise operators convert their operands from floating point to integers
338 and back, so they are not as efficient as in C or other languages. They
339 are rarely useful in browser applications. The similarity to the logical
340 operators can mask some programming errors. The <code>bitwise</code> <a href="#options">option</a>
341 prohibits the use of these operators: <code>&lt;&lt; &gt;&gt; &gt;&gt;&gt;
342 ~ &amp; |</code>.</p>
343 <h2 id=evil><code>eval</code> is evil</h2>
344 <p>The <code>eval</code> function (and its relatives, <code>Function</code>,
345 <code>setTimeout</code>, and <code>setInterval</code>) provide access
346 to the JavaScript compiler. This is sometimes necessary, but in most cases
347 it indicates the presence of extremely bad coding. The <code>eval</code>
348 function is the most misused feature of JavaScript.</p>
349
350 <h2 id=void><code>void</code></h2>
351 <p>In most C-like languages, <code>void</code> is a type. In
352 JavaScript, <code>void</code> is a prefix operator that always
353 returns <code>undefined</code>. <code>JSLint</code> does not expect to
354 see <code>void</code> because it is confusing and not very useful.</p>
355
356 <h2 id=regexp>Regular Expressions</h2>
357 <p>Regular expressions are written in a terse and cryptic notation. <code>JSLint</code>
358 looks for problems that may cause portability problems. It also attempts
359 to resolve visual ambiguities by recommending explicit escapement.</p>
360 <p>JavaScript's syntax for regular expression literals overloads the <code>/</code>
361 character. To avoid ambiguity, <code>JSLint</code> expects that the character
362 preceding a regular expression literal is a <code>(</code> or <code>=</code>
363 or <code>:</code> or <code>,</code> character. </p>
364 <h2 id=new>Constructors and <code>new</code></h2>
365 <p>Constructors are functions that are designed to be used with the <code>new</code>
366 prefix. The <code>new</code> prefix creates a new object based on the
367 function's <code>prototype</code>, and binds that object to the function's
368 implied <code>this</code> parameter. If you neglect to use the <code>new</code>
369 prefix, no new object will be made and <code>this</code> will be bound
370 to the global object. This is a <a href="http://yuiblog.com/blog/2006/11/13/javascript-we-hardly-new-ya/">serious
371 mistake</a>.</p>
372 <p><code>JSLint</code> enforces the convention that constructor functions
373 be given names with initial uppercase. <code>JSLint</code> does not expect
374 to see a function invocation with an initial uppercase name unless it
375 has the <code>new</code> prefix. <code>JSLint</code> does not expect to
376 see the <code>new</code> prefix used with functions whose names do not
377 start with initial uppercase. This can be controlled with the <code>newcap</code>
378 <a href="#options">option</a>.</p>
379 <p><code>JSLint</code> does not expect to see the wrapper forms <code>new Number</code>,
380 <code>new String</code>, <code>new Boolean</code>. </p>
381 <p><code>JSLint</code> does not expect to see <code>new Object</code> (use <code>{}</code>
382 instead). </p>
383 <p><code>JSLint</code> does not expect to see <code>new Array</code> (use <code>[]</code>
384 instead).</p>
385 <h2 id=unsafe>Unsafe Characters</h2>
386 <p> There are characters that are handled inconsistently in browsers, and
387 so must be escaped when placed in strings. </p>
388 <pre>\u0000-\u001f
389 \u007f-\u009f
390 \u00ad
391 \u0600-\u0604
392 \u070f
393 \u17b4
394 \u17b5
395 \u200c-\u200f
396 \u2028-\u202f
397 \u2060-\u206f
398 \ufeff
399 \ufff0-\uffff</pre>
400 <h2 id=not>Not Looked For</h2>
401
402 <p><code>JSLint</code> does not do flow analysis to determine that variables are assigned
403 values before used. This is because variables are given a value (<code>undefined</code>)
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404 that is a reasonable default for many applications.</p>
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405
406 <p><code>JSLint</code> does not do any kind of global analysis. It does
407 not attempt to determine that functions used with <code>new</code> are
408 really constructors (<a href="#new">except by enforcing capitalization
409 conventions</a>), or that property names are spelled correctly (<a href="#members">except
410 for matching against the <code>/*members */</code> comment</a>).</p>
411 <h2 id=html>HTML</h2>
412 <p><code>JSLint</code> is able to handle HTML text. It can inspect the JavaScript content
413 contained within <code>&lt;script&gt;</code>...<code>&lt;/script&gt;</code> tags. It
414 also inspects the HTML content, looking for problems that are known to interfere
415 with JavaScript:</p>
416 <ul>
417 <li>All tag names must be in lower case.</li>
418 <li>All tags that can take a close tag (such as <code>&lt;/p&gt;</code>)
419 must have a close tag.</li>
420 <li>All tags are correctly nested.</li>
421 <li>The entity <code>&amp;lt;</code> must be used for literal <code>'&lt;'</code>.</li>
422 </ul>
423 <p><code>JSLint</code> is less anal than the sycophantic conformity demanded
424 by XHTML, but more strict than the popular browsers. </p>
425 <p><code>JSLint</code> also checks for the occurrence of<code> '&lt;/' </code>in
426 string literals. You should always write<code> '&lt;\/' </code>instead.
427 The extra backslash is ignored by the JavaScript compiler but not by the
428 HTML parser. Tricks like this should not be necessary, and yet they are.</p>
429 <p>There is a <code>cap</code> <a href="#options">option</a> that allows
430 use of upper case tag names. There is also an <code>on</code> <a href="#options">option</a>
431 that allows the use of inline HTML event handlers.</p>
432 <p>There is a <code>fragment</code> <a href="#options">option</a> that can
433 inspect a well formed HTML fragment. If the <code>adsafe</code> <a href="#options">option</a>
434 is also used, then the fragment must be a <code>&lt;div&gt;</code> that
435 conforms to the <a href="http://www.ADsafe.org/">ADsafe</a> widget rules.</p>
436 <h2 id=css>CSS</h2>
437 <p><code>JSLint</code> can inspect CSS files. It expects the first line
438 of a CSS file to be </p>
439 <pre>@charset &quot;UTF-8&quot;;</pre>
440 <p>This feature is experimental. Please report any problems or limitations.
441 There is a <code>css</code> <a href="#options">option</a> that will tolerate
442 some of the non-standard-but-customary workarounds. <br>
443 </p>
444
445 <h2 id=options>Options</h2>
446 <p><code>JSLint</code> provides several options that control its operation and
447 its sensitivity. In the <a href="http://www.JSLint.com/">web edition</a>, the
448 options are selected with several checkboxes and two fields. Clicking on
449 the <a href="http://www.amazon.com/exec/obidos/ASIN/0596517742/wrrrldwideweb"
450 target="_blank"><input type="button" value="Good Parts"></a> button will give
451 you the ideal settings.
452 </p>
453 <p>It also provides assistance in constructing <code>/*jslint*/</code>
454 comments.
455 </p>
456 <p>When <code>JSLINT</code> is called as a function, it accepts an <code>option</code> object
457 parameter that allows you to determine the subset of JavaScript that is
458 acceptable to you. The web page version of <code>JSLint</code> at <a href="http://www.JSLint.com/">http://www.JSLint.com/</a>
459 does this for you. </p>
460 <p>Options can also be specified within a script with a <code>/*jslint */</code>
461 comment:</p>
462 <pre>/*jslint nomen: true, debug: true,
463 evil: false, onevar: true */</pre>
464 <p>An option specification starts with <code>/*jslint</code>. Notice that
465 there is no space before the <code>j</code>. The specification contains
466 a sequence of name value pairs, where the names are <code>JSLint</code>
467 options, and the values are <code>true</code> or <code>false</code>. The
468 <code>indent</code> <a href="#options">option</a> can take a number. A <code>/*jslint */</code>
469 comment takes precedence over the <code>option</code> object. </p>
470 <table>
471 <tbody>
472 <tr>
473 <th>Description</th>
474 <th><code>option</code></th>
475 <th>Meaning</th>
476 </tr>
477 <tr>
478 <td>ADsafe</td>
479 <td><code>adsafe</code></td>
480 <td><code>true</code> if <a href="http://www.ADsafe.org/">AD<span style="color: blue;">safe</span></a>
481 rules should be enforced. See <a href="http://www.ADsafe.org/">http://www.ADsafe.org/</a>.</td>
482 </tr>
483 <tr>
484 <td>Disallow bitwise operators </td>
485 <td><code>bitwise</code></td>
486 <td><code>true</code> if bitwise operators should not be allowed. <a href="#bitwise"><small>(more)</small></a></td>
487 </tr>
488 <tr>
489 <td>Assume a browser </td>
490 <td><code>browser</code></td>
491 <td><code>true</code> if the standard browser globals should be predefined.
492 <a href="#browser"><small>(more)</small></a> </td>
493 </tr>
494 <tr>
495 <td>Tolerate HTML case </td>
496 <td><code>cap</code></td>
497 <td><code>true</code> if upper case HTML should be allowed.</td>
498 </tr>
499 <tr>
500 <td>Tolerate CSS workarounds</td>
501 <td><code>css</code></td>
502 <td><code>true</code> if CSS workarounds should be tolerated. <a href="#css"><small>(more)</small></a></td>
503 </tr>
504 <tr>
505 <td>Tolerate debugger statements</td>
506 <td><code>debug</code></td>
507 <td><code>true</code> if <code>debugger</code> statements should be
508 allowed. Set this option to <code>false</code> before going into production.</td>
509 </tr>
510 <tr>
511 <td>Assume <code>console</code>, <code>alert</code>, ...</td>
512 <td><code>devel</code></td>
513 <td><code>true</code> if browser globals that are useful in development should be
514 predefined. (<a href="#devel">more</a>)</td>
515 </tr>
516 <tr>
517 <td>Disallow <code>==</code> and <code>!=</code></td>
518 <td><code>eqeqeq</code></td>
519 <td><code>true</code> if <code>===</code> should be required. <a href="#equal"><small>(more)</small></a></td>
520 </tr>
521 <tr>
522 <td>Tolerate ES5 syntax</td>
523 <td><code>es5</code></td>
524 <td><code>true</code> if ES5 syntax should be allowed.</a></td>
525 </tr>
526 <tr>
527 <td>Tolerate <code>eval</code> </td>
528 <td><code>evil</code></td>
529 <td><code>true</code> if <code>eval</code> should be allowed. <a href="#evil"><small>(more)</small></a></td>
530 </tr>
531 <tr>
532 <td>Tolerate unfiltered for in </td>
533 <td><code>forin</code></td>
534 <td><code>true</code> if unfiltered <code>for</code> <code>in</code>
535 statements should be allowed. <a href="#forin"><small>(more)</small></a></td>
536 </tr>
537 <tr>
538 <td>Tolerate HTML fragments </td>
539 <td><code>fragment</code></td>
540 <td><code>true</code> if HTML fragments should be allowed. <a href="#html"><small>(more)</small></a></td>
541 </tr>
542 <tr>
543 <td>Require parens around immediate invocations</td>
544 <td><code>immed</code></td>
545 <td><code>true</code> if immediate function invocations must be wrapped
546 in parens</td>
547 </tr>
548 <tr>
549 <td>Strict white space indentation</td>
550 <td><code>indent</code></td>
551 <td>The number of spaces used for indentation (default is 4)</td>
552 </tr>
553 <tr>
554 <td>Tolerate sloppy line breaking </td>
555 <td><code>laxbreak</code></td>
556 <td><code>true</code> if statement breaks should not be checked. <a href="#breaking"><small>(more)</small></a></td>
557 </tr>
558 <tr>
559 <td>Maximum number of errors</td>
560 <td><code>maxerr</code></td>
561 <td>The maximum number of warnings reported (default is 50)</td>
562 </tr>
563 <tr>
564 <td>Maximum line length</td>
565 <td><code>maxlen</code></td>
566 <td>The maximum number of characters in a line</td>
567 </tr>
568 <tr>
569 <td>Disallow dangling _ in identifiers </td>
570 <td><code>nomen</code></td>
571 <td><code>true</code> if names should be checked for initial or trailing underbars</td>
572 </tr>
573 <tr>
574 <td>Require Initial Caps for constructors </td>
575 <td><code>newcap</code></td>
576 <td><code>true</code> if Initial Caps must be used with constructor
577 functions. <a href="#new"><small>(more)</small></a></td>
578 </tr>
579 <tr>
580 <td>Tolerate HTML event handlers </td>
581 <td><code>on</code></td>
582 <td><code>true</code> if HTML event handlers should be allowed. <a href="#html"><small>(more)</small></a></td>
583 </tr>
584 <tr>
585 <td>Allow one <code>var</code> statement per function</td>
586 <td><code>onevar</code></td>
587 <td><code>true</code> if only one <code>var</code> statement per function
588 should be allowed. <a href="#scope"><small>(more)</small></a></td>
589 </tr>
590 <tr>
591 <td>Stop on first error </td>
592 <td><code>passfail</code></td>
593 <td><code>true</code> if the scan should stop on first error.</td>
594 </tr>
595 <tr>
596 <td>Disallow <code>++</code> and <code>--</code> </td>
597 <td><code>plusplus</code></td>
598 <td><code>true</code> if <code>++</code> and <code>--</code> should
599 not be allowed. <a href="#inc"><small>(more)</small></a></td>
600 </tr>
601 <tr>
602 <td>Predefined <small>( , separated)</small></td>
603 <td><code>predef</code></td>
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604 <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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605 with the <code>/*jslint */</code> comment. You can also use the <code>var</code>
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606 statement to declare global variables in a script file.</td>
607 </tr>
608 <tr>
609 <td>Disallow insecure <code>.</code> and <code>[^</code>...<code>]</code>. in /RegExp/ </td>
610 <td><code>regexp</code></td>
611 <td><code>true</code> if <code>.</code> and <code>[^</code>...<code>]</code> should not be allowed in RegExp
612 literals. These forms should not be used when validating in secure applications.</td>
613 </tr>
614 <tr>
615 <td>Assume Rhino </td>
616 <td><code>rhino</code></td>
617 <td><code>true</code> if the <a href="http://www.mozilla.org/rhino/">Rhino</a>
618 environment globals should be predefined. <a href="#rhino"><small>(more)</small></a></td>
619 </tr>
620 <tr>
621 <td>Safe Subset </td>
622 <td><code>safe</code></td>
623 <td><code>true</code> if the safe subset rules are enforced. These rules
624 are used by <a href="http://www.ADsafe.org/">ADsafe</a>. It enforces
625 the safe subset rules but not the widget structure rules.</td>
626 </tr>
627 <tr>
628 <td>Require <code>&quot;use strict&quot;;</code> </td>
629 <td><code>strict</code></td>
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630 <td><code>true</code> if the ES5 <code><a href="http://www.yuiblog.com/blog/2010/12/14/strict-mode-is-coming-to-town/">"use strict";</a></code> pragma
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631 is required. Do not use this option carelessly.</td>
632 </tr>
633 <tr>
634 <td>Tolerate inefficient subscripting<br>
635 </td>
636 <td><code>sub</code></td>
637 <td><code>true</code> if subscript notation may be used for expressions
638 better expressed in dot notation.</td>
639 </tr>
640 <tr>
641 <td>Disallow undefined variables </td>
642 <td><code>undef</code></td>
643 <td><code>true</code> if variables must be declared before used. <a href="#undefined"><small>(more)</small></a></td>
644 </tr>
645 <tr>
646 <td>Strict white space </td>
647 <td><code>white</code></td>
648 <td><code>true</code> if strict whitespace rules apply.</td>
649 </tr>
650 <tr>
651 <td>Assume a Yahoo Widget </td>
652 <td><code>widget</code></td>
653 <td><code>true</code> if the <a href="http://widgets.yahoo.com/gallery/view.php?widget=37484">Yahoo
654 Widgets</a> globals should be predefined. <a href="#widget"><small>(more)</small></a></td>
655 </tr>
656 <tr>
657 <td>AssumeWindows</td>
658 <td><code>windows</code></td>
659 <td><code>true</code> if the Windows</a> globals should be predefined. <a href="#windows"><small>(more)</small></a></td>
660 </tr>
661 </tbody>
662 </table>
663 <h2 id=members>Members</h2>
664 <p>Since JavaScript is a loosely-typed, dynamic-object language, it is not
665 possible to determine at compile time if property names are spelled correctly.
666 <code>JSLint</code> provides some assistance with this.</p>
667 <p>At the bottom of its report, <code>JSLint</code> displays a <code>/*members*/</code>
668 comment. It contains all of the names and string literals that were used
669 with dot notation, subscript notation, and object literals to name the
670 members of objects. You can look through the list for misspellings. Member
671 names that were only used once are shown in italics. This is to make misspellings
672 easier to spot.</p>
673 <p>You can copy the <code>/*members*/</code> comment into your script file.
674 <code>JSLint</code> will check the spelling of all property names against
675 the list. That way, you can have <code>JSLint</code> look for misspellings
676 for you.</p>
677 <h2 id=report>Report</h2>
678
679 <p>If <code>JSLint</code> is able to complete its scan, it generates a function
680 report. It lists for each function:</p>
681
682 <ul>
683 <li>The line number on which it starts.</li>
684 <li>Its name. In the case of anonymous functions, <code>JSLint</code>
685 will &quot;guess&quot; the name.</li>
686 <li>The parameters.</li>
687 <li><i>Closure</i>: The variables and parameters that are declared in
688 the function that are used by its inner functions.</li>
689 <li><i>Variables</i>: The variables that are declared in the function
690 that are used only by the function.</li>
691 <li><i>Exceptions</i>: The variables that are declared by try statements.</li>
692 <li><i>Unused</i>: The variables that are declared in the function that
693 are not used. This may be an indication of an error.</li>
694 <li><i>Outer</i>: Variables used by this function that are declared in
695 another function.</li>
696 <li><i>Global</i>: Global variables that are used by this function. Keep
697 these to a minimum.</li>
698 <li><i>Label</i>: Statement labels that are used by this function.</li>
699 </ul>
700 <p>The report will also include a list of all of the <a href="#members">member
701 names</a> that were used. There is a <a href="msgs.html">list of <code>JSLint</code>
702 messages</a>.</p>
703 <h2 id=feedback>Feedback</h2>
704 <p>Please let me know if <code>JSLint</code> is useful for you. Is it too
705 strict? Is there a check or a report that could help you to improve the
706 quality of your programs? <a href="mailto:douglas@crockford.com">douglas@crockford.com</a></p>
707
708 <p>I intend to continue to adapt <code>JSLint</code> based on your comments.
709 Keep watching for improvements. Updates are announced at <a href="http://tech.groups.yahoo.com/group/jslint_com/">http://tech.groups.yahoo.com/group/jslint_com/</a>.</p>
710
711 <h2 id=try>Try it</h2>
712
713 <p><a href="http://www.JSLint.com" target="_blank">Try it.</a> Paste your script
714 into the window and click the
715 <a href="http://www.JSLint.com" target=jslint><input type="button" value="JSLint"></a>
716 button. The analysis is done by a script running on your machine.
717 Your script is not sent over the network. You can set the options used.
718 The <a href="http://www.amazon.com/exec/obidos/ASIN/0596517742/wrrrldwideweb"
719 target="_blank"><input type="button" value="Good Parts"></a> button in the
720 Options area will preset the best options for you.
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721 </p>
722 <p>
723 JSLint is written entirely in JavaScript, so it can run anywhere that JavaScript can run. See for example <a href="http://tech.groups.yahoo.com/group/jslint_com/database?method=reportRows&tbl=1">http://tech.groups.yahoo.com/group/jslint_com/database?method=reportRows&amp;tbl=1</a>.</p>
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724 <h2 id=implementation>Implementation</h2>
725 <p><code>JSLint</code> uses a <a href="http://javascript.crockford.com/tdop/tdop.html">Pratt
726 Parser (Top Down Operator Precedence)</a>. It is written in JavaScript.
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727 The full source code is available: <a href="https://github.com/douglascrockford/JSLint">https://github.com/douglascrockford/JSLint</a>.</p>
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728 <a href="http://www.JSLint.com/"><img src="jslintpill.gif" width="36" height="17" border="0"></a>
729 <a href="http://www.ADsafe.org/"><img src="adsafepill.gif" width="36" height="17" border="0"></a>
730 <a href="http://www.JSON.org/"><img src="jsonpill.gif" width="36" height="17" border="0"></a>
731 </body>
732 </html>
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