JavaScript learning: using const to declare constants

const is used to declare one or more constants, which must be initialized when declaring, and the value cannot be modified after initialization.

const declaration of constants

Const-defined constants are similar to variables defined using let:

• both are block-level scopes
• and cannot have the same name as other variables or functions in the scope in which they are located.

There are two differences between the two:

• const declaration Constants must be initialized, but variables declared by let are not
• defined as const. The value of a constant cannot be modified through reassignment, nor can it be declared again. The variable values ​​defined by let can be modified.

Block-level scope

const definition constants also have block-level scope

var a = 10;
const x = 'world';
if (a > 0){
    const x = 'hello';
    console.log(x); // Output x here as hello
}
console.log(x); // The output x here is world

and cannot have the same name as other variables or functions in its scope

{
    var x = 'world';
    const x = 'hello'; // Error report}

Initialization

Constants declared by const must be initialized, but variables declared by let do not need to be initialized

// Wrong writing method const PI;
PI = 3.14

The following is the correct way to write, assign value at the same time of declaration

// Correct way to write const PI = 3.14;

the value cannot be modified after initialization

const PI = 3.14;
PI = PI + 1; // The error reported

is not a real constant

. Strings and numeric types defined using const are immutable. When an object or array is defined, the contents inside can be modified.

const defines the object to modify the properties

const defines the object to modify the properties

const person = {
    name: "yoyo",
    age: 20,
};
person.name = 'hello';
person.age = 30;
console.log(person.name); // hello
console.log(person.age); //age

but cannot reassign the object

const person = {
    name: "yoyo",
    age: 20,
};
person = {name: 'xx', age: 23}; // Error reported

const Define an array to modify members

const Define an array to modify the value of a member

const a = ['hello', 'world'];

//Modify element a[0] = "yoyo";
console.log(a); // ['yoyo', 'world']
a.shift('12');
console.log(a); // ['world']
a.unshift('xx');
console.log(a); // ['xx', 'world']
a.push('yy');
console.log(a); // ['xx', 'world', 'yy']

also cannot reassign constant arrays:

const a = ['hello', 'world'];
a = ['x', 'y']; // Error

summary: A constant is a quantity whose value (memory address) cannot be changed. For common definitions of const, an initial value is required.