JSLint
JSLint
is a JavaScript
program that looks for problems in JavaScript programs.
JSLint
?When C was
a young programming
language, there were several common programming errors that were not caught
by the primitive compilers, so an accessory program called lint
was developed which would scan a source file, looking for problems.
As the language matured, the definition of the language was
strengthened to eliminate some insecurities, and compilers got better
at issuing warnings. lint
is no longer needed.
JavaScript is a young 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 for larger projects. A lint
for JavaScript is needed: JSLint
, a JavaScript
syntax checker and validator.
JSLint
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. JSLint
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.
JSLint
defines a professional subset of JavaScript, a stricter
language than that defined by Edition
3 of the ECMAScript Language Specification. The subset is related
to recommendations found in Code
Conventions for the JavaScript Programming Language.
JavaScript is a sloppy language, but inside it there is an elegant, better language. JSLint helps you to program in that better language and to avoid most of the slop.
JavaScript uses a C-like syntax which requires the use of semicolons to delimit statements. JavaScript attempts to make semicolons optional with a semicolon insertion mechanism. This is dangerous.
Like C, JavaScript has ++
and --
and (
operators
which can be prefixes or suffixes. The disambiguation is done by the semicolon.
In Javascript, a linefeed can be whitespace or it can act as a semicolon. This replaces one ambiguity with another.
JSLint
expects that every statement be followed by ;
except
for for
, function
, if
, switch
, try
, and
while
. JSLint
does not expect to see unnecessary semicolons or the
empty statement.
As a further defense against the semicolon insertion mechanism, JSLint
expects long statements to be broken only after one of these punctuation
characters or operators:
, . ; : { } ( [ = < > ? ! + - * / % ~ ^ | &
== != <= >= += -= *= /= %= ^= |= &= << >>
|| &&
=== !== <<= >>= >>> >>>=
JSLint
does not expect to see a long statement broken after
an identifier, a string, a number, closer, or a suffix operator:
) ] ++ --
JSLint
allows you to turn on the Tolerate sloppy line breaking
option.
Semicolon insertion can mask copy/paste errors. If you always break lines after operators, then JSLint can do better at finding them.
The comma operator can lead to excessively tricky expressions. It can also mask some programming errors.
JSLint
expects to see the comma used as a separator, but not as an
operator (except in the initialization and incrementation parts of the for
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.
JSLint
expects that if
and for
statements will be made with blocks {that is, with statements
enclosed in braces}.
JavaScript allows an if
to be written like this:
if (condition)
statement;
That form is known to contribute to mistakes in projects where many programmers
are working on the same code. That is why JSLint
expects the use of
a block:
if (condition) { statement; }
Experience shows that this form is more resilient.
In many languages, a block introduces a scope. Variables introduced in a scope are not visible to other parts of the program.
In JavaScript, blocks do not introduce a scope. There is only function-scope. JavaScript's blocks confuse experienced programmers and lead to errors.
JSLint
expects blocks with function
,
if
, switch
, while
, for
,
do
, and try
statements and nowhere else.
An expression statement is expected to be an assignment or a function/method call. All other expression statements are considered to be errors.
var
JavaScript allows var
definitions to occur anywhere
within a function. JSLint
is more strict.
JSLint
expects that a var
will be declared only once, and
that it will be declared before it is used.
JSLint
expects that parameters will not also be declared
as vars.
JSLint
does not expect the arguments
array to be declared
as a var
.
JSLint
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.
switch
A common error in switch
statements is to forget to place a break
statement after each case
, resulting in unintended fall-through. JSLint
expects that the statement before a case
or default
is one
of these: break
, case
, continue
, return
,
switch
, or throw
.
with
The with
statement was intended to provide a shorthand in accessing
members in deeply nested objects. Unfortunately, it behaves very
badly when setting new members. Never use the with
statement. Use
a var
instead.
JSLint
does not expect to see a with
statement.
JSLint
does not expect to see an assignment statement in the condition
part of an if
or while
statement. This is because it is more
likely that
if (a = b) { ... }
was intended to be
if (a == b) { ... }
If you really intend an assignment, wrap it in another set of parens:
if ((a = b)) { ... }
The ==
and !=
operators do type coercion before comparing. This is bad because it causes '' == 0
to be true
. This can mask type errors.
When comparing to any of the following values, use the ===
or !==
operators, which do not do type coercion.
0 '' undefined null false true
If you want the type coercion, then use the short form. Instead of
(foo != 0)
just say
(foo)
and instead of
(foo == 0)
say
(!foo)
The ===
and !==
operators are preferred. There
is a Disallow == and !=
option which requires the use of
===
and !==
in all cases.
JavaScript allows any statement to have a label, and labels have a
separate name space. JSLint
is more strict.
JSLint
expects labels only on statements that interact
with break
: switch
, while
,
do
, and for
. JSLint
expects that labels
will be distinct from vars and parameters.
JSLint
expects that
a return
, break
, continue
,
or throw
statement will be followed by
a }
or case
or default
.
JSLint
expects that +
will not be followed by
+
or ++
, and that -
will not be followed
by -
or --
. A misplaced space can turn + +
into ++
, an error that is difficult to see. Use parens to avoid confusion..
++
and --
The ++
(increment) and --
(decrement)
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. There is an option that prohibits
the use of these operators.
JavaScript does not have an integer type, bit it does have bitwise operators. The bitwise operators convert their operands from floating point to integers and back, so they are not near 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. There is an option that prohibits the use of these operators.
eval
is evilThe eval
function (and its relatives, Function
, setTimeout
,
and setInterval
) provide access to the JavaScript compiler. This is
sometimes useful for parsing JSON text, but
in virtually all other cases it indicates the presences of extremely bad coding.
The eval
function is the most misused feature of JavaScript.
void
In most C-like languages, void
is a type. In
JavaScript, void
is a prefix operator that always
returns undefined
. JSLint
does not expect to
see void
because it is confusing and not very useful.
JavaScript's syntax for regular expression literals overloads the /
character. To avoid ambiguity, JSLint
expects that the character preceding
a regular expression literal is a (
or =
or :
or
,
character.
The ECMAScript specification requires that the /
character within
a regular expression literal be escaped.
In JavaScript, undefined variables are assumed to be implied global variables. JSLint provides an option for detecting these.
Use the /*extern
... */
declaration to indicate
symbols which are defined in other modules. See Extern
below.
new
Constructors are function which are designed to be used with the new
prefix. The new
prefix creates a new object based on the function's
prototype, and binds that object to the function's implied this
parameter. If you neglect to use the new
prefix, no new object
will be made and this
will be bound to the global object. This
is a serious mistake.
JSLint
enforces the convention that constructor functions be given
names with initial uppercase. JSLint
does not expect to see a function
invocation with an initial uppercase name unless it has the new
prefix. JSLint
does not expect to see the new
prefix used
with functions whose names do not start with initial uppercase.
JSLint
does not expect to see the wrapper forms new Number
,
new String
, new Boolean
.
JSLint
does not expect to see new Object
(use {}
instead).
JSLint
does not expect to see new Array
(use []
instead).
JSLint
does not do flow analysis to determine that variables are assigned
values before used. This is because variables are given a value (undefined
)
which is a reasonable default for many applications.
JSLint
does not do any kind of global analysis. It does not
attempt to determine that functions used with new
are really
constructors (except by enforcing capitalization conventions), or that
method names are spelled correctly.
JSLint
is able to handle HTML text. It can inspect the JavaScript content
contained within <script>
...</script>
tags. It
also inspects the HTML content, looking for problems that are known to interfere
with JavaScript:
</p>
) must have
a close tag.'<'
.JSLint
is less anal than the sycophantic conformanity demanded by
XHTML, but more strict than the popular browsers.
JSLint
also checks for the occurence of '</'
in
string literals. You should always write '<\/'
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.
There is an option that allows use of upper case tagnames.
If the source is as JSLint
expects, it generates a
function report. It lists for each function:
JSLint
will "guess"
the name.arguments
if used).These suffixes may appear in the report:
(closure) | The parameter or var is used by an inner function. |
(outer) | The var is defined by an outer function. |
(unused) | The var is defined but not used. This may be an indication of an error. |
(?) | The global var is defined by default within a function. This may be an
indication of an error. It can also be a normal occurrence because JSLint
does not know about global vars or functions that are defined by the browser
or by other files. Select the Detect undefined variables option
to flag these as errors. |
The report will also include a list of all of the member names that were used. There is a list of JSLint messages.
You can include in your program a comment that lists the global functions and objects that your program depends on, but that are not defined in your program or script file. Including this information can improve the quality of the report that is generated.
An external declaration can look like this:
/*extern getElementByClass, breakCycles, JSONRequest */
A global declaration starts with /*extern
. Notice that there is no
space before the e
.
Select the Assume a browser
option to predefine the standard
global properties that are supplied by web browsers, such as window
and document
and alert
. Select the Assume
Rhino
option to predefine the global properties provided by the
Rhino environment. Select the Assume a Yahoo Widget
option
to predefine the global properties provided by the Yahoo! Widgets environment.
The implementation of JSLint
accepts an option object which
allows you to determine the subset of JavaScript that is acceptable to
you. It is also possible to set those options within the source of a script.
An option specification can look like this:
/*jslint nomen: true, evil: false */
An option specification starts with /*jslint
. Notice that
there is no space before the j
. The specification contains
a sequence of name value pairs, where the names are JSLint
options, and the values are true
or false
. An
option specification takes precedence over the option object.
These are the current options:
adsafe |
true if ADSAFE rules should be enforced |
bitwise |
true if bitwise operators should not be allowed |
browser |
true if the standard browser globals should be predefined
|
cap |
true if upper case HTML should be allowed |
debug |
true if debugger statements should be allowed |
eqeqeq |
true if === should be required |
evil |
true if eval should be allowed |
fragment |
true if HTML fragments should be allowed |
laxbreak |
true if line breaks should not be checked |
nomen |
true if names should be checked |
passfail |
true if the scan should stop on first error |
plusplus |
true if increment/decrement should not be allowed |
rhino |
true if the Rhino environment globals should be predefined
|
undef |
true if undefined variables are errors |
white |
true if strict whitespace rules apply |
widget |
true if the Yahoo Widgets globals should be predefined |
Please let me know if JSLint
is useful for you. Is it too
strict? Is there a check or a report that could speed up your
debugging? douglas@crockford.com
I intend to continue to adapt JSLint
based on your comments. Keep
watching for improvements.
Try it. Paste your script into the window and click the button. The analysis is done by a script running on your machine. Your script is not sent over the network.
It is also available as a Konfabulator widget. You can check a file by dragging it and dropping it on the widget. You can recheck the file by double-clicking the widget.
It is also available in a WSH Command Line version.
It is also available in a Rhino Command Line version.
JSLint
uses a Pratt
Parser (Top Down Operator Precedence). It is written in JavaScript.
The full source code is available: