What are the differences between JavaScript constants and variables?

Difference: Variables can be redeclared and reassigned; while constants cannot be changed once declared and cannot be redeclared or reassigned. Variables can be declared without assignment, and the return value is undefined; constant declarations must be assigned a value.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 5, Dell G3 computer.

Variables and constants in JavaScript:

Variables

Variables are used to store data The container can change or be assigned again during the running of the program.

We can use the var keyword to declare variables, and the value of the variable can be of any type. For better reading, it is recommended to use camel case naming for variables:

var myName="zhangpeiyue";//myName为变量

In ES6, there is a new let keyword to declare variables. Its function is similar to var, except that the declared variables are only valid in the area where they are located:

let str="es6";//str为变量

Constant

Constant classification:

1. Integer constant

• Binary (0b Beginning; e.g. 0b1001,0b1010)

• Decimal (9,-10,0) Octal (beginning with 0; e.g. 011, 012)

• Hexadecimal (starting with 0x; 0x10, 0x11)

2, real constant

• Decimal (3.14, 9.8)

3. String constants

• Use one or more characters enclosed in single quotes (') or double quotes (")

4. Boolean value

• Boolean constants have only two states: true or false. Special characters

In ES6, const is newly added to define constants:

Constants, like variables, are containers used to store data, but the value of the constant cannot be changed during the running of the program. .

It is recommended that constants be named in all capital letters. If they consist of multiple words, they can be separated by underscores:

//NUM只能在初始化的时候被赋值，赋值以后不允许被再次赋值。
const NUM=12;//NUM是常量，

The difference between JavaScript constants and variables

Variables:

• can be redeclared;

• can be reassigned;

• has variables Name in advance

• can only be declared, without assignment, the return value is undefined;

• No declaration, direct assignment uses global variables;

• If it is used directly without declaration or assignment, an error will be reported;

• Variables can be declared multiple times at the same time;

Constant:

• A quantity that cannot be changed once declared;

• The declaration must be assigned a value;

• Cannot be reassigned;

• Cannot be redeclared;

• It is customary to use uppercase identifiers; (js is case sensitive)

• No declaration in advance;

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