http://www.w3schools.com/js/js_datatypes.asp
JavaScript Data Types
String, Number, Boolean, Array, Object.
JavaScript Data Types
JavaScript variables can hold many data types: numbers, strings, arrays, objects and more:
var length = 16; // Number
var lastName = "Johnson"; // String
var cars = ["Saab", "Volvo", "BMW"]; // Array
var x = {firstName:"John", lastName:"Doe"}; // Object
The Concept of Data Types
In programming, data types is an important concept.
To be able to operate on variables, it is important to know something about the type.
Without data types, a computer cannot safely solve this:
var x = 16 + "Volvo";
Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?
JavaScript will treat the example above as:
var x = "16" + "Volvo";
When adding a number and a string, JavaScript will treat the number as a string. |
Example
var x = 16 + "Volvo";
Example
var x = "Volvo" + 16;
자바 스크립트는 왼쪽에서 오른쪽으로 식을 푼다.
같은 값이라도 다른 배열일 경우 결과값이 다를 수 있다.
JavaScript:
var x = 16 + 4 + "Volvo";
Result:
20Volvo
JavaScript:
var x = "Volvo" + 16 + 4;
Result:
Volvo164
In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".
In the second example, since the first operand is a string, all operands are treated as strings.
JavaScript Has Dynamic Types
JavaScript has dynamic types. This means that the same variable can be used as different types:
Example
var x; // Now x is undefined
var x = 5; // Now x is a Number
var x = "John"; // Now x is a String
JavaScript Strings
A string (or a text string) is a series of characters like "John Doe".
Strings are written with quotes. You can use single or double quotes:
Example
var carName = "Volvo XC60"; // Using double quotes
var carName = 'Volvo XC60'; // Using single quotes
You can use quotes inside a string, as long as they don't match the quotes surrounding the string:
Example
var answer = "It's alright"; // Single quote inside double quotes
var answer = "He is called 'Johnny'"; // Single quotes inside double quotes
var answer = 'He is called "Johnny"'; // Double quotes inside single quotes
You will learn more about strings later in this tutorial.
JavaScript Numbers
JavaScript has only one type of numbers.
Numbers can be written with, or without decimals:
Example
var x1 = 34.00; // Written with decimals
var x2 = 34; // Written without decimals
Extra large or extra small numbers can be written with scientific (exponential) notation:
Example
var y = 123e5; // 12300000
var z = 123e-5; // 0.00123
You will learn more about numbers later in this tutorial.
JavaScript Booleans
Booleans can only have two values: true or false.
Example
var x = true;
var y = false;
Booleans are often used in conditional testing.
You will learn more about conditional testing later in this tutorial.
JavaScript Arrays
JavaScript arrays are written with square brackets.
Array items are separated by commas.
The following code declares (creates) an array called cars, containing three items (car names):
Example
var cars = ["Saab", "Volvo", "BMW"];
Array indexes are zero-based, which means the first item is [0], second is [1], and so on.
You will learn more about arrays later in this tutorial.
JavaScript Objects
JavaScript objects are written with curly braces.
Object properties are written as name:value pairs, separated by commas.
Example
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.
You will learn more about objects later in this tutorial.
The typeof Operator
You can use the JavaScript typeof operator to find the type of a JavaScript variable:
Example
typeof "John" // Returns string
typeof 3.14 // Returns number
typeof false // Returns boolean
typeof [1,2,3,4] // Returns object
typeof {name:'John', age:34} // Returns object
In JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object. |
Undefined
In JavaScript, a variable without a value, has the value undefined. The typeof is also undefined.
Example
var person; // Value is undefined, type is undefined
Any variable can be emptied, by setting the value to undefined. The type will also be undefined.
Example
person = undefined; // Value is undefined, type is undefined
Empty Values
An empty value has nothing to do with undefined.
An empty string variable has both a value and a type.
Example
var car = ""; // The value is "", the typeof is string
Null
In JavaScript null is "nothing". It is supposed to be something that doesn't exist.
Unfortunately, in JavaScript, the data type of null is an object.
You can consider it a bug in JavaScript that typeof null is an object. It should be null. |
You can empty an object by setting it to null:
Example
var person = null; // Value is null, but type is still an object
You can also empty an object by setting it to undefined:
Example
var person = undefined; // Value is undefined, type is undefined
Difference Between Undefined and Null
typeof undefined // undefined
typeof null // object
null === undefined // false
null == undefined // true
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