Minimum example:

function test() {
  console.log(arguments.join(','));
}

test(1,2,3);

I then get:

TypeError: undefined is not a function

However, when I do the same for an array:

console.log([1,2,3].join(','));

I get

"1,2,3"

As expected.

What's wrong with arugments? It's suppose to be an array:

(function () {
  console.log(typeof [] == typeof arguments)
})();

true

Minimum example:

function test() {
  console.log(arguments.join(','));
}

test(1,2,3);

I then get:

TypeError: undefined is not a function

However, when I do the same for an array:

console.log([1,2,3].join(','));

I get

"1,2,3"

As expected.

What's wrong with arugments? It's suppose to be an array:

(function () {
  console.log(typeof [] == typeof arguments)
})();

true

• javascript
• node.js
• v8
• ecmascript-5

2 Answers 2

Arguments is not an array.

(function(){
   console.log(typeof arguments);
})();
// 'object'

It is an array-like structure with a length and numeric properties, but it is not actually an array. If you want to, you may use the array function on it though.

function test() {
    console.log(Array.prototype.join.call(arguments, ','));

    // OR make a new array from its values.
    var args = Array.prototype.slice.call(arguments);
    console.log(args.join(','));
}

test(1,2,3);

Note, your example works because array is not a type. typeof [] === 'object' also. You can however check if an object is an array by using

Array.isArray(arguments) // false
Array.isArray([]) // true

The problem is that arguments is NOT a javascript Array per se. It behaves like an array in some ways but no in others.

Why don't you try converting it into an pure javascript array. This can be done the following way:

(function () {
   var args = Array.prototype.slice.call(arguments, 0);
   console.log(typeof [] === typeof args);
}());