The arguments object

The arguments object is a local variable available within all non-arrow functions. You can refer to a function's arguments inside that function by using its arguments object. It has entries for each argument the function was called with, with the first entry's index at 0.

For example, if a function is passed 3 arguments, you can access them as follows:

arguments[0]; // first argument
arguments[1]; // second argument
arguments[2]; // third argument

The arguments object is useful for functions called with more arguments than they are formally declared to accept, called variadic functions, such as Math.min(). This example function accepts any number of string arguments and returns the longest one:

function longestString() {
  let longest = "";
  for (let i = 0; i < arguments.length; i++) {
    if (arguments[i].length > longest.length) {
      longest = arguments[i];
    }
  }
  return longest;
}

You can use arguments.length to count how many arguments the function was called with. If you instead want to count how many parameters a function is declared to accept, inspect that function's length property.

Each argument index can also be set or reassigned:

arguments[1] = "new value";

Non-strict functions that only have simple parameters (that is, no rest, default, or destructured parameters) will sync the new value of parameters with the arguments object, and vice versa:

function func(a) {
  arguments[0] = 99; // updating arguments[0] also updates a
  console.log(a);
}
func(10); // 99

function func2(a) {
  a = 99; // updating a also updates arguments[0]
  console.log(arguments[0]);
}
func2(10); // 99

Non-strict functions that are passed rest, default, or destructured parameters will not sync new values assigned to parameters in the function body with the arguments object. Instead, the arguments object in non-strict functions with complex parameters will always reflect the values passed to the function when the function was called.

function funcWithDefault(a = 55) {
  arguments[0] = 99; // updating arguments[0] does not also update a
  console.log(a);
}
funcWithDefault(10); // 10

function funcWithDefault2(a = 55) {
  a = 99; // updating a does not also update arguments[0]
  console.log(arguments[0]);
}
funcWithDefault2(10); // 10

// An untracked default parameter
function funcWithDefault3(a = 55) {
  console.log(arguments[0]);
  console.log(arguments.length);
}
funcWithDefault3(); // undefined; 0

This is the same behavior exhibited by all strict-mode functions, regardless of the type of parameters they are passed. That is, assigning new values to parameters in the body of the function never affects the arguments object, nor will assigning new values to the arguments indices affect the value of parameters, even when the function only has simple parameters.

arguments is an array-like object, which means that arguments has a length property and properties indexed from zero, but it doesn't have Array's built-in methods like forEach() or map(). However, it can be converted to a real Array, using one of slice(), Array.from(), or spread syntax.

const args = Array.prototype.slice.call(arguments);
// or
const args = Array.from(arguments);
// or
const args = [...arguments];

For common use cases, using it as an array-like object is sufficient, since it both is iterable and has length and number indices. For example, Function.prototype.apply() accepts array-like objects.

function midpoint() {
  return (
    (Math.min.apply(null, arguments) + Math.max.apply(null, arguments)) / 2
  );
}

console.log(midpoint(3, 1, 4, 1, 5)); // 3

arguments.callee Deprecated

Reference to the currently executing function that the arguments belong to. Forbidden in strict mode.

arguments.length

The number of arguments that were passed to the function.

arguments[Symbol.iterator]()

Returns a new array iterator object that contains the values for each index in arguments.

This example defines a function that concatenates several strings. The function's only formal argument is a string containing the characters that separate the items to concatenate.

function myConcat(separator) {
  const args = Array.prototype.slice.call(arguments, 1);
  return args.join(separator);
}

You can pass as many arguments as you like to this function. It returns a string list using each argument in the list:

myConcat(", ", "red", "orange", "blue");
// "red, orange, blue"

myConcat("; ", "elephant", "giraffe", "lion", "cheetah");
// "elephant; giraffe; lion; cheetah"

myConcat(". ", "sage", "basil", "oregano", "pepper", "parsley");
// "sage. basil. oregano. pepper. parsley"

This example defines a function that creates a string containing HTML for a list. The only formal argument for the function is a string that is "u" if the list is to be unordered (bulleted), or "o" if the list is to be ordered (numbered). The function is defined as follows:

function list(type) {
  let html = `<${type}l><li>`;
  const args = Array.prototype.slice.call(arguments, 1);
  html += args.join("</li><li>");
  html += `</li></${type}l>`; // end list
  return html;
}

You can pass any number of arguments to this function, and it adds each argument as a list item to a list of the type indicated. For example:

list("u", "One", "Two", "Three");
// "<ul><li>One</li><li>Two</li><li>Three</li></ul>"

The typeof operator returns 'object' when used with arguments

console.log(typeof arguments); // 'object'

The type of individual arguments can be determined by indexing arguments:

console.log(typeof arguments[0]); // returns the type of the first argument

• Functions guide
• Functions
• Rest parameters