Variables

Variables are containers that store values. You start by declaring a variable with the let keyword, followed by the name you give to the variable.

let

The let declaration declares re-assignable, block-scoped local variables, optionally initializing each to a value.

let myVariable = "Lite";

// change vars without a declaration
myVariable = 'Rank';

// slight shortcut for multiple vars
let a = 'hello', b = 'world'

const

The const declaration declares block-scoped local constant variables. The value of a constant can't be changed through reassignment, but if a constant is an object, its properties can be added, updated, or removed.

const number = 42;

// [Error]: Assignment to constant variable.
number = 99;

Scope

The scope is the current context of execution in which values and expressions are "visible" or can be referenced. If a variable or expression is not in the current scope, it will not be available for use.

JavaScript has the following kinds of scopes:

• Global scope: The default scope for all code running in script mode.
• Module scope: The scope for code running in module mode.
• Function scope: The scope created with a function.

In addition, variables declared with let or const can belong to an additional scope:

• Block scope: The scope created with a pair of curly braces (a block).

// Variables in  Global scope are accessible everywhere
let g = 'global';

function updateVars() {
    g = 'useVars';
}
updateVars();
console.log(g); // useVars

// A new local function scope is established in functions
// Local variables are only accessible inside their scope
function f1() {
  let v1 = 'apple';
  {
    // A new block scope is established in curly braces
    let v1 = 'banana';
    let v2 = 'cherry';
    console.log(v1); // banana
  }
  // Block scope ends here, come back to function scope
  console.log(v1); // apple

  // [Error]: v2 is not defined
  console.log(v2);
}

Code Challenge

Try to swap two variables' values without using a third variable.