Flash cards
Review the key moves
What is the main idea behind JavaScript Const?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ PI = 3.141592653589793;Put the learning moves in the order that makes the concept easiest to apply.
The const keyword was introduced in ES6 (2015)
Variables defined with const cannot be Redeclared
Variables defined with const cannot be Reassigned
Variables defined with const have Block Scope
Cannot be Reassigned
A variable defined with the const keyword cannot be reassigned:
Example
const PI = 3.141592653589793;
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an errorMust be Assigned
JavaScript const variables must be assigned a value when they are declared:
Correct
const PI = 3.14159265359;Incorrect
const PI;
PI = 3.14159265359;When to use JavaScript const?
Always declare a variable with const when you know that the value should not be changed.
Use const when you declare
- A new Array
- A new Object
- A new Function
- A new RegExp
Constant Objects and Arrays
The keyword const is a little misleading.
It does not define a constant value. It defines a constant reference to a value.
Because of this you can NOT
- Reassign a constant value
- Reassign a constant array
- Reassign a constant object
- Change the elements of constant array
- Change the properties of constant object
Constant Arrays
You can change the elements of a constant array:
Example
// You can create a constant array: const cars = ["Saab", "Volvo", "BMW"]; // You can change an element: cars[0] = "Toyota"; // You can add an element: cars.push("Audi");But you can NOT reassign the array:
Example
const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; // ERRORConstant Objects
You can change the properties of a constant object:
Example
// You can create a const object: const car = {type:"Fiat", model:"500", color:"white"}; // You can change a property: car.color = "red"; // You can add a property: car.owner = "Johnson";But you can NOT reassign the object:
Example
const car = {type:"Fiat", model:"500", color:"white"};
car = {type:"Volvo", model:"EX60", color:"red"} // ERRORDifference Between var, let and const
| Scope | Redeclare | Reassign | Hoisted | Binds this | |
|---|---|---|---|---|---|
| var | No | Yes | Yes | Yes | Yes |
| let | Yes | No | Yes | No | No |
| const | Yes | No | No | No | No |
What is Good?
let and const have block scope .
let and const can not be redeclared .
let and const must be declared before use.
let and const does not bind to this .
let and const are not hoisted .
What is Not Good?
var does not have to be declared.
var is hoisted.
var binds to this.
Browser Support
The let and const keywords are not supported in Internet Explorer 11 or earlier.
The following table defines the first browser versions with full support:
| Chrome 49 | Edge 12 | Firefox 36 | Safari 11 | Opera 36 |
|---|---|---|---|---|
| Mar, 2016 | Jul, 2015 | Jan, 2015 | Sep, 2017 | Mar, 2016 |
Block Scope
Declaring a variable with const is similar to let when it comes to Block Scope .
The x declared in the block, in this example, is not the same as the x declared outside the block:
Example
const x = 10;
// Here x is 10 {
const x = 2;
// Here x is 2
}
// Here x is 10You can learn more about block scope in the chapter JavaScript Scope .
Redeclaring
Redeclaring a JavaScript var variable is allowed anywhere in a program:
Example
var x = 2; // Allowed
var x = 3; // Allowed
x = 4; // AllowedRedeclaring 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
}Hoisting
Variables defined with var are hoisted to the top and can be initialized at any time.
Meaning: You can use the variable before it is declared:
Example
carName = "Volvo";
var carName;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
alert (carName);
const carName = "Volvo";