What is a Function Declaration?

According to the MDN docs, "the function declaration (function statement) defines a function with the specified parameters".

  function addNums(a, b) {
    return a + b;
  }

  console.log(addNums(5, 6));  //11

What is a Function Expression?

According to the MDN docs, "the function keyword can be used to define a function inside an expression".

  const addNums = function (a, b) {
    return a + b;
  };

  console.log(addNums(5, 6)); //11

Difference - Hoisting

The biggest difference between function declaration and function expression is hoisting.

• Function Declaration - Hoisted (Hoisted to the top of the enclosing function or global scope)
• Function Expression - Not hoisted

  //Function Declaration
  hoisted(); //"foo"

  function hoisted() {
    console.log('foo');
  }

  //Function Expression
  notHoisted(); // TypeError: notHoisted is not a function

  const notHoisted = function () {
    console.log('bar');
  };

Reference

MDN Function declaration

MDN Function expression

When to use a function declaration vs. a function expression

When to Use a Function Expression vs. Function Declaration