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In computer programming, an anonymous function (function literal, expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to higher-order functions or used for constructing the result of a higher-order function that needs to return a function. [ 1 ]
In ES6, arrow function syntax was added, allowing functions that return a value to be more concise. They also retain the this of the global object instead of inheriting it from where it was called / what it was called on, unlike the function() {} expression.
Arrow functions were first introduced in 6th Edition – ECMAScript 2015. They shorten the syntax for writing functions in JavaScript. Arrow functions are anonymous, so a variable is needed to refer to them in order to invoke them after their creation, unless surrounded by parenthesis and executed immediately. Example of arrow function:
Immediately invoked function expressions may be written in a number of different ways. [3] A common convention is to enclose the function expression – and optionally its invocation operator – with the grouping operator, [4] in parentheses, to tell the parser explicitly to expect an expression.
For example, when writing :, the intent is that and are types, while the arrow is a type constructor, specifically, the function type or arrow type. Similarly, the Cartesian product X × Y {\displaystyle X\times Y} of types is constructed by the product type constructor × {\displaystyle \times } .
The term closure is often used as a synonym for anonymous function, though strictly, an anonymous function is a function literal without a name, while a closure is an instance of a function, a value, whose non-local variables have been bound either to values or to storage locations (depending on the language; see the lexical environment section below).
MDN Web Docs, previously Mozilla Developer Network and formerly Mozilla Developer Center, is a documentation repository and learning resource for web developers. It was started by Mozilla in 2005 [ 2 ] as a unified place for documentation about open web standards, Mozilla's own projects, and developer guides.
Alternatives, in situations where a function is required, are: Use the Function.prototype() built-in function, that accepts any arguments and returns undefined; [11] Use a NOP function available in a third-party library —see below; Define a custom NOP function, as in the following example (using the ES6 arrow function syntax):