Higher Order Functions

A higher-order function either takes in a function as an argument, or returns a function.

Callback Functions

When taking a function as a parameter, this is called a callback function. This is used in Javascript built-ins like setTimeout, filter, map, and forEach.

var arr = [1,2,3,4];
var add2 = function (num) {
  return num + 2;
};
var newArr = arr.map(add2); // result: [3,4,5,6]

Closures

Functions that return other functions can create closures, which means that the variables within which the function is defined will remain accessable by that function; the variables are enclosed within that function's own scope.

In the example below, a function called counter keeps track of how many times it is called. Even though the function runs and fully completes, the reference to this execution context remains with the function that is returned at the conclusion of that invocation. We cannot directly access count from outside of the scope of counter, but with the returned function, we can add to it and see what count's value' is at that point.

var counter = function () {
  var count = 0;
  console.log(count);
  var add1 = function () {
    count++;
    console.log(count);
  };
  return add1;
};

// newCounter will now reference the `add1` function within the closure of that invocation of counter
var newCounter = counter(); // 0
// This calls `add1` within that same closure, incrementing `count` by 1
newCounter(); // 1

References

1. https://en.wikipedia.org/wiki/Higher-order_function

Last modified: 202401040446