TypeScript Variable Declarations

TypeScript has introduced two new types of variable declarations let and const. Here let is similar to var in some respects and const is an augmentation of let and prevents re-assignment to a variable.

Here you will learn both types of variable declarations and why they're preferable over to var.

var Declarations

This is similar to JavaScript variable declarations using var keyword. For Example:

var a = 210;
var message = "Some string;

var Scoping Rules

The var declarations are accessible anywhere within their containing function, module, namespace, or global scope.

function Display(setValue: boolean) {
    if (setValue) {
        var counter = 10;
    }
    return counter; //Accessible outside if block
}
Display(true);  // returns '10'
Display(false); // returns 'undefined'

Here, variable counter was declared within the if block, but still accessible outside the if block.

let Declarations

The let variable declarations are similar to var variable declarations. But still has some semantics key difference from var. For Example:

let a = 210;
let message = "Some string;

let Scoping Rules

The let variables are block-scoped variables and are not visible outside of their nearest containing block or for-loop.

function Display(setValue: boolean) {
	let x = 10;
    if (setValue) {
        let counter = 10;
    }
    return counter; //Not Accessible, Error : counter not exist here
}

Here, variable counter was declared within the if block and not accessible outside the if block. But x declared in function block and is accessible anywhere within function block.

const Declarations

The const variable declarations are another way of variable declaration and similar to let but, const value can not be changed. For Example:

const x = 9;
const Customer = {
    fname: "Jimi",
    lname: "Scott",
}

const Scoping Rules

The const follow the same scoping rules as let, but you can't re-assign to them.

const x = 9;
const Customer = {
    fname: "Jimi",
    lname: "Scott",
}

// Error : can not change
Customer = {
    fname: "John",
    lname: "Wood"
};

Here, variable counter was declared within the if block and not accessible outside the if block. But x declared in function block and is accessible anywhere within function block.

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