See the example code below :
function Test1() {
// 1) Declare using var
if (true) {
var x = 10;
}
console.log(x); // 10
// 2) Declare using let
if (true) {
let x = 5;
console.log(x); // 5
}
console.log(x); // 10
// 3) Declare using const
if (true) {
const x = 8;
console.log(x); // 8
}
console.log(x); // 10
}
1. Declare variable using var
If you declare variable in JavaScript using var keyword, variable is accessible through out
the function even thought you declared it inside any loop (e.g if, while...). See the above
example where x is declared using var keyword inside if loop still it is accessible
through out the function scope. If you declare variable without giving any keyword then it becomes global variable even if you declare it inside the function (throw ReferenceError in strict mode).
the function even thought you declared it inside any loop (e.g if, while...). See the above
example where x is declared using var keyword inside if loop still it is accessible
through out the function scope. If you declare variable without giving any keyword then it becomes global variable even if you declare it inside the function (throw ReferenceError in strict mode).
2. Declare variable using let
let keyword allows you to declare variable withing the limited scope of the block,
statement or expression. Variables declared using let have their scope to the block
in which they are defined, as well as in any contained sub-blocks. Unlike var, let does not create a property on the global object. E.g.
statement or expression. Variables declared using let have their scope to the block
in which they are defined, as well as in any contained sub-blocks. Unlike var, let does not create a property on the global object. E.g.
var x = 'global';
let y = 'global';
console.log(this.x); // "global"
console.log(this.y); // undefined
You can not re-declare same variable in same scope using let. E.g
switch (x) {
case 0:
let x;
break;
case 1:
let x; // SyntaxError for redeclaration.
break;
}
See the second example in first code variable scope is limited to block even if you change
the value it won’t reflect outside the scope.
the value it won’t reflect outside the scope.
1. Declare variable using const
Const keyword is block scoped much like let. It creates a constant that can be either global
or local to the function in which it is declared. Constant initialization is required. You must
specify its value while declaring the constant. Constants once created never change their
value and can’t be re-declared. The const declaration creates a read-only reference to a
value but not all values are immutable. If you create object you can alter the values of
object but can not reassign. E.g.
const A = 10;
// A = 12 // throw error;
const myObj = { "Name": "AP" };
// myObj = { "City": "AHD" }; // throw error;
// Object.freeze(myObj);
myObj.Name = "Ajay" // works fine if object is not freezed 
