var vs let vs const

ES2015 (or ES6) introduced two new ways to create variables, let and const.

var

1. var is function scoped
2. This will give undefined when accessing a variable before it's declared
3. it creates a global property on window with the same name.
4. is reassignable.
5. is redeclarable.

Ex: var <<variable_name>>;

Example:
--------

var city = "Coral Springs";

console.log(window.city);

Example2:
--------

var city = "Florence";

function bubble() {
  var city = "Siena";
  console.log(city);
}

bubble(); // "Siena"

console.log(city); // "Florence"

let

1. let is block scoped.
2. ReferenceError when accessing a variable before it's declared
3. it does not create any global property on window.
4. is reassignable.
5. is not redeclarable.

Ex: let <<variable_name>>;

What that means is that a variable created with the let keyword is available inside the “block” that it was created in as well as any nested blocks. When I say “block”, I mean anything surrounded by a curly brace like in a for loop or an if statement.

const

const is almost exactly the same as let. However, the only difference is that once you’ve assigned a value to a variable using const, you can’t reassign it to a new value.

1. block scoped
2. ReferenceError when accessing a variable before it's declared
3. it does not create any global property on window.
4. is not reassignable.
5. is not redeclarable.

Ex: const handle = 'test';

tip

The take away above is that variables declared with let can be re-assigned, but variables declared with const can’t be.