Get a function's arity

> zero.length
0
> one.length
1
> two.length
2

Source

A function can determine its own arity (length) like this:

// For IE, and ES5 strict mode (named function)
function foo(x, y, z) {
    return foo.length; // Will return 3
}

// Otherwise
function bar(x, y) {
    return arguments.callee.length; // Will return 2
}

A function's arity is stored in its .length property.

function zero() {
    return arguments.callee.length;
}

function one(x) {
    return arguments.callee.length;
}

function two(x, y) {
    return arguments.callee.length;
}

> console.log("zero="+zero() + " one="+one() + " two="+two())
zero=0 one=1 two=2

As is covered in other answers, the length property tells you that. So zero.length will be 0, one.length will be 1, and two.length will be 2.

As of ES2015, we have two wrinkles:

• Functions can have a "rest" parameter at the end of the parameter list which gathers up any arguments given at that position or afterward into a true array (unlike the arguments pseudo-array)
• Function parameters can have default values

The "rest" parameter isn't counted when determining the arity of the function:

function stillOne(a, ...rest) { }
console.log(stillOne.length); // 1

Similarly, a parameter with a default argument doesn't add to the arity, and in fact prevents any others following it from adding to it even if they don't have explicit defaults (they're assumed to have a silent default of undefined):

function oneAgain(a, b = 42) { }
console.log(oneAgain.length);    // 1

function oneYetAgain(a, b = 42, c) { }
console.log(oneYetAgain.length); // 1