JavaScript var 변수

https://www.w3schools.com/js/js_variables.asp

JavaScript Variables

 

JavaScript Variables

There are 3 ways to declare a JavaScript variable:

• Using var
• Using let
• Using const

This chapter uses var.

The let and const keywords are explained in the next chapters.

Variables

Variables are containers for storing data (values).

In this example, x, y, and z, are variables, declared with the var keyword:

Example

var x = 5;
var y = 6;
var z = x + y;

Try it Yourself »

From the example above, you can expect:

• x stores the value 5
• y stores the value 6
• z stores the value 11

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Much Like Algebra

In this example, price1, price2, and total, are variables:

Example

var price1 = 5;
var price2 = 6;
var total = price1 + price2;

Try it Yourself »

In programming, just like in algebra, we use variables (like price1) to hold values.

In programming, just like in algebra, we use variables in expressions (total = price1 + price2).

From the example above, you can calculate the total to be 11.

JavaScript variables are containers for storing data values.

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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 identifiers are case-sensitive.

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The Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, not an "equal to" operator.

This is different from algebra. The following does not make sense in algebra:

x = x + 5

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.

(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)

The "equal to" operator is written like == in JavaScript.

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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.

Example

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

Try it Yourself »

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Declaring (Creating) JavaScript Variables

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

You declare a JavaScript variable with the var keyword:

var carName;

After the declaration, the variable has no value (technically it has the value of undefined).

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

carName = "Volvo";

You can also assign a value to the variable when you declare it:

var carName = "Volvo";

In the example below, we create a variable called carName and assign the value "Volvo" to it.

Then we "output" the value inside an HTML paragraph with id="demo":

Example

<p id="demo"></p>

<script>
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>

Try it Yourself »

It's a good programming practice to declare all variables at the beginning of a script.

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One Statement, Many Variables

You can declare many variables in one statement.

Start the statement with var and separate the variables by comma:

var person = "John Doe", carName = "Volvo", price = 200;

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A declaration can span multiple lines:

var person = "John Doe",
carName = "Volvo",
price = 200;

Try it Yourself »

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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.

The variable carName will have the value undefined after the execution of this statement:

Example

var carName;

Try it Yourself »

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Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable, it will not lose its value.

The variable carName will still have the value "Volvo" after the execution of these statements:

Example

var carName = "Volvo";
var carName;

Try it Yourself »

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JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

Example

var x = 5 + 2 + 3;

Try it Yourself »

You can also add strings, but strings will be concatenated:

Example

var x = "John" + " " + "Doe";

Try it Yourself »

Also try this:

Example

var x = "5" + 2 + 3;

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If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.

Now try this:

Example

var x = 2 + 3 + "5";

Try it Yourself »

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JavaScript Dollar Sign $

Remember that JavaScript identifiers (names) must begin with:

• A letter (A-Z or a-z)
• A dollar sign ($)
• Or an underscore (_)

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

Example

var $$$ = "Hello World";
var $ = 2;
var $myMoney = 5;

Try it Yourself »

Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library.

In the JavaScript library jQuery, for instance, the main function $ is used to select HTML elements. In jQuery $("p"); means "select all p elements".

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JavaScript Underscore (_)

Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:

Example

var _lastName = "Johnson";
var _x = 2;
var _100 = 5;

Try it Yourself »

Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for "private (hidden)" variables.