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JavaScript Const

Cannot be Reassigned

A const variable cannot be reassigned:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript const</h2>

<p id=”demo”></p>

<script>

try {

  const PI = 3.141592653589793;

  PI = 3.14;

}

catch (err) {

 document.getElementById(“demo”).innerHTML = err;

}

</script>

</body>

</html>

Output

JavaScript const

Must be Assigned

JavaScript const variables should be assigned a value when they are declared:

Correct

const PI = 3.14159265359;

Incorrect

const PI;

PI = 3.14159265359;

Constant Objects and Arrays

The keyword const is a little misleading.

A constant value is not defined. It specifies a constant reference to a value.

Because of this you can NOT:

  • Reassigning a constant value.
  • Reassigning a constant array.
  • Reassigning a constant object.

But you CAN:

  • Changing the elements of constant array.
  • Changing the properties of constant object.
  • Constant Arrays.

Constant Arrays

The elements of a constant array can be changed:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript const</h2>

<p>Declaring a constant array does NOT make the elements unchangeable:</p>

<p id=”demo”></p>

<script>

// Create an Array:

const cars = [“Saab”, “Volvo”, “BMW”];

// Change an element:

cars[0] = “Toyota”;

// Add an element:

cars.push(“Audi”);

// Display the Array:

document.getElementById(“demo”).innerHTML = cars;

</script>

</body>

</html>

Output

JavaScript const

Declaring a constant array does NOT make the elements unchangeable:

Toyota,Volvo,BMW,Audi

But the array cannot be reassigned:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript const</h2>

<p>You can NOT reassign a constant array:</p>

<p id=”demo”></p>

<script>

try {

  const cars = [“Saab”, “Volvo”, “BMW”];

  cars = [“Toyota”, “Volvo”, “Audi”];

}

catch (err) {

document.getElementById(“demo”).innerHTML = err;

}

</script>

</body>

</html>

Output

JavaScript const

You can NOT reassign a constant array:

Constant Objects

The properties of a constant object can be changed:

Example

<!DOCTYPE html>

<html>

<body>

 

<h2>JavaScript const</h2>

 

<p>Declaring a constant object does NOT make the objects properties unchangeable:</p>

<p id=”demo”></p>

<script>

// Create an object:

const car = {type:”Fiat”, model:”500″, color:”white”};

// Change a property:

car.color = “red”;

// Add a property:

car.owner = “Johnson”;

// Display the property:

document.getElementById(“demo”).innerHTML = “Car owner is ” + car.owner;

</script>

</body>

</html>

Output

JavaScript const

Declaring a constant object does NOT make the objects properties unchangeable:

Car owner is Johnson

The object cannot be reassigned :

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript const</h2>

<p>You can NOT reassign a constant object:</p>

<p id=”demo”></p>

<script>

try {

  const car = {type:”Fiat”, model:”500″, color:”white”};

car = {type:”Volvo”, model:”EX60″, color:”red”};

}

catch (err) {

 document.getElementById(“demo”).innerHTML = err;

}

</script>

</body>

</html>

Output

JavaScript const

You can NOT reassign a constant object:

Block Scope

Declaring a variable with const is similar to let when used in Block Scope.

The x declared in the block, in this example, is not the same as the x declared outside the block:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScropt const variables has block scope</h2>

<p id=”demo”></p>

<script>

const  x = 10;

// Here x is 10

const x = 2;

// Here x is 2

}

// Here x is 10

document.getElementById(“demo”).innerHTML = “x is ” + x;

</script>

</body>

</html>

Output

JavaScropt const variables has block scope

x is 10

Redeclaring

Redeclaring a JavaScript var variable can be done anywhere in a program:

Example

var x = 2;     // Allowed

var x = 3;     // Allowed

x = 4;         // Allowed

Redeclaring an existing var or let variable to const, in the same scope, is not allowed:

Example

var x = 2;     // Allowed

const x = 2;   // Not allowed

{

let x = 2;     // Allowed

const x = 2;   // Not allowed

}

{

const x = 2;   // Allowed

const x = 2;   // Not allowed

}

Reassigning an existing const variable, in the same scope, is not allowed:

Example

const x = 2;     // Allowed

x = 2;           // Not allowed

var x = 2;       // Not allowed

let x = 2;       // Not allowed

const x = 2;     // Not allowed

{

  const x = 2;   // Allowed

  x = 2;         // Not allowed

  var x = 2;     // Not allowed

  let x = 2;     // Not allowed

  const x = 2;   // Not allowed

}

Redeclaring a variable with const, in another scope, or in another block, is allowed:

Example

const x = 2;       // Allowed

{

  const x = 3;   // Allowed

}

{

const x = 4;   // Allowed

}

Const Hoisting

Variables specified with var are hoisted to the top and can be initialized at any time.

Meaning: Before declaring the variable, it can be used:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript Hoisting</h2>

<p>With <b>var</b>, you can use a variable before it is declared:</p>

<p id=”demo”></p>

<script>

carName = “Volvo”;

document.getElementById(“demo”).innerHTML = carName;

var carName;

</script>

</body>

</html>

Output

JavaScript Hoisting

With var, you can use a variable before it is declared:

Volvo

Variables defined with const are also hoisted to the top, but not initialized.

Meaning: Using a const variable before it is declared will result in a ReferenceError:

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript Hoisting</h2>

<p>With <b>const</b>, you cannot use a variable before it is declared:</p>

<p id=”demo”></p>

<script>

try {

  alert(carName);

  const carName = “Volvo”;

}

catch (err) {

 document.getElementById(“demo”).innerHTML = err;

}

</script>

</body>

</html>

Output

JavaScript Hoisting

With const, you cannot use a variable before it is declared:

ReferenceError: Cannot access 'carName' before initialization


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