JavaScript Variables - Var, Let And Const

Hey Guys! In this blog post I am going to explain in detail about javascript variables,

Javascript variables are containers that holding pieces of data.

There are three keywords used when declaring the variable in Javascript, namely,var let and.const They follow this pattern or syntax.

var variableName = variableValue;

1	var variableName = variableValue;

Javascript variables are dynamic typing means

that they can change from one data type to another. Below, the variable technology change from string to number and then boolean.

var technology = 'javascript'; // javascript
  javascript = 100; // 100
  javascript = false; // false

var technology = 'javascript'; // javascript

javascript = 100; // 100

javascript = false; // false

Three ways of declaring variables

=> var

This was the only way to declare variable before ES6. Here you can declare the same variables more than one time and can be updated.

var technology = 'javascript';
  var technology = 'jquery';
  console.log(technology);

var technology = 'javascript';

var technology = 'jquery';

console.log(technology);

If you declare variable inside the block statement, the variable will leak outside.

  var bodyWeight = 50;
  if (bodyWeight &gt; 49) {
    var water = 1.4;
    console.log(`For body weight of ${bodyWeight}kg, you should drink water atleast ${water}litre`);
  }
  console.log(water); // 1.4

var bodyWeight = 50;

if (bodyWeight > 49) {

var water = 1.4;

console.log(`For body weight of ${bodyWeight}kg, you should drink water atleast ${water}litre`);

}

console.log(water); // 1.4

=> let and const

let and const are the new ways for declaring variables introduced in ES6. In let and const you cannot declare the variable twice.

let technology = 'javascript';
  let technology = 'jquery'; // Uncaught SyntaxError: Identifier 'jquery' has already been declared

1 2	let technology = 'javascript'; let technology = 'jquery'; // Uncaught SyntaxError: Identifier 'jquery' has already been declared

In most case let and const are almost same, the only difference I known, const cannot be updated but let can.

// let can be updated
  let technology = 'javascript';
  technology = 'jquery';
  console.log(technology); // jquery

  // const cannot be updated
  const othertechnology = 'node';
  othertechnology = 'java'; // Uncaught TypeError: Assignment to constant variable.

// let can be updated

let technology = 'javascript';

technology = 'jquery';

console.log(technology); // jquery

// const cannot be updated

const othertechnology = 'node';

othertechnology = 'java'; // Uncaught TypeError: Assignment to constant variable.

The variable is not leaked outside of the block statement if you use let or const.

const bodyWeight = 50;
  if (bodyWeight &gt; 49) {
    const water = 1.4;
    console.log(`For body weight of ${bodyWeight}kg, you should drink water atleast ${water}litre`);
  }
  console.log(water); // Uncaught ReferenceError: water is not defined

const bodyWeight = 50;

if (bodyWeight > 49) {

const water = 1.4;

console.log(`For body weight of ${bodyWeight}kg, you should drink water atleast ${water}litre`);

}

console.log(water); // Uncaught ReferenceError: water is not defined

When to use var, let and const

Always use const when declaring variable, use only let when you want update the variable. var shouldn’t be used in ES6 and above.

I hope you will find this blog useful. Please comments to make it even more better.

JavaScript Variables – Var, Let And Const

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