Passing arguments to function in javascript

JavaScript Function Parameters

A JavaScript function does not perform any checking on parameter values (arguments).

Function Parameters and Arguments

Earlier in this tutorial, you learned that functions can have parameters:

Function parameters are the names listed in the function definition.

Function arguments are the real values passed to (and received by) the function.

Parameter Rules

JavaScript function definitions do not specify data types for parameters.

JavaScript functions do not perform type checking on the passed arguments.

JavaScript functions do not check the number of arguments received.

Default Parameters

If a function is called with missing arguments (less than declared), the missing values are set to undefined .

Sometimes this is acceptable, but sometimes it is better to assign a default value to the parameter:

Example

Default Parameter Values

ES6 allows function parameters to have default values.

Example

If y is not passed or undefined, then y = 10.

Function Rest Parameter

The rest parameter (. ) allows a function to treat an indefinite number of arguments as an array:

Example

function sum(. args) <
let sum = 0;
for (let arg of args) sum += arg;
return sum;
>

let x = sum(4, 9, 16, 25, 29, 100, 66, 77);

The Arguments Object

JavaScript functions have a built-in object called the arguments object.

The argument object contains an array of the arguments used when the function was called (invoked).

This way you can simply use a function to find (for instance) the highest value in a list of numbers:

Example

x = findMax(1, 123, 500, 115, 44, 88);

function findMax() let max = -Infinity;
for (let i = 0; i < arguments.length; i++) if (arguments[i] > max) max = arguments[i];
>
>
return max;
>

Or create a function to sum all input values:

Example

x = sumAll(1, 123, 500, 115, 44, 88);

function sumAll() let sum = 0;
for (let i = 0; i < arguments.length; i++) sum += arguments[i];
>
return sum;
>

If a function is called with too many arguments (more than declared), these arguments can be reached using the arguments object.

Arguments are Passed by Value

The parameters, in a function call, are the function’s arguments.

JavaScript arguments are passed by value: The function only gets to know the values, not the argument’s locations.

If a function changes an argument’s value, it does not change the parameter’s original value.

Changes to arguments are not visible (reflected) outside the function.

Objects are Passed by Reference

In JavaScript, object references are values.

Because of this, objects will behave like they are passed by reference:

If a function changes an object property, it changes the original value.

Changes to object properties are visible (reflected) outside the function.

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The arguments object

arguments is an array-like object accessible inside functions that contains the values of the arguments passed to that function.

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Description

Note: In modern code, rest parameters should be preferred.

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:

[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.

Assigning to indices

Each argument index can also be set or reassigned:

Non-strict functions that only has 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.

Note: You cannot write a «use strict»; directive in the body of a function definition that accepts rest, default, or destructured parameters. Doing so will throw a syntax error.

arguments is an array-like object

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 

Properties

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

The number of arguments that were passed to the function.

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

Examples

Defining a function that concatenates several strings

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" 

Defining a function that creates HTML lists

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:

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:

Using typeof with arguments

The typeof operator returns ‘object’ when used with arguments

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

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

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

Specifications

Browser compatibility

BCD tables only load in the browser

See also

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This page was last modified on Apr 5, 2023 by MDN contributors.

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