Author: Michal Szymanski
ECMAScript 2015
ES2015 intoduced two important new JavaScript keywords: let and const.
Variables defined with const behave like let
variables, except they
cannot be reassigned:
const PI = 3.141592653589793;
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
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:
var x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
Assigned when Declared
JavaScript const variables must be assigned a value when they are declared.
Incorrect:
const PI;
PI = 3.14159265359;
Correct
const PI = 3.14159265359;
Not Real Constants
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, we cannot change constant primitive values, but we can change the properties of constant objects.
Primitive Values
If we assign a primitive value to a constant, we cannot change the primitive value:
const PI = 3.141592653589793;
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
Constant Objects can Change
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 a constant object:
const car = {type:"Fiat", model:"500", color:"white"};
car = {type:"Volvo", model:"EX60", color:"red"}; // ERROR
Constant Arrays can Change
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 a constant array:
const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; // ERROR
Redeclaring
Redeclaring a JavaScript var variable is allowed
anywhere in a program:
var x = 2; // Allowed
var x = 3; // Allowed
x = 4; // Allowed
Redeclaring or reassigning an existing var or let
variable to const, in the same scope, or in
the same block, is not allowed:
var x = 2; // Allowed
const x = 2; // Not allowed
{
let x = 2; // Allowed
const x = 2; // Not allowed
}
Redeclaring or reassigning an existing const variable, in the same scope, or in the same block, is not allowed:
const x = 2; // Allowed
const x = 3; // Not allowed
x = 3; // Not allowed
var x = 3; // Not allowed
let x = 3; // Not allowed
{
const x = 2; // Allowed
const x = 3; // Not allowed
x = 3; // Not allowed
var x = 3; // Not allowed
let x = 3; // 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
}
Hoisting
Variables defined with const are not hoisted to the top.
A const
variable cannot be used before it is declared:
carName = "Volvo"; // You can NOT use carName here
const carName = "Volvo";
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