Higher order functions in Javascript

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.

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

Example:

Strings as arguments:

      sayHi = (name) => `Hi, ${name}!`;
      result = sayHi('Khalil');
      console.log(result); // 'Hi, Khalil!

Numbers as arguments:

      double = (x) => x * 2;
      result = double(4);
      console.log(result); // 8

Booleans as arguments:

      getClearance = (allowed) => allowed ?
      'Access granted' : 'Access denied';
      result1 = getClearance(true);
      result2 = getClearance(false);
      console.log(result1); // 'Access granted'
      console.log(result2); // 'Access denied'

Objects as arguments:

      getFirstName = (obj)  => obj.firstName;
      result = getFirstName({
        firstName: 'Khalil'
      });
      console.log(result); // 'Khalil'

Arrays as arguments:

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

Functions as Arguments:

     isEven = (num) => num % 2 === 0;
     result = [1, 2, 3, 4].filter(isEven);
     console.log(result); // [2, 4]

Returning Functions::

    add = (x) => (y) => x + y;
    result = add(10)(20);
    console.log(result); // 30