JavaScript: Function Argument Default Value

New in JS2015.

Function's parameters can have default value, like this:

function f(param=value) {body}

// function with default parameter values
function f(x = 4, y = 2) { return [x, y]; }
console.log(f()); // [ 4, 2 ]

Default Value Expression Evaluated at Call Time

Default value expression are evaluated at call time. For example,

function f(x=m) { return x; }

the m is evaluated when f is called.

// function, param default value expression is eval'd at call time
let m = 2;
function f(x=m) { return x; }
m = 3;
console.log( f() === 3 ); // true

Later Value Expression Have Access to Previous Value Expression

Later value expression have access to previous value expression.

// function with default value for parameter
// later expression can refer previous
function f(x=4, y=x) { return [x , y]; }
console.log( f() ); // [4, 4]

Passing undefined as Argument

Passing undefined as arguments is equivalent to not passing.

// passing undefined is equivalent as not passing

function f(x=4) { return x; }

console.log( f(undefined) === 4 ); // true

Passing null as Argument

Passing null has no special effect. The argument just got the value of null.

function f (x=3) { return x; }

console.log( f(null) === null ); // true