function func1(a, b, c) { console.log(arguments[0]); // Expected output: 1 console.log(arguments[1]); // Expected output: 2 console.log(arguments[2]); // Expected output: 3 } func1(1, 2, 3);
The arguments object is a local variable available within all non-
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:
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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
, such as
. This example function accepts any number of string arguments and returns the longest one:
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function longestString() { let longest = ""; if (arguments.length === 0) { throw new TypeError("At least one string is required"); } for (const arg of arguments) { if (arg.length > longest.length) { longest = arg; } } return longest; } You can use
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
property.
Each argument index can also be set or reassigned:
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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:
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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
,
, or
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.
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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
, 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
.
arguments is an array-like object
arguments is an array-like object, which means that arguments has a
property and properties indexed from zero, but it doesn't have
's built-in methods like
or
. However, it can be converted to a real Array, using one of
,
, or
.
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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
and has length and number indices. For example,
accepts array-like objects.
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function midpoint() { return ( (Math.min.apply(null, arguments) + Math.max.apply(null, arguments)) / 2 ); } console.log(midpoint(3, 1, 4, 1, 5)); // 3
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
object that contains the values for each index in arguments.
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.
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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:
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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
, or "o" if the list is to be
. The function is defined as follows:
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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:
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list("u", "One", "Two", "Three"); // "<ul><li>One</li><li>Two</li><li>Three</li></ul>"
The
operator returns 'object' when used with arguments
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console.log(typeof arguments); // 'object' The type of individual arguments can be determined by indexing arguments:
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console.log(typeof arguments[0]); // returns the type of the first argument
Specification
ECMAScript® 2027 Language Specification# sec-arguments-exotic-objects
guide