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C # tutorial
The const keyword was introduced inES6 (2015)
A const variable cannot be reassigned:
const PI = 3.141592653589793;
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
JavaScript const variables must be assigned a value when they are declared:
const PI = 3.14159265359;
const PI;
PI = 3.14159265359;
As a general rule, always declare a variable with const unless you know that
the value will change.
Use const when you declare:
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:
But you CAN:
You can change the elements of a constant array:
// 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:
const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; // ERROR
You can change the properties of a constant object:
// 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:
const car = {type:"Fiat", model:"500", color:"white"};
car = {type:"Volvo", model:"EX60", color:"red"}; //
ERROR
The const keyword is not supported in Internet Explorer 10 or earlier.
The following table defines the first browser versions with full support for the const keyword:
| Chrome 49 | IE 11 / Edge | Firefox 36 | Safari 10 | Opera 36 |
| Mar, 2016 | Oct, 2013 | Feb, 2015 | Sep, 2016 | Mar, 2016 |
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:
const x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
You can learn more about block scope in the chapter JavaScript Scope.
Redeclaring a JavaScript var variable is allowed
anywhere in a program:
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:
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:
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:
const x = 2; // Allowed
{
const x = 3; // Allowed
}
{
const x = 4; // Allowed
}
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:
This is OK:
carName = "Volvo";
var carName;
If you want to learn more about hoisting, study the chapter JavaScript Hoisting.
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:
alert (carName);
const carName = "Volvo";