Higher-Order Functions Fundamentals

Higher-Order Functions

A function that accepts and/or returns another function is called a higher-order function.

It's higher-order because instead of strings, numbers, or booleans, it goes higher to operate on functions. Pretty meta.

With functions in JavaScript, you can

Store them as variables

Use them in arrays

Assign them as object properties(methods)

Pass them as arguments

Return them from other functions

Functions Operate on Data

Strings Are Data

sayHi = (name) => `Hi, ${name}!`;
result = sayHi('Unity');

console.log(result);	// Hi, Unity!

Numbers Are Data

double = (x) = > x * 2;
result = double(4);

console.log(result);	// 8

Booleans Are Data

getClearance = (allowed) => (allowed ? 'Access granted' : 'Access denied');

result1 = getClearance(true);
result2 = getClearance(false);

console.log(result1);	// Access granted
console.log(result2);	// Access denied

Objects Are Data

getFirstName = (obj) => obj.firstName;

result = getFirstName({
   firstName: 'Bjarne'
});

console.log(result);	// 'Bjarne'

Arrays Are Data

len = (array) => array.length;
result = len([1, 2, 3]);

console.log(result);	// 3

What makes them first-class? You can pass them around, store them in variables and arrays, use them as inputs for calculations. You can use them like any piece of data.