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What are variables in TypeScript?

Gutha Vamsi Krishna

A variable is a named location in memory to store data. In TypeScript, an extension of Javascript, there are three different keywords to define variables: var, let, and const.

Visual representation of variable in code

var

var a = 11;
  • The var keyword is used to define a variable and has scope outside the block in which it is used. This is known as var scoping or function scoping.
  • Once we define var, we can access it anywhere in the program.
  • var can be redeclared many times, but this may lead to bugs.
//var redeclaration example

function f(x:any) {

  //declaring first time
  var x:any = 1;
  console.log(x);

  //redeclaring second time
  var x:any = 2;
  console.log(x)
  if (true) {

    //redeclaring third time
    var x:any = 3;
    console.log(x)

  }
}

//calling the function
f(0)

let

let a = 11;
  • The let keyword is used to define a variable whose scope is limited to the block it is defined in.
  • If you run the code snippet below, you will get the error Cannot find name x because we define and declare x in the if block with the let keyword, so we cannot access it from outside the if block.
function f(isLoggedIn: boolean) {
  //defining variable x with let keyword in if block
  if (isLoggedIn) {
    let x = 10;
  }
  return x;
}

//calling the function
f(true)
  • To resolve the error in the code above, we need to define the variable x outside of the if block and inside the function with the let keyword so it has function scope.
  • Check the code snippet below for the modified code.
function f(isLoggedIn: boolean) {
  //defining variable x with let keyword in function block
  let x;
  if (isLoggedIn) {
    x = 10;
  }
  return x;
}

//calling the function
console.log(f(true))

Unlike var, variables that are declared with let cannot be redeclared, as shown in the code below.

function f(){
  let x = 5;

  //redeclaring x
  let x = 10;
  return x;
}

f()

const

const pi = 3.14
  • const is a keyword that is used to declare variables where the value of the variable is restricted to change.
  • const is useful when you want to define some constant values and you don’t want other programmers to change that value in the rest of the program.
const  pi = 3.14;

// changing const value throws error
pi = 1;

Rules

  • Variable names can contain alphabets and digits.
  • Variable names cannot start with a digit.
  • Allowed special characters are _ and .

Below are the example code snippets for variable rules.

//variable can contain alphabets and digits
var abc123 = "Example"
console.log(abc123)
//variable names cannot start with a digit
//It will throw error
var 4abc = "Example";
console.log(4abc);
//Allowed special character `_` and `$`.

var a_b = "Example";
console.log(a_b)

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