JavaScript Style
Fall 2008, David Matuszek

Many of the following points are drawn from JavaScript: The Good Parts by Douglas Crockford.

Rules Reasons

Don't use global variables.

All variables should be declared inside methods, using var.

Global variables can be changed anywhere, making debugging harder.

Global variables make conflicts more likely if you try to run independent subprograms in the same program.

 

Variables should be declared at the top of each function. JavaScript has no "block scope." Variables declared anywhere within a function are available throughout the function.
End every statement with a semicolon. JavaScript's automatic semicolon insertion may put semicolons where you don't want them, leading to hard-to-find errors.
Use === and !== instead of == and !=. The shorter versions try to convert their operands to the same type, using some complicated and unmemorable rules.
Don't use the with statement. with automatically inserts a prefix before some variables but not others. It usually does the "right" thing, but that's hard to tell.
Don't use the bitwise operators & and |. These work correctly for integers. JavaScript doesn't have integers. Also don't use them in place of the logical operators && and ||.
Before using eval, look for some other way of doing the same thing. eval is insecure, inefficient, and harder to read. It's useful when you really need to create code dynamically, but it's a poor substitute for knowing how to do things in JavaScript.
If you deliberately fall through to the next case in a switch statement, say so in a comment. This is done more often by accident than deliberately.
Always use braces with control statements, even if only one statement is being controlled. As in Java, this helps to avoid future mistakes when code is added.
Put open braces, {, at the end of lines, not at the beginning (i.e. use Java style) Otherwise JavaScript is likely to insert a semicolon at the end of the line.
Use ++ and -- only by themselves, not in more complex expressions. Pre/post increment and decrement are confusing. When they are used just to increment or decrement (as a statement by themselves, or as the only part of the "increment" of a for loop), they are okay.
Declare your functions in the <head> of the page. This ensures functions are loaded before you try to use them.
If you declare a function dynamically, use the form
var foo = function foo(...){...};
instead of just
function foo(...){...};
You can dynamically declare functions in, for example, if-then statements. Functions declared in the latter form may (or may not) be "hoisted"--declared statically at the top level.
Don't use new Boolean, new Number, or new String. Don't use void, either. These are useless.