【JavaScript Tutorial】JavaScript Variables

JavaScript Variables

Variables are "containers" used to store information.

Example

var x=5;
var y=6;
var z=x+y;

Just like in algebra

x=5
y=6
z=x+y

In algebra we use letters (like x) to hold values ​​(like 5).

With the above expression z=x+y, we can calculate the value of z to be 11.

In JavaScript, these letters are called variables.

You can think of variables as containers for storing data.

JavaScript Variables

Like algebra, JavaScript variables can be used to store values ​​(such as x=5) and expressions (such as z=x+y).

Variables can have short names (such as x and y) or more descriptive names (such as age, sum, totalvolume).

Variables must start with a letter

Variables can also start with $ and _ symbols (but we don’t recommend this)

Variable names are case-sensitive (y and Y are different variables)

JavaScript Both statements and JavaScript variables are case-sensitive.

JavaScript data types

JavaScript variables can also hold other data types, such as text values ​​(name="Bill Gates").

In JavaScript, a text like "Bill Gates" is called a string.

JavaScript variables come in many types, but for now, we’ll just focus on numbers and strings.

When you assign a text value to a variable, you should surround the value with double or single quotes.

Don’t use quotes when the value you assign to a variable is a numeric value. If you surround a numeric value with quotes, the value is treated as text.

Example

var pi=3.14;
var person="John Doe";
var answer='Yes I am!';

Declaring (Creating) JavaScript Variables

Creating variables in JavaScript is often called "declaring" a variable.

We use var keyword to declare a variable:

var carname;

After a variable is declared, the variable is empty (it has no value).

To assign a value to a variable, use the equal sign:

carname="Volvo";

However, you can also assign a value to a variable when you declare it:

var carname="Volvo";

In the example below, we create a variable called carname and assign it Assign "Volvo" and put it in the HTML paragraph with id="demo":

Example

var carname="Volvo";
document.getElementById("demo").innerHTML=carname;

A good programming habit is to declare the required variables uniformly at the beginning of the code.

One statement, multiple variables

You can declare many variables in one statement. The statement starts with var and uses commas to separate variables:

var lastname="Doe", age=30, job="carpenter";

The declaration can also span multiple lines:

var lastname="Doe",
age=30,
job="carpenter";

Value = undefined

In computer programs, variables with no value are often declared. The value of a variable declared without a value is actually undefined.

After executing the following statement, the value of the variable carname will be undefined:

var carname;

重新声明 JavaScript 变量

如果重新声明 JavaScript 变量，该变量的值不会丢失：

在以下两条语句执行后，变量 carname 的值依然是 "Volvo"：

var carname="Volvo"; 
var carname;

JavaScript 算数

您可以通过 JavaScript 变量来做算数，使用的是 = 和 + 这类运算符：

实例

y=5;
x=y+2;

您将在本教程稍后的章节学到更多有关 JavaScript 运算符的知识。

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