Get a function's arity

JasonSmith picture JasonSmith · Jan 31, 2011 · Viewed 8k times · Source

In Javascript, how can one determine the number of formal parameters defined for a function?

Note, this is not the arguments parameter when the function is called, but the number of named arguments the function was defined with.

function zero() {
    // Should return 0
}

function one(x) {
    // Should return 1
}

function two(x, y) {
    // Should return 2
}

Answer

Josh Lee picture Josh Lee · Jan 31, 2011
> 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
}