JS: 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

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

Parameter Default Value

Parameter can have default value, like this:

function f(param=value) {…}

[see JS: Function Argument Default Value]

Rest Parameters

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

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

That is, 3 dots followed by a name. In function body, the name is received as a array.

The syntax ...name must be the last item in parameter declaration.

[see JS: Function Rest Parameters]

Function Argument Destructure

[see JS: Function Argument Destructure]

the 「arguments」 Object

[see JS: Function's “arguments” Object]

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