JSLint: Appendix C - JavaScript: The Good Parts
by Douglas CrockfordWhen 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 that would scan a source file, looking for problems.

This excerpt is from JavaScript: The Good Parts . This authoritative book scrapes away these bad features to reveal a subset of JavaScript that's more reliable, readable, and maintainable than the language as a whole-a subset you can use to create truly extensible and efficient code.

As C 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-for-its-age language. It was originally
intended to do small tasks in web pages, 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 verifier.
JSLint is a code quality tool for JavaScript. It takes a source text 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 the third edition of the ECMAScript Language Specification. The subset is closely related to the style recommendations from Chapter 9, Style.
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.
JSLint can be found at http://www.JSLint.com/.
JavaScript's biggest problem is its dependence on global
variables, particularly implied global variables. If a variable is not
explicitly declared (usually with the var
statement), then JavaScript assumes that
the variable was global. This can mask misspelled names and other
problems.
JSLint expects that all variables and functions will be 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.
Sometimes a file is dependent on global variables and functions that are defined elsewhere. You can identify these to JSLint by including a comment in your file that lists the global functions and objects that your program depends on, but that are not defined in your program or script file.
A global declaration comment can be used to list all of the names
that you are intentionally using as global variables. JSLint can use
this information to identify misspellings and forgotten var
declarations. A global declaration can
look like this:
/*global getElementByAttribute, breakCycles, hanoi */
A global declaration starts with /*global
. Notice that there is no space before
the g
. You can have as many /*global
comments as you like. They must
appear before the use of the variables they specify.
Some globals can be predefined for you (see the later section
"the section called “Options”"). Select the "Assume a browser" (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" (rhino
) option to predefine the global
properties provided by the Rhino environment. Select the "Assume a Yahoo
Widget" (widget
) option to predefine
the global properties provided by the Yahoo! Widgets environment.
Since JavaScript is a loosely typed dynamic-object language, it is not possible to determine at compile time if property names are spelled correctly. JSLint provides some assistance with this.
At the bottom of its report, JSLint displays a /*members*/
comment. It contains all of the
names and string literals that were used with dot notation, subscript
notation, and object literals to name the members of objects. You can
look through the list for misspellings. Member names that were used only
once are shown in italics. This is to make misspellings easier to
spot.
You can copy the /*members*/
comment into your script file. JSLint will check the spelling of all
property names against the list. That way, you can have JSLint look for
misspellings for you:
/*members doTell, iDoDeclare, mercySakes, myGoodness, ohGoOn, wellShutMyMouth */
The implementation of JSLint accepts an option object that 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. All of the options default to
false
. Table C.1, “JSLint options” lists the options available in using
JSLint.
Table C.1. JSLint options
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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 masking of errors by 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, a closer, or a suffix operator:
) ] ++ −−
JSLint allows you to turn on the "Tolerate sloppy line breaking"
(laxbreak
) option.
Semicolon insertion can mask copy/paste errors. If you always break lines after operators, then JSLint can do a better job of finding those errors.
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) { statements; }
Experience shows that this form is more resilient.
In many languages, a block introduces a scope. Variables introduced in a block are not visible outside of the block.
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.
JSLint expects blocks with function
, if
, switch
,
while
, for
, do
,
and try
statements and nowhere else.
An exception is made for an unblocked if
statement on an else
or for
in
.
An expression statement is expected to be an assignment, a
function/method call, or delete
. All
other expression statements are considered errors.
The for in
statement allows for
looping through the names of all of the properties of an object.
Unfortunately, it also loops through all of the members that were
inherited through the prototype chain. This has the bad side effect of
serving up method functions when the interest is in the data
members.
The body of every for in
statement should be wrapped in an if
statement that does filtering. if
can
select for a particular type or range of values, it can exclude
functions, or it can exclude properties from the prototype. For
example:
for (name in object) { if (object.hasOwnProperty(name)) { .... } }
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 the
next case or default is one of these: break
, return
, or throw
.
JavaScript allows var
definitions to occur anywhere within a function. JSLint is
stricter.
JSLint expects that:
A
var
will be declared only once, and that it will be declared before it is used.A function will be declared before it is used.
Parameters will not also be declared as
var
s.
JSLint does not expect:
The
arguments
array to be declared as avar
.That a variable will be declared in a block. This is because JavaScript blocks do not have block scope. This can have unexpected consequences, so define all variables at the top of the function body.
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 parentheses:
if ((a = b)) { ... }
The ==
and !=
operators do type coercion before
comparing. This is bad because it causes ' \f\r
\n\t ' == 0
to be true
.
This can mask type errors.
When comparing to any of the following values, always 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)
Use of the ===
and !==
operators is always preferred. There is a
"Disallow ==
and !=
" (eqeqeq
) option, which requires the use of
===
and !==
in all cases.
JavaScript allows any statement to have a label, and labels have a separate namespace. JSLint is stricter.
JSLint expects labels only on statements that interact with
break
: switch
, while
, do
,
and for
. JSLint expects that labels
will be distinct from variables 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 parentheses to avoid confusion.
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 viruses and other
security menaces. The JSLint option plusplus
prohibits the use of these
operators.
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 nearly as efficient
as they are in C or other languages. They are rarely useful in browser
applications. The similarity to the logical operators can mask some
programming errors. The bitwise
option prohibits the use of these operators.
The eval
function and its
relatives (Function
, setTimeout
, and setInterval
) provide access to the JavaScript
compiler. This is sometimes useful, but in most cases it indicates the
presence of extremely bad coding. The eval
function is the most misused feature of
JavaScript.
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.
Regular expressions are written in a terse and cryptic notation. JSLint looks for problems that may cause portability problems. It also attempts to resolve visual ambiguities by recommending explicit escapement.
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.
Constructors are functions that 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 letters. 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
, or 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 they are used. This is because variables are
given a value (undefined
) that 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 and
event handlers. It also inspects the HTML content, looking for problems
that are known to interfere with JavaScript:
All tag names must be in lowercase.
All tags that can take a close tag (such as
</p>
) must have a close tag.All tags are correctly nested.
The entity
<
must be used for literal<
.
JSLint is less anal than the sycophantic conformity demanded by XHTML, but more strict than the popular browsers.
JSLint also checks for the occurrence 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 uppercase tag names. There is also an option that allows the use of inline HTML event handlers.
JSLint can also check that JSON data structures are well formed.
If the first character JSLint sees is {
or [
,
then it strictly enforces the JSON rules. See Appendix E, JSON.
If JSLint is able to complete its scan, it generates a function report. It lists the following for each function:
The line number on which it starts.
Its name. In the case of anonymous functions, JSLint will "guess" the name.
The parameters.
Closure: the variables and parameters that are declared in the function that are used by its inner functions.
Variables: the variables declared in the function that are used only by the function.
Unused: the variables that are declared in the function that are not used. This may be an indication of an error.
Outer: variables used by this function that are declared in another function.
Global: global variables that are used by this function.
Label: statement labels that are used by this function.
The report will also include a list of all of the member names that were used.
If you enjoyed this excerpt, buy a copy of JavaScript: The Good Parts .