JS: Function Parameters

By Xah Lee. Date: . Last updated: .

Number of Arguments Not Checked

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

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]

How to find the number of arguments passed?

The number of arguments passed is arguments.length.

With ES2015 onward, you should avoid using this feature.

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

How to find out how many parameters are declared?

Function has a .length property. Its value is the number of parameters declared in the function.

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

With ES2015 onward, you should avoid using this feature.

Function Topic

  1. Define Function
  2. Functional Programing
  3. Arrow Function
  4. Function Parameters
  5. f Declaration vs Expression
  6. Closure
  7. Function Call, Apply, Bind
  8. Function Argument Default Value
  9. Function Rest Parameters
  10. Function Argument Destructure
  11. Function Object
  12. Function.prototype
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