JavaScript variable declarations

In computer programming, to declare a variable simply means to initialize it with a name and, optionally, a value.

The keywords const and let

We use the keywords const and let to declare what kind of statement that we wish to make:

• const is a variable but up until the point we declare it
  • we can change its value at its line of declaration
    • but we cannot re-assign it at any other point in the program
• let is a variable in its truest form
  • we can change its value after its declaration
    • at any other point in the program

// we can change "100" here...
const x = 100 

// ...but we cannot change it here
x = 200 

// we can change "100" here...
let y = 100 

// ...and we can change it here because of let
y = 200

The var keyword

There also exists another keyword var which is similar to let but:

• let has block scope
  • i.e. it exists only within the confines of its closest curly braces
• var has function scope and global scope
  • if declared inside a function, it exists only within the confines of its parent function
  • if declared outside a function, it will have global scope
  • the same variable can also be re-declared
  • has no block scoping

function doSomething() {
// function scope for var

  const x = 10

  if (x > 0) {
  // block scope for let
    let z = 100
    z = 200    // we can change it here
    var t = 1
  }

  // "1" (this call exists inside the function scope)
  console.log(t) 
  
  // undefined (this call exists outside the block scope)
  console.log(z) 

}

The var keyword has largely phased out, in favour of both let and const but it still remains important to know when working with old JavaScript code!

Reserved keywords

When we declare variables, we cannot use some reserved keywords as variable names, e.g.:

let let = 1

...won't work because the language won't let us use the second let as a variable name! This tries to make our code less confusing!

For a full list of reserved keywords, we have this w3schools page!