Function parameter

As explained in Secrets of the JavaScript Ninja, Second Edition

A variable listed in a function definition that receives a corresponding argument when the function is invoked.

A parameter is a variable named in a function definition. When the function is invoked, arguments—the values passed to it—are assigned to parameters in order. section 3.4

Parameters appear across JavaScript function forms:

• Function declarations and function expressions can list parameters in their definitions. section 3.4
• Arrow functions use a comma-separated parameter list; parentheses are optional for one parameter but required for none or more than one. section 3.3.2

Definition versus invocation

In skulk(ninja), ninja is the parameter. Calling skulk("Hattori") passes "Hattori" as an argument, assigning that value to ninja. section 3.4

Arguments are matched to parameters by position. Extra arguments are not assigned to named parameters; access to those excess values is deferred. JavaScript permits calls with different numbers of arguments and parameters. section 3.4

Figure 3.7. The difference between function parameters and function arguments

Use the diagram to distinguish a parameter in a definition from an argument in a call. section 3.4

Figure 3.8. Arguments are assigned to function parameters in the order specified. Excess arguments aren’t assigned to any parameters.

The figure illustrates positional assignment and what happens to excess arguments. section 3.4

How Secrets of the JavaScript Ninja, Second Edition covers it

How deep it goes

Defines parameters, explains positional assignment and excess arguments, and defers how to access excess values.

Examples

Uses skulk and practice calls to demonstrate parameter assignment, and arrow functions to show parameter-list syntax.

What it assumes

Assumes enough object fundamentals to get by and defines the parameter–argument distinction directly.

What you can do after

Understand how function definitions name parameters and how calls assign arguments to them.

How it connects

Connects parameters with function arguments, function declarations, function expressions, arrow functions, and first-class functions.

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Related concepts

• function argumentcontrasts with · A function argument is a value or expression supplied when a function or method is invoked, providing input to the called operation.
• function declarationpart of · A function declaration is a source-language construct that specifies a function’s name and parameter interface, and may include its implementation.
• default parameterincludes · A default parameter is a function parameter assigned a specified value when its argument is omitted or, in JavaScript, undefined.
• rest operatorincludes · In JavaScript, rest syntax uses three dots to collect remaining function arguments, array elements, or object properties into a single binding.
• argumentsused by · The JavaScript arguments object is an array-like object available inside non-arrow functions that exposes the values passed to the current function invocation.
• pointer-sized integeruses · A pointer-sized integer is a signed or unsigned integer type whose width matches the target platform’s pointer size.

This explainer was generated by Manning's knowledge graph from the text of Secrets of the JavaScript Ninja, Second Edition. The section links point to the parts of the book it draws on.