In computer science, a first-class citizen is an entity that supports all operations available to other entities. Some of the available operations are:

• They may be named by variables;
• They may be passed as arguments to procedures;
• They may be returned as the results of procedures;
• They may be included in data structures.

It was the British computer scientist Christopher Strachey (1916-1975) who first coined this notion of first-class citizen status of elements in a programming language in the 1960s.

In JavaScript, for example, functions are first-class citizens, as all of the operations cited above can be applied to them. Let's see some examples:

A simple function definition in JavaScript

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function sum(a, b) {
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  return a + b
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}

Assigning a constant to a function

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const sum = (a, b) => a + b
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// or
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// 
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// const sum = function (a, b) {
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//   a + b
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// }

Passing a function as an argument

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function sum(a, b, callback) {
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  const result = a + b
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  if (typeof callback === 'function') {
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    callback(result) // pass the result as an argument of `callback`
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  }
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  return result
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}
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//        Pass `console.log` as the callback function
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// -------\/
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sum(2, 2, console.log) // => 4

Return a function

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function sum(a, b, callback) {
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  const result = a + b
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  if (callback) {
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    return () => callback(result)
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  }
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  return result
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}
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//            The callback is the sum of the result with 2.
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// ------------------\/
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const fn = sum(2, 2, (result) => sum(2, result))
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//    ^---- Store the returned function in a variable
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//          Execute the function
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// ---------\/
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console.log(fn()) // => 6

Including a function in a data structure

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// Store the basic operations in an object
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const operations = {
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  sum: (a, b) => a + b,
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  sub: (a, b) => a - b,
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  mul: (a, b) => a * b,
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  div: (a, b) => a / b,
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}