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The difference between let, var and const...Javascript

The difference between let, var and const...Javascript

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While developing with Javascript and going through pre-written code, you often come across let, var and const all used while declaring variables in Javascript.

The three are Javascript keywords that precede a variable name/identifier. These keywords are used to describe a variable's use, scope, and the ability to be re-declared or updated. Using either of the identifiers tells the compiler on how to handle the variable.

To differentiate the three keywords we will use three common aspects:

Scope

The scope of a variable defines the areas within the program from which the variable can be accessed.

Let and const have a block scope in that their use and accessibility are restrained to the block wherein the variable was declared.

A block is any part of the code enclosed within curly braces. ({}).

Var on the other hand is either on a global scope or a functional scope. If the variable is declared within a function, the variable is only accessible within the function. On the other hand, when defined outside a function the variable is visible and accessible anywhere throughout the program.

Ability to be updated

Any variable declared using the keywords let and var can be updated.

var name="faith"; 
name="ian"; 
console.log(name);
//ian
let name="faith"; 
name="ian"; 
console.log(name);
//ian

Values for variables declared using the const keyword cannot be updated.

const pi=3.14; 
pi=3.142; 
//error:Assignment to constant variable

However, for objects declared using the const keyword, the object elements value can be updated.

const person = { name: "Ali", age:20}; 
person.age=25;

Ability to be re-declared.

Variables declared using let and const cannot be re-declared. i.e:

let name="faith"; 
let name="ian"; 
//error: Identifier 'name' has already been declared
const pi=3.14; 
const pi=3.142; 
//error: Identifier 'pi' has already been declared

Variables declared using var keyword can be re-declared. i.e:

var name="faith"; 
var name="faith"; //acceptable

This is hazardous as it can lead to bugs in a program when the programmer re-declares the variable unknowingly. It is therefore safer to use let or const.

Below is a table that summarizes the differences between let, var, and const.

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