Difference between var and let in TypeScript


Their are some java script errors like consider the following example.

class difference {
    getVar(): void {
        var a = 1010;
        if (true) {
            var a = 2020;
        alert("You are in var function: "+ a); // 2020

var obj = new difference();

In above example you will get result as 2020. Because { } this block does not create a new variable scope in if condition. The variable is remains same as it is.

Your output will be

let keyword :

The let keyword is actually a newer JavaScript construct that TypeScript makes available. TypeScript (and ES6) introduces the let keyword.

What is ES6?

  • ECMAScript = ES:
  • ECMAScript (or ES) is a trademarked scripting-language specification standardized by Ecma International in ECMA-262 and ISO/IEC 16262.
  • It was created to standardize JavaScript, so as to foster multiple independent implementations. JavaScript has remained the best-known implementation of ECMAScript
  • Languages like Javascript are based on the ECMAScript standard.
  • ECMA Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft).
  • ECMA means European Computer Manufacturer’s Association

For more info https://en.wikipedia.org/wiki/ECMAScript

Consider below let block.

class difference {

getLet(): void {
        let b = 1010;
   //let b = 2525;    // you can not re-declare b in same scope
        if (true) {
            let b = 2020;
            alert("You are in if condition: " + b); // 1010
        alert("You are in let function: " + b); // 1010

After compilation of code into java script as bellow

var difference = (function () {
    function difference() {
    difference.prototype.getLet = function () {
        var b = 1010;
        //let b = 2525;    // you can not re-declare b in same scope
        if (true) {
            var b_1 = 2020;
            alert("You are in if condition: " + b_1); // 1010
        alert("You are in let function: " + b); // 1010
    return difference;
var objDiff = new difference();

In this above case for each let keyword will has a unique elements depends upon a condition i.e if block is true then it will create unique element & in alert you will get result as.

Summary :
In this article you learn difference between var and let in TypeScript


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