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let in TypeScript

TypeScript (and ES6) introduces the let keyword to allow you to define variables with true block scope. That is if you use let instead of var you get a true unique element disconnected from what you might have defined outside the scope.

Following example code example demonstrates use of “let” which is truly block scoped.

Function getUserNameWithLet() returns username "nmk" which is block scoped.

Typescript Code:

function getUserNameWithVar()
{
    var userName = "ken";
    if (true) {
       var userName = "kendre";
    }
    return userName;
}
 
function getUserNameWithLet() {
    let userName = "nmk";
    if (true) {
        let userName = "kendre";
    }
    return userName;
}
 
window.onload = () => {
    var elNameWithVar = document.getElementById('lblFirstNameWithVar');
    var elNameWithLet = document.getElementById('lblFirstNameWithVar');
    elNameWithVar.innerHTML =getUserNameWithVar();
     elNameWithLet.innerHTML =getUserNameWithLet();
};

HTML code:

<html lang="en">
<head>
    <meta charset="utf-8" />
    <title>TypeScript HTML App</title>
    <link rel="stylesheet" href="app.css" type="text/css" />
    <script src="LetExample.js"></script>
</head>
<body>
    <h1>TypeScript HTML App</h1>
 
<label id="lblFirstNameWithVar"></label>
<label id="lblFirstNameWithLet"></label>
</body>