A function is a set of statements that performs a task or calculates a value.
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Functions are one of the fundamental building blocks in JavaScript. It is similar to a procedure—a set of statements that performs a task or calculates a value (1). It can accept some parameters, and may define some local variables necessary for performing the desired operation. These parameters and local variables are contained in the scope of the function, and cannot be accessed from the outside. However, a function can access variables external to itself (as long as they are not overridden), such as global variables or variables in the scope this function is encapsulated in. A nested function can access free variables that are defined in outer scopes, even when it is being invoked away from where it is defined. It does this by capturing the free variables (by reference) into a closure (1). Closure, arrow functions, and more, are functional language features based on lambda calculus.

This package includes a number of methods to transform functions. This enables you to obtain a desired function by transforming the behavior of existing functions (without actually executing them). The result of a function can be manipulated with negate. In case a pure function is expensive, its results can cached with memoize. Parameters of a function can be manipulated with reverse, spread, unspread. reverse flips the order of parameters, spread spreads the first array parameter of a function, and unspread combines all parameters into the first parameter (array). If you want some functional behavior, compose, composeRight, curry, and curryRight can be used. composeRight is also known as pipe-forward operator or function chaining. If you are unfamiliar, Haskell is a great purely functional language, and there is great haskell beginner guide to learn from.

To control invocation time of a function, use delay. A function can be rate controlled with restrict, debounce, debounceEarly, throttle, throttleEarly. debounce and debounceEarly prevent the invocation of a function during hot periods (when there are too many calls), and can be used for example to issue AJAX request after user input has stopped (for certain delay time). throttle and throttleEarly can be used to limit the rate of invocation of a function, and can be used for example to minimize system usage when a user is constantly refreshing a webpage. Except restrict, all rate/time control methods can be flushed (flush()) to invoke the target function immediately, or cleared (clear()) to disable invocation of the target function.

In addition, is, isAsync, isGenerator, name, and length obtain metadata (about) information on a function. To attach a this to a function, use bind. A few generic functions are also included: NOOP, FALSE, TRUE, IDENTITY, COMPARE, ARGUMENTS.

This package is available in Node.js and Web formats. To use it on the web, simply use the extra_function global variable after loading with a <script> tag from the jsDelivr CDN.

Stability: Experimental.

const xfunction = require('extra-function');
// import * as xfunction from "extra-function";
// import * as xfunction from "https://unpkg.com/extra-function/index.mjs"; (deno)

var a = xfunction.composeRight(x => x*x, x => x+2);
a(10);
// → 102

var a = xfunction.curry((x, y) => x+y);
a(2)(3);
// → 7

var a = xfunction.unspread(Math.max);
a([2, 3, 1]);
// → 1.25

• MDN Web docs
• Lodash documentation
• Underscore.js documentation
• Function composition
• Debouncing and Throttling Explained Through Examples by David Corbacho
• Learn You a Haskell for Great Good!: Higher order functions by Miran Lipovaca
• How to know if a function is async?
• Check if function is a generator
• Haskell composition (.) vs F#'s pipe forward operator (|>)
• JavaScript Detect Async Function by David Walsh
• is-function package by Stephen Sugden
• is-async-function package by Jordan Harband
• is-callback-function package by Charlike Mike Reagent
• is-generator-fn package by Sindre Sorhus
• is-generator-function package by Jordan Harband
• fn-name package by Sindre Sorhus
• memoizee package by Mariusz Nowak
• memoizerific package by @thinkloop
• compose-function package by Christoph Hermann
• chain-function package by Jason Quense
• @spudly/curry package by Stephen Sorensen
• one-time package by Arnout Kazemier
• onetime package by Sindre Sorhus
• once package by Isaac Z. Schlueter
• debounce package by @component
• throttle-debounce package by Ivan Nikolić
• throttleit package by @component