Tips for golfing in JavaScript

Splitting with numbers to save the quotemarks:

"alpha,bravo,charlie".split(",") // before
"alpha0bravo0charlie".split(0)   // after

Fancy For Loops

you can use the standard for loop in non-standard ways

for ( a; b; c )

is essentially equivalent to:

a;
while ( b )
{
  ...
  c;
}

so a good trick is to write your code with a while loop, and then split it into the a,b,c parts in a for loop.

A couple examples I've written:

for(x=y=n;!z;x--,y++)z=i(x)?x:i(y)?y:0
for(a=b=1;b<n;c=a+b,a=b,b=c);

Chain your setters

If you're initializing or resetting multiple values, chain the value to all the variables that need it:

a=b=1;

Implicit Casting

Don't check your types, just use them as they are. parseInt() costs 10 characters. If you need to cast out of a string, be creative:

a='30';
b='10';
c = a + b; //failure
c = parseInt(a) + parseInt(b) //too long

c = -(-a-b); //try these
c = ~~a+~~b;
c = +a+ +b;
c = a- -b;

Avoid Semicolons

JavaScript has automatic semi-colon insertion. Use it often and well.

One-liners

Save on brackets by shoving as much as possible into single lines, or parameters:

a( realParam1, realParam2, fizz='buzz' )

Increment/Decrement operators

a = a - 1;
foo(a);

and

foo(a);
a = a - 1;

can easily be rewritten as

foo(--a);

and

foo(a--);

respectively.

Use this or self instead of window in global context

self explanatory 2 character savings.

Use bracket notation for repeat property access

This is definitely a balancing act between property name length and number of accesses. Instead of calling a.longFunctionName() with dot notation twice, it's shorter to save the name and call the function via bracket notation:

a.longFunctionName(b)
a.longFunctionName(c)
//42

-vs-

a[f='longFunctionName'](b)
a[f](c)
//34

this is especially effective with functions like document.getElementById which can be reduced to d[e].

Note:

With bracket notation, the cost is 6 + name.length characters the first time. Each subsequent access has a cost of 3 characters.

For dot notation, all accesses cost name.length + 1 (+1 for the .) characters.

Use this method if 6 + name.length + (3 * (accesses - 1)) < accesses * (name.length + 1).

len = length of property name
i = minimum accesses to take advantage

len | i 
========
1   | ∞ 
2   | ∞ 
3   | 7 
4   | 4 
5   | 3 
6   | 3 
7   | 3 
8+  | 2 

The number of accesses can also span multiple objects. If you access .length 4 or more times on different arrays, you can use the same variable holding the string 'length'.

Use the comma operator to avoid braces (also applies to C):

if(i<10)m+=5,n-=3;

Instead of

if(i<10){m+=5;n-=3}

which is one character longer.