The arguments object

JavaScript course · Beginner · Lesson 84/133

Learn the legacy arguments object you often see in old code and why rest is preferred.

The arguments object

In old JavaScript code you will see an object named arguments. It holds all arguments passed to the function and is available even without parameters. Today the rest parameter is preferred, but you must recognize arguments when reading old code.

Example: function add() { return arguments[0] + arguments[1]; }. Here arguments[0] is the first argument and arguments[1] the second. The call add(3, 5) returns 8.

Rest is preferred

The arguments object is array-like but not a real array. So array methods like forEach, map and filter do not work on it. Also, arrow functions do not have arguments.

For these reasons modern code prefers a rest parameter like ...nums. Rest gives a real array, all array methods work, and it works in arrow functions too. When writing new code, always use rest.

Exercise Sum the first two arguments

Define a function called add. Do not write any parameters. Inside, use the arguments object to return the sum of the first two arguments with return arguments[0] + arguments[1]. Then print the result with console.log(add(3, 5)).

Show example solution

function add() {
  return arguments[0] + arguments[1];
}

console.log(add(3, 5));

Test yourself

Which is true about the arguments object?
```
function add() {
  return arguments[0] + arguments[1];
}
```
- It is not a real array
- It only exists in arrow functions
- It was added in ES2023
- It is more modern than the rest parameter
Show answer

It is not a real array — Correct. arguments is array-like but not a real array; array methods do not work.

Access the first argument

function add() {
  return ____ + arguments[1];
}
console.log(add(3, 5));

Show answer

function add() {
  return arguments[0] + arguments[1];
}
console.log(add(3, 5));

arguments and rest

arguments

Not an array, array-like
...nums

A real array
arguments.length

Number of arguments
nums.forEach

Array method works

All lessons in Beginner

1 Variables and data types

The arguments object