First-class functions

An important concept in functional programming (FP) is to treat functions as first-class. A function is a first-class function provided it has the following properties:

1. Can be assigned to a variable.
2. Can be stored in a data structure.
3. Can be passed as an argument to another function.
4. Can be returned as a result from another function.

First-class functions are treated like a primitive data type such as string, number, boolean, and null. Data in JavaScript are first-class because they have the above 4 properties. For example, we can store a string to a variable, store a number in a data structure such as an array, pass a boolean as an argument to a function, and return null as a result from a function.

Which features of JavaScript are not first-class? Reconsider the above 4 properties and think about how they relate to loops and conditionals. Can we pass a loop to a function? Can we manipulate a loop at runtime? Rather than focus on which features of JavaScript are not first-class, it is helpful to concentrate on those aspects that are first-class. Data, objects, and functions in JavaScript are first-class. The above 4 properties allow us to manipulate functions as if they are data.

A higher-order function is a function that has the following properties:

1. Takes a function as an argument.
2. Returns a function as a result.

In order for a language to support FP, functions in that language must be first-class and higher-order. If a function is first-class only, it cannot be passed as an argument to another function because there would not be functions that take other functions as arguments. First-class functions are data. Higher-order functions operate on first-class functions and treat them like any other primitive data.