Var, Let and Const in javascript

We can now better define our variables depending on our needs with the introduction of let and const in javascript after ES6.

Before exploring the let and const let us first see why there was need for it.

var in javascript

var are function scoped, which means they are still accessible outside the block scope even though we have declared them inside it.

//for loop is block scoped
for (var i = 0; i < 10; i++) {
  var iAmInside = "I am available outside of the loop";
}

console.log(iAmInside);
// I am available outside of the loop

//block scope
if(true){
  var inside = "Inside";
}
console.log(inside);
// Inside

//Function scoped
function myFunc(){
  var functionScoped = "I am available inside this function";
  console.log(functionScoped);
}
myFunc();
// I am available inside this function
console.log(functionScoped);
// ReferenceError: functionScoped is not defined

Explanation

In the first and second example, the value of the var leaked out of the block-scope and could be accessed from outside of it, whereas in the third example var was confined inside a function-scope and we could not access it from outside.

This happens because of Hoisting.

var are treated as if they are at the top of the function (or global scope) regardless of where the actual declaration occurs, this is called hoisting. For a demonstration see the following example.

var inside;  // hoisted on the top of the function. As there is no function so it is present in the global scope.
//block scope
if(true){
  var inside = "Inside";
}
console.log(inside);
//Inside

//Function scoped In this case value is hoisted inside the function
function getValue(condition) {
  if (condition) {
      var value = "blue";
      return value;
  } else {
      // value exists here with a value of undefined
      return value;
  }

  // value exists here with a value of undefined
}

console.log(getValue(true));   // blue
console.log(getValue(false));  // undefined


//While execution it is hoisted like this internally
function getValue(condition) {
  var value;  //value is hoisted as there is no value attached, so it is undefined.
  if (condition) {
      var value = "blue";
      return value;
  } else {
      // value exists here with a value of undefined
      return value;
  }

  // value exists here with a value of undefined
}
console.log(getValue(true));   // blue
console.log(getValue(false));  // undefined

Block-Level Declarations in javascript

Block or lexical scopes are the boundaries in which declared variables are not accessible outside it. This means the variables declared inside it are available inside given block and its sub-blocks.

Many c-based languages work with block scoping and with its introduction in ES6 will bring the same flexibility to the Javascript.

Block are indicated by { and } characters.

let in javascript

let are declared same as var but it limits the variable scope to the given block. That is why we should declare let at the top of the block so that is accessible throughout the block and its sub-blocks.

Example

function getValue(condition) {
  if (condition) {
      let value = "blue";   // accessible inside the given scope only
      return value;
  } else {
      // value doesnot exits here
      return value;
  }

  // value doesnot exits here
}
console.log(getValue(true));   // blue
console.log(getValue(false));  // ReferenceError: value is not defined

let x = 10;
if(x == 10){
  let x = 11;
  console.log(x);   //11  value of x inside if block
}
console.log(x); //10 value of x

No Redeclaration's

We cannot redeclare a given variable with the same name again in the given block. Doing so will result is an error.

var c = 0;
let c = 0;  //SyntaxError: Identifier 'c' has already been declared

var c = 10;
if(c == 10){
  let c = 11;
  console.log(c);  //11 This will work as it is declared in another scope
}

const in javascript

Like let const is also block scoped. But it differs from the fact that their variable cannot be redeclared or change by re-assigning the value. The value remains Constant.

const abc = "XYZ";
let abc; //SyntaxError: Identifier 'abc' has already been declared
abc = "pqr"; //TypeError: Assignment to constant variable.

For this reason the const variable should be initialised while declaring.

//Should be initialised while declaring
const XYZ; //SyntaxError: Missing initializer in a const declaration 

Just like let, const is also blocked scoped.

if(true){
  const a = "I am inside";
  console.log(a);  // I am inside
}
console.log(a); //ReferenceError: a is not defined

However, the value a constant holds may be modified if it is an object.

const person = {
  name: 'Prashant',
  age: 25,
}

person.age = 26;
console.log(person.age); // 26

const declarations for objects do not prevent modification of those objects. A const declaration prevents modification of the binding and not of the value of the binding.

But this will result in the error.

const person = {
  name: 'Prashant',
  age: 25,
}

person = 26;
console.log(person); //  TypeError: Assignment to constant variable.

The Temporal Dead Zone (TDZ)

According to MDN:

In ECMAScript 2015, let bindings are not subject to Variable Hoisting, which means that let declarations do not move to the top of the current execution context. Referencing the variable in the block before the initialization results in a ReferenceError (contrary to a variable declared with var, which will just have the undefined value). The variable is in a “temporal dead zone” from the start of the block until the initialization is processed.

function do_something() {
  console.log(bar); // undefined
  console.log(foo); // ReferenceError: foo is not defined
  var bar = 1;
  let foo = 2;
}
do_something();

The Temporal Dead Zone and typeof

Using the typeof operator to check the type of a variable in it's tdz will throw a ReferenceError. But for the undeclared variables it will throw undefined.

console.log(typeof iAmNotDeclared) // undefined

console.log(typeof iAmDeclared)// ReferenceError: iAmDeclared is not defined
//Above this is it's temporal dead zone
let iAmDeclared = 10;

Temporal Dead Zone with lexical or block scoping

function test(){
   var foo = 33;
   if (true) {
      let foo = (foo + 55); // ReferenceError: foo is not defined
   }
}
test();

Due to lexical or block scoping let foo = (foo + 55) access the foo of the current block that is inside the if condition. It does not access the var foo = 33; as let is blocked scope. let foo is declared but it is not initialized that is why it is still in temporal dead zone.

When to use Var, Let and Const

There is no as such rule stating where to use each of them, Everyone has different opinions.
But according to the properties of these three, it should be used as follows.

• Use var for top-level variables that are shared across many (especially larger) scopes.
• let can used for localized variables in smaller scopes.
• If there is no need to re-assign any variable. like const PI = 3.14; then use const.

Do your own research and figure out which one to use where.