Understanding callback functions and closures in JavaScript

Learn what are Higher order functions, callback functions, and closures.

Hello Everyone,

In this post, we will explore the HOF (Higher order function), Callbacks, and the crazy JavaScript Closures that made us all pull our hairs at some point in our learning journey.

Higher Order Function

A function that accepts a function definition as a parameter and/or returns a function is HOF (Higher Order Function).

For instance:

function isEven(n) {
  return n % 2 === 0;
}
 
function printMsg(evenFunc, num) {
  const isEven = evenFunc(num);
  console.log(`The number ${num} is even: ${isEven}`);
}
 
printMsg(isEven, 2);

In the above snippet printMsg is the Higher Order Function, because it accepts the isEven function as parameter.

Callbacks

A callback is a function passed into another function as an argument to be executed later. In the last code snippet isEven is the callback function.

It is called callback because it calls back to the place where it was created.

Let's see one more example.

let numbers = [1, 2, 4, 7, 3, 5, 6];
 
function isOddNumber(number) {
  return number % 2 !== 0;
}
 
function oddNumbers = numbers.filter(isOddNumber);
console.log(oddNumbers); // [1, 7, 3, 5]

In the above code snippet isOddNumber is the callback function. Because isOddNumber is passed as argument to the filter function.

Callback functions can be used to handle Async JavaScript.

Closures

From execution context, we know that after a function is returned, it is removed from the call stack and deleted from execution context. So, keeping that in mind, here is the definition of a closure.

A closure is a function which has access to the outer variables even after the outer function is returned.

Once, the outer function is returned, it is removed from the callstack and deleted from execution context. But the inner function can still access the outer function variables. And this concept is known as closures.

let x = 10;
 
function foo() {
  let y = 20;
 
  function bar() {
    let z = 15;
    return x + y + z;
  }
 
  return bar;
}
 
let test = foo();
test(); // 45

In the above code, bar is the closure function, which has access to outer variables (x and y) even after outer function is returned.

Simplified Closure Definitions:

• A closure is function which remembers the environment in which it was created.
• When a function is returned from an outer function, it carries the environment with itself.

Closures are possible in JS because of Lexical Scoping. Lexical scoping means, If a variable is not found in the local scope, it looks in the outer scope, and so on till the global scope.

Exception in closures

There is just one exeption in closures. In JavaScript every function is a closure except for functions created via "new Function" syntax.

Usually a function have access to the lexical environment where it was created. But when a function is created via the "new Function" syntax, it doesn't have access to the lexical environment but the global one.

function getFunc() {
  let value = "test";
 
  let func = new Function("value", "alert(value)");
 
  return func;
}
 
getFunc()(); // Undefined

So, functions such as func (in above code) doesn’t have access to outer variables, only to the global ones.

That's it for this one.

-- Swastik Yadav Software Engineer