firstClassVsHigherOrderFunction.md

[JavaScript] - First Class vs Higher Order Functions

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First Class Functions

• JS treats function as first class citizens.

• This means that functions are simply values.

• Functions are just another "type" of object.

  • Store functions in variables or properties:

  const add = (a, b) => a + b;
  
  const counter = {
    value: 23;
    inc: function() { this.value++; }
  }
  

  • Pass functions as arguments to OTHER functions:

  const greet = () => console.log('Hello Jonas');
  btnClose.addEventListener('click', greet);
  

  • Return functions FROM functions.
  • Call methods on function:

  counter.inc.bind(someOtherObject);
  

Higher Order Functions

• A function that receives another function as an argument, that returns a new function, or both.
• This is only possible because of first class function.
  • Function that receives another function:

  const greet = () => console.log('Hello Jonas');
  btnClose.addEventListener('click', greet);
  

  • Function that returns a new function

  function count() {
    let counter = 0;
    return function() {
      counter++;
    };
  }