JavaScript Array Const
ECMAScript 2015 (ES6)
In 2015, JavaScript introduced an important new keyword:const.
It has become a common practice to declare arrays usingconst:
Cannot be Reassigned
An array declared withconst cannot be reassigned:
Example
cars = ["Toyota", "Volvo", "Audi"]; // ERROR
Arrays are Not Constants
The keywordconst is a little misleading.
It does NOT define a constant array. It defines a constant reference to an array.
Because of this, we can still change the elements of a constant array.
Elements Can be Reassigned
You can change the elements of a constant array:
Example
const cars = ["Saab", "Volvo", "BMW"];
// You can change an element:
cars[0] = "Toyota";
// You can add an element:
cars.push("Audi");
Assigned when Declared
JavaScriptconst variables must be assigned a value when they are declared:
Meaning: An array declared withconst must be initialized when it is declared.
Usingconst without initializing the array is a syntax error:
Example
This will not work:
cars = ["Saab", "Volvo", "BMW"];
Arrays declared withvar can be initialized at any time.
You can even use the array before it is declared:
Const Block Scope
An array declared withconst hasBlock Scope.
An array declared in a block is not the same as an array declared outside the block:
Example
// Here cars[0] is "Saab"
{
const cars = ["Toyota", "Volvo", "BMW"];
// Here cars[0] is "Toyota"
}
// Here cars[0] is"Saab"
An array declared withvar does not have block scope:
Example
// Here cars[0] is "Saab"
{
var cars = ["Toyota", "Volvo", "BMW"];
// Here cars[0] is "Toyota"
}
// Here cars[0] is"Toyota"
You can learn more about Block Scope in the chapter:JavaScript Scope.
Redeclaring Arrays
Redeclaring an array declared withvar is allowed anywhere in a program:
Example
var cars = ["Toyota", "BMW"]; // Allowed
cars = ["Volvo", "Saab"]; // Allowed
Redeclaring or reassigning an array toconst, in the same scope, or in the same block, is not allowed:
Example
const cars = ["Volvo", "BMW"]; // Not allowed
{
var cars = ["Volvo", "BMW"]; // Allowed
const cars = ["Volvo", "BMW"]; // Not allowed
}
Redeclaring or reassigning an existingconst array, in the same scope, or in the same block, is not allowed:
Example
const cars = ["Volvo", "BMW"]; // Not allowed
var cars = ["Volvo", "BMW"]; // Not allowed
cars = ["Volvo", "BMW"]; // Not allowed
{
const cars = ["Volvo", "BMW"]; // Allowed
const cars = ["Volvo", "BMW"]; // Not allowed
var cars = ["Volvo", "BMW"]; // Not allowed
cars = ["Volvo", "BMW"]; // Not allowed
}
Redeclaring an array withconst, in another scope, or in another block, is allowed:
Example
{
const cars = ["Volvo", "BMW"]; // Allowed
}
{
const cars = ["Volvo", "BMW"]; // Allowed
}
Complete Array Reference
For a complete Array reference, go to our:
Complete JavaScript Array Reference
The reference contains descriptions and examples of all Array properties and methods.
