JavaScript Variables

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

JavaScript Variables

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JavaScript Variables

JavaScript variables are containers for storing data values.

In this example, x, y, and z, are variables:

Example

var x = 5;
var y = 6;
var z = x + y;

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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;

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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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자바스크립트 식별자

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:

• 이름은 문자, 숫자. 밑줄, 달러 기호를 포함 할 수 있다.
• 이름은 문자로 시작해야 한다.
• 이름의 시작에 _와 $가 사용될 수 있따.
• 대소문자를 구분한다.
• 예약어는 이름으로 사용할 수 없다.

	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.

문자열은 " "나 ' '안에 적는다. 숫자는 따옴표없이 쓴다.

만일 숫자에 따옴표를 사용하면 문자로 인식된다. (3 - 숫자 , "3" - 문자)

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 »

	스크립트 시작 부분에 모든 변수를 모아두는 것이 좋다.

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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;

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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;

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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;

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You can also add strings, but strings will be concatenated (added end-to-end):

문자열 연결도 된다~

Example

var x = "John" + " " + "Doe";

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Also try this:

Example

var x = "5" + 2 + 3;

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	숫자를 문자열로 추가한 경우 (5가 아니라 "5"인 경우) 수는 문자열과 연결된 걸로 처리 한다.

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Test Yourself with Exercises!

Exercise 1 »  Exercise 2 »  Exercise 3 »  Exercise 4 »  Exercise 5 »  Exercise 6 »