JavaScript: Function Parameters

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 parameters have values of undefined.

function f (x) { return x;}

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

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

Rest Parameters

ES2015 added Rest Parameters feature. It allows you to explicitly declare optional parameters, and they are passed in as a array.

〔►see JavaScript: Function Rest Parameters〕

Declare Parameter Default Value

To declare default value for parameter, use ES2015 feature for default arguments.

〔►see JavaScript: Function Argument Default Value〕

For pre-ES2015, to define default values for optional parameter, you can just check if a argument is undefined, then set value.

// define function with optional parameter and default value

function f (x, y) {
    if ( y === undefined ) {
        y = 3; // default value
    }
    return x + y;
}

console.log(f(2)); // 5

the 「arguments」 Object

〔►see JavaScript: Function's “arguments” Object〕

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

ES2015 Features

ES2015 has many features that obsolete the need for the argument object. See:

• JavaScript: Function Argument Default Value
• JavaScript: Function Rest Parameters
• JavaScript: Arrow Function