Variables in JavaScript - JavaScript Tutorial - w3Schools - Ch#07 English

Описание: 4 Ways to Declare a JavaScript Variable:
Using var
Using let
Using const
Using nothing

Variables are containers for storing data (storing data values).

When to Use JavaScript var?
Always declare JavaScript variables with var,let, orconst.

The var keyword is used in all JavaScript code from 1995 to 2015.

The let and const keywords were added to JavaScript in 2015.

If you want your code to run in older browser, you must use var.

When to Use JavaScript const?
If you want a general rule: always declare variables with const.

If you think the value of the variable can change, use let.
JavaScript Identifiers
All JavaScript variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter
Names can also begin with $ and _ (but we will not use it in this tutorial)
Names are case sensitive (y and Y are different variables)
Reserved words (like JavaScript keywords) cannot be used as names.

JavaScript Data Types
JavaScript variables can hold numbers like 100 and text values like "John Doe".

In programming, text values are called text strings.

JavaScript can handle many types of data, but for now, just think of numbers and strings.

Strings are written inside double or single quotes. Numbers are written without quotes.

If you put a number in quotes, it will be treated as a text string.

Declaring a JavaScript Variable
Creating a variable in JavaScript is called "declaring" a variable.

You declare a JavaScript variable with the var or the let keyword

One Statement, Many Variables
You can declare many variables in one statement.

Start the statement with let and separate the variables by comma

Value = undefined
In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.

A variable declared without a value will have the value undefined.

JavaScript Dollar Sign $
Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names.

JavaScript Underscore (_)
Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names.