JavaScript Var, Let, and Const

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To declare a variable in JavaScript either var, let or const is used.

Let's see the differences between the three below.

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var and let

Block Scope

A Block scope contains a group of code within curly braces {}.

A variable created with the let keyword in a block scope is only available within it.

let greeting = "Hi John!!!"; // Global Variable

if (true) {
    let greeting = "Hello Bello!!!"; // Local Variable
    console.log(greeting);// Hello Bello!!!
}
console.log(greeting) // "Hi John!!!"

let creates either a global or a local variable. If it is out of scope, it's globally scoped else it is locally scoped if in scope.

While;

var always creates global variables.

if (true) {
    var greeting = "Hello Bello!!!";
}
console.log(greeting) // "Hello Bello!!!"

if (true) {
    let hello = "Hello Bello!!!";
}
console.log(hello) // ReferenceError: hello is not defined

Update and redeclaration

Variables declared with either var or let can get updated at any time in a program.

var name = 'Mary';
name = 'Nadia';
console.log(name); // Nadia

let myName = 'Bob';
myName = 'Richard';
console.log(myName); // Richard

Both var and let can be updated as shown above, but only var can be redeclared.

var firstName = 'John';
var firstName = 'Osagie';
console.log(firstName); // John

let lastName = 'Bello';
let lastName = 'Bob';
console.log(firstName); 
// SyntaxError: Identifier 'lastName' has already been declared

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Hoisting

Hoisting is JavaScript's default behavior of moving declarations to the top.

A variable may get to be declared after it has been used. It's only peculiar to the var keyword.

console.log('My name is ' + name); // My name is Michael

var name = 'Michael';

The below example shows how the JavaScript engine will interpret the above code when var keyword is used for declaration.

var name = 'Michael';
console.log('My name is ' + name); // My name is Michael

JavaScript in strict mode ('use strict') does not allow a variable to be used if not declared first.

See the example below:

'use strict';
console.log(name);

var name = 'Jerry'; // no output, no error

When let is used, it is impossible to use a variable before it has been declared.

console.log('My name is ' + name); // ReferenceError: Cannot access 'name' before initialization

let name = 'Michael';

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const

Block scope

const has the same feature as let because it also maintains its scope.

const greeting = "Hi John!!!"; // Global Variable

if (true) {
    const greeting = "Hello Bello!!!"; // Local Variable
    console.log(greeting);// Hello Bello!!!
}
console.log(greeting) // "Hi John!!!"

Update and redeclaration

The const keyword is also used to create a variable but can not be updated unlike let and var.

const birthday = '01/20/2020';
birthday = '01/19/2020';
console.log(birthday); // TypeError: Assignment to constant variable.

It is, of course, impossible to update someone's birthday. So use const only when a value will not be updated or changed.

const birthday = '01/20/2020';
console.log(birthday); // 01/20/2020

Since it can not be updated, it can not be redeclared.

const birthday = '01/20/2020';
const birthday = '01/10/2020';
console.log(birthday); // SyntaxError: Identifier 'birthday' has already been declared

Undefined

A const variable must be initialized to a value. If a constant variable is undefined, it leads to an error.

const name; // undefined variable
name = 'Jack';
console.log(name); // SyntaxError: Missing initializer in const declaration

If you plan to undefined a constant variable, use the value undefined.

const name = undefined;

function myName() {
  if (!name) {
  return 'Jack';
}

return name;
}
console.log( myName() ); // Jack

Hosting

const has another similarity with let in terms of hoisting a variable. That is it also doesn't support hoisting.

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Conclusion

• It is advisable to use let and not var has it is the modern way to create a variable in JavaScript.

• Use const only when a value is constant (immutable variable).

• It is advisable to always declare all variables at the beginning of every scope with let when necessary to avoid bugs (errors).

Happy Coding!!!

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Comments

[Sana Michael Yves]

Hi, i don't understand why hello variable is not defined.
We know it has been declared like greeting variable.

[Bello Osagie]

Because when let is used to create a variable within a block scope ({ ... }), the variable is only available in it. This type of variables are called local variables; while when var is used to create a variable within a block scope ({ ... }), the variable is available everywhere in the program (within or outside the scope). This type of variables are called global variables