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<title>JSLint: The JavaScript Code Quality Tool</title>
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<td><img src="jslint.gif" width="383" height="120" alt="JSLint"> </td>
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<p><big><code>JSLint</code>: The
<a href="http://javascript.crockford.com/">JavaScript</a> Code Quality Tool</big></p>
<p><a href="http://www.crockford.com/" target="_top">&copy;2002 Douglas Crockford</a></p>
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<h2 id=warning>Warning!</h2>
<p><a href="http://www.JSLint.com" target="_blank"><code>JSLint</code></a>
will hurt your feelings.</p>
<h2 id=what>What is <code>JSLint</code>?</h2>
<p><a href="http://www.JSLint.com" target="_blank"><code>JSLint</code></a>
is a JavaScript program that looks for problems in JavaScript programs.
It is a code quality tool.</p>
<p>When <a href="http://en.wikipedia.org/wiki/C_programming_language">C</a>
was a <a href="http://cm.bell-labs.com/cm/cs/who/dmr/chist.html">young</a>
programming language, there were several common programming errors that
were not caught by the primitive compilers, so an accessory program called
<code><a href="http://en.wikipedia.org/wiki/Lint_programming_tool">lint</a></code>
was developed that would scan a source file, looking for problems.</p>
<p>As the language matured, the definition of the language was
strengthened to eliminate some insecurities, and compilers got better
at issuing warnings. <code>lint</code> is no longer needed.</p>
<p><a href="http://javascript.crockford.com/">JavaScript</a> is a young-for-its-age
language. It was originally intended to do small tasks in webpages, tasks
for which Java was too heavy and clumsy. But JavaScript is a very capable
language, and it is now being used in larger projects. Many of the features
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>,
a JavaScript syntax checker and validator.</p>
<p><code>JSLint</code> takes a JavaScript source and scans it. If it finds
a problem, it returns a message describing the problem and an approximate
location within the source. The problem is not necessarily a syntax error,
although it often is. <code>JSLint</code> looks at some style conventions
as well as structural problems. It does not prove that your program is
correct. It just provides another set of eyes to help spot problems.</p>
<p><code>JSLint</code> defines a professional subset of JavaScript, a stricter
language than that defined by <a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm" target="ecma">Third
Edition of the <i>ECMAScript Programming Language Standard</i></a>. The
subset is related to recommendations found in <a href="http://javascript.crockford.com/code.html" target="sun"><i>Code
Conventions for the JavaScript Programming Language</i></a>. </p>
<p>JavaScript is a sloppy language, but inside it there is an elegant, better
language. <code>JSLint</code> helps you to program in that better language
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>
<p><code>JSLint</code> can operate on JavaScript source, HTML source, CSS source, or <a href="http://www.JSON.org/">JSON</a>
text.</p>
<h2 id=global>Global Variables</h2>
<p>JavaScript's <a href="http://yuiblog.com/blog/2006/06/01/global-domination/">biggest
problem</a> is its dependence on global variables, particularly implied
global variables. If a variable is not explicitly declared (usually with
the <code>var</code> statement), then JavaScript assumes that the variable
was global. This can mask misspelled names and other problems.</p>
<p><code>JSLint</code> expects that all variables and functions are declared
before they are used or invoked. This allows it to detect implied global
variables. It is also good practice because it makes programs easier to
read.</p>
<p>Sometimes a file is dependent on global variables and functions that
are defined elsewhere. You can identify these to <code>JSLint</code> with a <code>var</code> statement that lists the global functions and objects
that your program depends on. </p>
<p>A global declaration can look like this:</p>
<pre>var getElementByAttribute, breakCycles, hanoi;</pre>
<p>The declaration should appear near the top of the file. It must appear before the use of the variables
it declares. </p>
<p>It is necessary to use a <code>var</code> statement to declare a variable before that variable is assigned to. </p>
<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>
<p id=browser>Some globals can be predefined for you. Select the <i>Assume
a browser</i> (<code>browser</code>) <a href="#options">option</a> to
predefine the standard global properties that are supplied by web browsers,
such as <code>document</code> and <code>addEventListener</code>. It has the same
effect as this comment:</p>
<blockquote>
<code>/*global
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
*/</code></blockquote>
<p>Select the
<em>Assume console, alert, ...</em>
(<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
effect as this comment:</p>
<pre>/*global alert: false, confirm: false, console: false, Debug: false, opera: false, prompt: false */</pre>
<p id=node>Select the
<em>Assume Node.js</em>
(<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
effect as this comment:</p>
<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>
<p id=rhino>Select the <i>Assume Rhino</i> (<code>rhino</code>) <a href="#options">option</a>
to predefine the global properties provided by the Rhino environment.
It has the same effect as this statement:</p>
<blockquote>
<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>
</blockquote>
<p id=widget>Select the <i>Assume a Yahoo Widget</i> (<code>widget</code>)
<a href="#options">option</a> to predefine the global properties provided
by the Yahoo! Widgets environment. It has the same effect as this statement:</p>
<blockquote>
<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>
</blockquote>
<p id=windows>Select the <i>Assume Windows</i> (<code>windows</code>)
<a href="#options">option</a> to predefine the global properties provided by Microsoft Windows. It has the same effect as this statement:</p>
<blockquote>
<p><code>/*global ActiveXObject: false, CScript: false, Debug: false, Enumerator: false, System: false, VBArray: false, WScript: false */</code></p>
</blockquote>
<h2 id=semicolon>Semicolon</h2>
<p>JavaScript uses a C-like syntax which requires the use of semicolons to delimit certain
statements. JavaScript attempts to make those semicolons optional with a semicolon
insertion mechanism. This is dangerous because it can mask errors.</p>
<p>Like C, JavaScript has <code>++</code> and <code>--</code> and <code>(</code> operators
which can be prefixes or suffixes. The disambiguation is done by the semicolon.</p>
<p>In JavaScript, a linefeed can be whitespace or it can act as a semicolon.
This replaces one ambiguity with another. </p>
<p><code>JSLint</code> expects that every statement be followed by <code>;</code> except
for <code>for</code>, <code>function</code>, <code>if</code>, <code>switch</code>, <code>try</code>, and
<code>while</code>. <code>JSLint</code> does not expect to see unnecessary semicolons or the
empty statement.</p>
<h2 id=comma>Comma</h2>
<p>The comma operator can lead to excessively tricky expressions. It can also
mask some programming errors.</p>
<p><code>JSLint</code> expects to see the comma used as a separator, but not as an
operator (except in the initialization and incrementation parts of the <code>for</code>
statement). It does not expect to see elided elements in array literals. Extra
commas should not be used. A comma should not appear after the last element
of an array literal or object literal because it can be misinterpreted by some
browsers. </p>
<h2 id=scope>Scope</h2>
<p>In many languages, a block introduces a scope. Variables introduced in
a block are not visible outside of the block.</p>
<p>In JavaScript, blocks do not introduce a scope. There is only function-scope.
A variable introduced anywhere in a function is visible everywhere in
the function. JavaScript's blocks confuse experienced programmers and
lead to errors because the familiar syntax makes a false promise.</p>
<p><code>JSLint</code> expects blocks with <code>function</code>, <code>if</code>,
<code>switch</code>, <code>while</code>, <code>for</code>, <code>do</code>,
and <code>try</code> statements and nowhere else. </p>
<p>In languages with block scope, it is usually recommended that variables
be declared at the site of first use. But because JavaScript does not
have block scope, it is wiser to declare all of a function's variables
at the top of the function. It is recommended that a single <code>var</code>
statement be used per function. This can be declined with the <code>vars</code>
<a href="#options">option</a>.</p>
<h2 id=required>Required Blocks</h2>
<p><code>JSLint</code> expects that <code>if</code>, <code>while</code>,
<code>do</code> and <code>for</code> statements will be made with blocks
<code>{</code>that is, with statements enclosed in braces<code>}</code>.</p>
<p>JavaScript allows an <code>if</code> to be written like this:</p>
<pre>if (<i>condition</i><code>)
</code><i>statement</i>;</pre>
<p>That form is known to contribute to mistakes in projects where many programmers
are working on the same code. That is why <code>JSLint</code> expects the use of
a block:</p>
<pre>if (<i>condition</i>) {
<i>statements</i>;
}</pre>
<p>Experience shows that this form is more resilient.</p>
<h2 id=expression>Expression Statements</h2>
<p>An expression statement is expected to be an assignment or a function/method
call or <code>delete</code>. All other expression statements are considered
to be errors.</p>
<h2 id=forin><code>for</code> <code>in</code></h2>
<p>The <code>for</code> <code>in</code> statement allows for looping through
the names of all of the properties of an object. <a href="http://yuiblog.com/blog/2006/09/26/for-in-intrigue/">Unfortunately,
it also loops through all of the properties that were inherited through
the prototype chain.</a> This has the bad side effect of serving up method
functions when the interest is in data properties. If a program is written without awareness of this situation, then it can fail.</p>
<p>The body of every <code>for</code> <code>in</code> statement should be
wrapped in an <code>if</code> statement that does filtering. It can select
for a particular type or range of values, or it can exclude functions,
or it can exclude properties from the prototype. For example,</p>
<pre>for (name in object) {
if (object.hasOwnProperty(name)) {
....
}
}</pre>
<h2 id=switch><code>switch</code></h2>
<p>A <a href="http://yuiblog.com/blog/2007/04/25/id-rather-switch-than-fight/">common
error</a> in <code>switch</code> statements is to forget to place a <code>break</code>
statement after each case, resulting in unintended fall-through. <code>JSLint</code>
expects that the statement before the next <code>case</code> or <code>default</code>
is one of these: <code>break</code>, <code>return</code>, or <code>throw</code>.
</p>
<h2 id=var><code>var</code></h2>
<p>JavaScript allows <code>var</code> definitions to occur anywhere
within a function. <code>JSLint</code> is more strict.</p>
<p><code>JSLint</code> expects that a <code>var</code> will be declared
only once, and that it will be declared before it is used.</p>
<p><code></code><code>JSLint</code> expects that a <code>function</code>
will be declared before it is used.</p>
<p><code>JSLint</code> expects that parameters will not also be declared
as vars. </p>
<p><code>JSLint</code> does not expect the <code>arguments</code> array to be declared
as a <code>var</code>.</p>
<p><code>JSLint</code> does not expect that a var will be defined in a block.
This is because JavaScript blocks do not have block scope. This can have
unexpected consequences. Define all variables at the top of the function.</p>
<h2 id=with><code>with</code></h2>
<p>The <code>with</code> statement was intended to provide a shorthand in accessing
properties in deeply nested objects. Unfortunately, it behaves <a href="http://yuiblog.com/blog/2006/04/11/with-statement-considered-harmful/">very
badly</a> when setting new properties. Never use the <code>with</code> statement. Use
a <code>var</code> instead.</p>
<p><code>JSLint</code> does not expect to see a <code>with</code> statement.</p>
<h2 id=assignment>=</h2>
<p><code>JSLint</code> does not expect to see an assignment statement in
the condition part of an <code>if</code> or <code>for</code> or <code>while</code>
<code></code> or <code>do</code> statement. This is because it is more
likely that </p>
<pre>if (a = b) {
...
}</pre>
<p>was intended to be </p>
<pre>if (a == b) {
...
}</pre>
<p>It is difficult to write correct programs while using idioms that are
hard to distinguish from obvious errors.</p>
<h2 id=eqeq>== and !=</h2>
<p>The <code>==</code> and <code>!=</code> operators do type coercion before
comparing. This is bad because it causes <code>' \t\r\n' == 0</code> to
be <code>true</code>. This can mask type errors. JSLint cannot reliably determine if == is being used correctly, to it is best to not use <code>==</code> and != and always use the more reliable <code>===</code> and <code>!==</code> operators instead. </p>
<p align="left">If you only care that a value is <i>truthy</i> or <i>falsy</i>,
then use the short form. Instead of </p>
<pre align="left">(foo != 0)</pre>
<p align="left">just say </p>
<pre align="left">(foo)</pre>
<p align="left">and instead of</p>
<pre align="left">(foo == 0)</pre>
<p align="left"> say</p>
<pre align="left">(!foo)</pre>
<p>There is an <code>eqeq</code> <a href="#options">option</a> that allows the use of <code>==</code> and <code>!=</code>.</p>
<h2 id=labels>Labels</h2>
<p>JavaScript allows any statement to have a label, and labels have a
separate name space. <code>JSLint</code> is more strict.</p>
<p><code>JSLint</code> expects labels only on statements that interact
with <code>break</code>: <code>switch</code>, <code>while</code>,
<code>do</code>, and <code>for</code>. <code>JSLint</code> expects that labels
will be distinct from vars and parameters.</p>
<h2 id=unreachable>Unreachable Code</h2>
<p><code>JSLint</code> expects that
a <code>return</code>, <code>break</code>, <code>continue</code>,
or <code>throw</code> statement will be followed by
a <code>}</code> or <code>case</code> or <code>default</code>.</p>
<h2 id=pluses>Confusing Pluses and Minuses</h2>
<p><code>JSLint</code> expects that <code>+</code> will not be followed by
<code>+</code> or <code>++</code>, and that <code>-</code> will not be followed
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>
<h2 id=inc><code>++</code> and <code>--</code></h2>
<p>The <code>++</code> <small>(increment)</small> and <code>--</code> <small>(decrement)</small>
operators have been known to contribute to bad code by encouraging excessive
trickiness. They are second only to faulty architecture in enabling to
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>
that allows the use of these operators.</p>
<h2 id=bitwise>Bitwise Operators</h2>
<p>JavaScript does not have an integer type, but it does have bitwise operators.
The bitwise operators convert their operands from floating point to integers
and back, so they are not as efficient as in C or other languages. They
are rarely useful in browser applications. The similarity to the logical
operators can mask some programming errors. The <code>bitwise</code> <a href="#options">option</a>
allows the use of these operators: <code>&lt;&lt; &gt;&gt; &gt;&gt;&gt;
~ &amp; |</code>.</p>
<h2 id=evil><code>eval</code> is evil</h2>
<p>The <code>eval</code> function (and its relatives, <code>Function</code>,
<code>setTimeout</code>, and <code>setInterval</code>) provide access
to the JavaScript compiler. This is sometimes necessary, but in most cases
it indicates the presence of extremely bad coding. The <code>eval</code>
function is the most misused feature of JavaScript.</p>
<h2 id=void><code>void</code></h2>
<p>In most C-like languages, <code>void</code> is a type. In
JavaScript, <code>void</code> is a prefix operator that always
returns <code>undefined</code>. <code>JSLint</code> does not expect to
see <code>void</code> because it is confusing and not very useful.</p>
<h2 id=regexp>Regular Expressions</h2>
<p>Regular expressions are written in a terse and cryptic notation. <code>JSLint</code>
looks for problems that may cause portability problems. It also attempts
to resolve visual ambiguities by recommending explicit escapement.</p>
<p>JavaScript's syntax for regular expression literals overloads the <code>/</code>
character. To avoid ambiguity, <code>JSLint</code> expects that the character
preceding a regular expression literal is a <code>(</code> or <code>=</code>
or <code>:</code> or <code>,</code> character. </p>
<h2 id=new>Constructors and <code>new</code></h2>
<p>Constructors are functions that are designed to be used with the <code>new</code>
prefix. The <code>new</code> prefix creates a new object based on the
function's <code>prototype</code>, and binds that object to the function's
implied <code>this</code> parameter. If you neglect to use the <code>new</code>
prefix, no new object will be made and <code>this</code> will be bound
to the global object. This is a <a href="http://yuiblog.com/blog/2006/11/13/javascript-we-hardly-new-ya/">serious
mistake</a>.</p>
<p><code>JSLint</code> enforces the convention that constructor functions
be given names with initial uppercase. <code>JSLint</code> does not expect
to see a function invocation with an initial uppercase name unless it
has the <code>new</code> prefix. <code>JSLint</code> does not expect to
see the <code>new</code> prefix used with functions whose names do not
start with initial uppercase. This can be disabled with the <code>newcap</code>
<a href="#options">option</a>.</p>
<p><code>JSLint</code> does not expect to see the wrapper forms <code>new Number</code>,
<code>new String</code>, <code>new Boolean</code>. </p>
<p><code>JSLint</code> does not expect to see <code>new Object</code> (use <code>{}</code>
instead). </p>
<p><code>JSLint</code> does not expect to see <code>new Array</code> (use <code>[]</code>
instead).</p>
<h2 id=unsafe>Unsafe Characters</h2>
<p> There are characters that are handled inconsistently in browsers, and
so must be escaped when placed in strings. </p>
<pre>\u0000-\u001f
\u007f-\u009f
\u00ad
\u0600-\u0604
\u070f
\u17b4
\u17b5
\u200c-\u200f
\u2028-\u202f
\u2060-\u206f
\ufeff
\ufff0-\uffff</pre>
<h2 id=not>Not Looked For</h2>
<p><code>JSLint</code> does not do flow analysis to determine that variables are assigned
values before used. This is because variables are given a value (<code>undefined</code>)
that is a reasonable default for many applications.</p>
<p><code>JSLint</code> does not do any kind of global analysis. It does
not attempt to determine that functions used with <code>new</code> are
really constructors (<a href="#new">except by enforcing capitalization
conventions</a>), or that property names are spelled correctly (<a href="#properties">except
for matching against the <code>/*properties */</code> comment</a>).</p>
<h2 id=html>HTML</h2>
<p><code>JSLint</code> is able to handle HTML text. It can inspect the JavaScript content
contained within <code>&lt;script&gt;</code>...<code>&lt;/script&gt;</code> tags. It
also inspects the HTML content, looking for problems that are known to interfere
with JavaScript:</p>
<ul>
<li>All tag names must be in lower case.</li>
<li>All tags that can take a close tag (such as <code>&lt;/p&gt;</code>)
must have a close tag.</li>
<li>All tags are correctly nested.</li>
<li>The entity <code>&amp;lt;</code> must be used for literal <code>'&lt;'</code>.</li>
</ul>
<p><code>JSLint</code> is less anal than the sycophantic conformity demanded
by XHTML, but more strict than the popular browsers. </p>
<p><code>JSLint</code> also checks for the occurrence of<code> '&lt;/' </code>in
string literals. You should always write<code> '&lt;\/' </code>instead.
The extra backslash is ignored by the JavaScript compiler but not by the
HTML parser. Tricks like this should not be necessary, and yet they are.</p>
<p>There is a <code>cap</code> <a href="#options">option</a> that allows
use of uppercase tag names. There is also an <code>on</code> <a href="#options">option</a>
that allows the use of inline HTML event handlers.</p>
<p>There is a <code>fragment</code> <a href="#options">option</a> that can
inspect a well formed HTML fragment. If the <code>adsafe</code> <a href="#options">option</a>
is also used, then the fragment must be a <code>&lt;div&gt;</code> that
conforms to the <a href="http://www.ADsafe.org/">ADsafe</a> widget rules.</p>
<h2 id=css>CSS</h2>
<p><code>JSLint</code> can inspect CSS files. It expects the first line
of a CSS file to be </p>
<pre>@charset &quot;UTF-8&quot;;</pre>
<p>This feature is experimental. Please report any problems or limitations.
There is a <code>css</code> <a href="#options">option</a> that will tolerate
some of the non-standard-but-customary workarounds. <br>
</p>
<h2 id=options>Options</h2>
<p><code>JSLint</code> provides several options that control its operation and
its sensitivity. In the <a href="http://www.JSLint.com/">web edition</a>, the
options are selected with several checkboxes and two fields. </p>
<p>It also provides assistance in constructing <code>/*jslint*/</code>
comments.
</p>
<p>When <code>JSLINT</code> is called as a function, it accepts an <code>option</code> object
parameter that allows you to determine the subset of JavaScript that is
acceptable to you. The web page version of <code>JSLint</code> at <a href="http://www.JSLint.com/">http://www.JSLint.com/</a>
does this for you. </p>
<p>Options can also be specified within a script with a <code>/*jslint */</code>
directive:</p>
<pre>/*jslint nomen: true, debug: true,
evil: false, vars: true */</pre>
<p>An option specification starts with <code>/*jslint</code>. Notice that
there is no space before the <code>j</code>. The specification contains
a sequence of name value pairs, where the names are <code>JSLint</code>
options, and the values are <code>true</code> or <code>false</code>. The
<code>indent</code> <a href="#options">option</a> can take a number. A <code>/*jslint */</code>
comment takes precedence over the <code>option</code> object. The directive respects function scope.</p>
<table>
<tbody>
<tr>
<th>Description</th>
<th><code>option</code></th>
<th>Meaning</th>
</tr>
<tr>
<td>ADsafe</td>
<td><code>adsafe</code></td>
<td><code>true</code> if <a href="http://www.ADsafe.org/">AD<span style="color: blue;">safe</span></a>
rules should be enforced. See <a href="http://www.ADsafe.org/">http://www.ADsafe.org/</a>.</td>
</tr>
<tr>
<td>Tolerate bitwise operators </td>
<td><code>bitwise</code></td>
<td><code>true</code> if bitwise operators should be allowed. <a href="#bitwise"><small>(more)</small></a></td>
</tr>
<tr>
<td>Assume a browser </td>
<td><code>browser</code></td>
<td><code>true</code> if the standard browser globals should be predefined.
<a href="#browser"><small>(more)</small></a> </td>
</tr>
<tr>
<td>Tolerate HTML case </td>
<td><code>cap</code></td>
<td><code>true</code> if uppercase HTML should be allowed.</td>
</tr>
<tr>
<td>Tolerate <code>continue</code></td>
<td><code>continue</code></td>
<td><code>true</code> if the <code>continue</code> statement should be allowed.</td>
</tr>
<tr>
<td>Tolerate CSS workarounds</td>
<td><code>css</code></td>
<td><code>true</code> if CSS workarounds should be tolerated. <a href="#css"><small>(more)</small></a></td>
</tr>
<tr>
<td>Tolerate debugger statements</td>
<td><code>debug</code></td>
<td><code>true</code> if <code>debugger</code> statements should be
allowed. Set this option to <code>false</code> before going into production.</td>
</tr>
<tr>
<td>Assume <code>console</code>, <code>alert</code>, ...</td>
<td><code>devel</code></td>
<td><code>true</code> if browser globals that are useful in development should be
predefined. (<a href="#devel">more</a>)</td>
</tr>
<tr>
<td>Tolerate <code>==</code> and <code>!=</code></td>
<td><code>eqeq</code></td>
<td><code>true</code> if the <code>==</code> and <code>!=</code> operators should be tolerated. (<a href="#eqeq">more</a>).</td>
<tr>
<td>Tolerate ES5 syntax</td>
<td><code>es5</code></td>
<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>
</tr>
<tr>
<td>Tolerate <code>eval</code> </td>
<td><code>evil</code></td>
<td><code>true</code> if <code>eval</code> should be allowed. <a href="#evil"><small>(more)</small></a></td>
</tr>
<tr>
<td>Tolerate unfiltered for in </td>
<td><code>forin</code></td>
<td><code>true</code> if unfiltered <code>for</code> <code>in</code>
statements should be allowed. <a href="#forin"><small>(more)</small></a></td>
</tr>
<tr>
<td>Tolerate HTML fragments </td>
<td><code>fragment</code></td>
<td><code>true</code> if HTML fragments should be allowed. <a href="#html"><small>(more)</small></a></td>
</tr>
<tr>
<td>Strict white space indentation</td>
<td><code>indent</code></td>
<td>The number of spaces used for indentation (default is 4). If 0, then no indentation checking takes place.</td>
</tr>
<tr>
<td>Maximum number of errors</td>
<td><code>maxerr</code></td>
<td>The maximum number of warnings reported. (default is 50)</td>
</tr>
<tr>
<td>Maximum line length</td>
<td><code>maxlen</code></td>
<td>The maximum number of characters in a line.</td>
</tr>
<tr>
<td>Tolerate uncapitalized constructors</td>
<td><code>newcap</code></td>
<td><code>true</code> if Initial Caps with constructor
functions is optional. <a href="#new"><small>(more)</small></a></td>
</tr>
<tr>
<td>Assume Node.js</td>
<td><code>node</code></td>
<td><code>true</code> if Node.js globals should be predefined. <a href="#node"><small>(more)</small></a></td>
</tr>
<tr>
<td>Tolerate dangling _ in identifiers </td>
<td><code>nomen</code></td>
<td><code>true</code> if names should not be checked for initial or trailing underbars.</td>
</tr>
<tr>
<td>Tolerate HTML event handlers </td>
<td><code>on</code></td>
<td><code>true</code> if HTML event handlers should be allowed. <a href="#html"><small>(more)</small></a></td>
</tr>
<tr>
<td>Stop on first error </td>
<td><code>passfail</code></td>
<td><code>true</code> if the scan should stop on first error.</td>
</tr>
<tr>
<td>Tolerate <code>++</code> and <code>--</code> </td>
<td><code>plusplus</code></td>
<td><code>true</code> if <code>++</code> and <code>--</code> should
be allowed. <a href="#inc"><small>(more)</small></a></td>
</tr>
<tr>
<td>Predefined <small>( , separated)</small></td>
<td><code>predef</code></td>
<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
with the <code>/*jslint */</code> comment. You can also use the <code>var</code>
statement to declare global variables in a script file.</td>
</tr>
<tr>
<td>Tolerate <code>.</code> and <code>[^</code>...<code>]</code>. in /RegExp/ </td>
<td><code>regexp</code></td>
<td><code>true</code> if <code>.</code> and <code>[^</code>...<code>]</code> should be allowed in RegExp
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>
</tr>
<tr>
<td>Assume Rhino </td>
<td><code>rhino</code></td>
<td><code>true</code> if the <a href="http://www.mozilla.org/rhino/">Rhino</a>
environment globals should be predefined. <a href="#rhino"><small>(more)</small></a></td>
</tr>
<tr>
<td>Safe Subset </td>
<td><code>safe</code></td>
<td><code>true</code> if the safe subset rules are enforced. These rules
are used by <a href="http://www.ADsafe.org/">ADsafe</a>. It enforces
the safe subset rules but not the widget structure rules.</td>
</tr>
<tr>
<td> Tolerate missing&nbsp;<code>'use strict'</code>&nbsp;pragma </td>
<td><code>sloppy</code></td>
<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
is not required. Do not use this pragma unless you know what you are doing.</td>
</tr>
<tr>
<td>Tolerate inefficient subscripting<br>
</td>
<td><code>sub</code></td>
<td><code>true</code> if subscript notation may be used for expressions
better expressed in dot notation.</td>
</tr>
<tr>
<td>Tolerate type inconsistency<br>
</td>
<td><code>type</code></td>
<td><code>true</code> if variables are allowed to contain more than one type of value.</td>
</tr>
<tr>
<td> Tolerate misordered definitions </td>
<td><code>undef</code></td>
<td><code>true</code> if variables and functions need not be declared before used. <a href="#undefined"><small>(more)</small></a></td>
</tr>
<tr>
<td> Tolerate unused parameters</td>
<td><code>unparam</code></td>
<td><code>true</code> if warnings should not be given for unused parameters.</td>
</tr>
<tr>
<td>Tolerate many&nbsp;<tt>var</tt>&nbsp;statements per function</td>
<td><code>vars</code></td>
<td><code>true</code> if multiple <code>var</code> statement per function
should be allowed. <a href="#scope"><small>(more)</small></a></td>
</tr>
<tr>
<td> Tolerate messy white space</td>
<td><code>white</code></td>
<td><code>true</code> if strict whitespace rules should be ignored.</td>
</tr>
<tr>
<td>Assume a Yahoo Widget </td>
<td><code>widget</code></td>
<td><code>true</code> if the <a href="http://widgets.yahoo.com/gallery/view.php?widget=37484">Yahoo
Widgets</a> globals should be predefined. <a href="#widget"><small>(more)</small></a></td>
</tr>
<tr>
<td>Assume Windows</td>
<td><code>windows</code></td>
<td><code>true</code> if the Windows</a> globals should be predefined. <a href="#windows"><small>(more)</small></a></td>
</tr>
</tbody>
</table>
<h2 id=properties>Properties</h2>
<p>Since JavaScript is a loosely-typed, dynamic-object language, it is not
possible to determine at compile time if property names are spelled correctly.
<code>JSLint</code> provides some assistance with this.</p>
<p>At the bottom of its report, <code>JSLint</code> displays a <code>/*properties*/</code>
comment. It contains all of the names and string literals that were used
with dot notation, subscript notation, and object literals to name the
properties of objects. You can look through the list for misspellings. Property
names that were only used once are shown in italics. This is to make misspellings
easier to spot.</p>
<p>You can copy the <code>/*properties*/</code> comment into your script file.
<code>JSLint</code> will check the spelling of all property names against
the list. That way, you can have <code>JSLint</code> look for misspellings
for you. The directive respects function scope.</p>
<h2 id=report>Report</h2>
<p>If <code>JSLint</code> is able to complete its scan, it generates a function
report. It lists for each function:</p>
<ul>
<li>The line number on which it starts.</li>
<li>Its name. In the case of anonymous functions, <code>JSLint</code>
will 'guess' the name.</li>
<li>The parameters.</li>
<li><i>Closure</i>: The variables and parameters that are declared in
the function that are used by its inner functions.</li>
<li><i>Variables</i>: The variables that are declared in the function
that are used only by the function.</li>
<li><i>Exceptions</i>: The variables that are declared by try statements.</li>
<li><i>Unused</i>: The variables that are declared in the function that
are not used. This may be an indication of an error.</li>
<li><i>Outer</i>: Variables used by this function that are declared in
another function.</li>
<li><i>Global</i>: Global variables that are used by this function. Keep
these to a minimum.</li>
<li><i>Label</i>: Statement labels that are used by this function.</li>
</ul>
<p>The report will also include a list of all of the <a href="#properties">property
names</a> that were used. There is a <a href="msgs.html">list of <code>JSLint</code>
messages</a>.</p>
<h2 id=feedback>Feedback</h2>
<p>Please let me know if <code>JSLint</code> is useful for you. Is it too
strict? Is there a check or a report that could help you to improve the
quality of your programs? <a href="mailto:douglas@crockford.com">douglas@crockford.com</a></p>
<p>I intend to continue to adapt <code>JSLint</code> based on your comments.
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>
<h2 id=try>Try it</h2>
<p><a href="http://www.JSLint.com" target="_blank">Try it.</a> Paste your script
into the window and click the
<a href="http://www.JSLint.com" target=jslint><input type="button" value="JSLint"></a>
button. The analysis is done by a script running on your machine.
Your script is not sent over the network. You can set the options used.
</p>
<p>
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>
<h2 id=implementation>Implementation</h2>
<p><code>JSLint</code> uses a <a href="http://javascript.crockford.com/tdop/tdop.html">Pratt
Parser (Top Down Operator Precedence)</a>. It is written in JavaScript.
The full source code is available: <a href="https://github.com/douglascrockford/JSLint">https://github.com/douglascrockford/JSLint</a>.</p>
<a href="http://www.JSLint.com/"><img src="jslintpill.gif" width="36" height="17" border="0"></a>
<a href="http://www.ADsafe.org/"><img src="adsafepill.gif" width="36" height="17" border="0"></a>
<a href="http://www.JSON.org/"><img src="jsonpill.gif" width="36" height="17" border="0"></a>
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