JS: Function parameters default values

Parameter default value

Function's parameters can specify default values, like this:

function f (param=value) {body}

it works with Arrow Function and function (keyword).

// function with default parameter values
const ff = (x = 4, y = 2) => [x, y];
console.log(ff());
// [ 4, 2 ]

// 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