JavaScript: Function Parameters

Number of Arguments Not Checked

JavaScript doesn't check the number of arguments passed in a function call.

• If there are more arguments passed in a function call than declared parameters, they are ignored.
• If less, then extra parameter variables have values of undefined.

function f (x) { return x;}

// extra arguments are ignored
console.log( f(2,3,4) === 2 ); // true

const gg = (x => x);

// extra arguments are ignored
console.log( gg(2,3,4) === 2 ); // true

// unfilled parameters have value of 「undefined」
function gg (x, y) { return y;}
console.log( gg(3) === undefined ); // true

// unfilled parameters have value of 「undefined」
const gg = ((x, y) => y);
console.log( gg(3) === undefined ); // true

Parameter Default Value

Parameter can have default value, like this:

function f(param=value) {…}

Function Argument Default Value

Rest Parameters

A function can have unspecified number of parameters, like this:

function f (...name) { return name; }

Function Rest Parameters

Function Argument Destructure

Function Argument Destructure

the 「arguments」 Object

arguments Object

console.log( 
    (function (x,y) {return x + y;}) .length
);
// 2