JavaScript basic data types (introduction)

In JavaScript, the basic data types include the following five numbers, strings, Boolean values, null values ​​and undefined values. (ES6 adds a new Symbol type value, which will not be introduced in this article).

Number

Integer or floating point number

var numInt = 1;
var numFloat = 1.1;

String

A string is a sequence of characters representing a text value

var stringSingle = 'a';
var string = 'abscmj';

Boolean value

true / false

var t = true;
var f = false;

5. undefined

undefined indicates the attribute when the variable is undefined

var x;
if (x === undefined) {
    console.log(1)  //1
} 
// 这里 var a  等价于 var a = undefined

null

A special keyword indicating a null value

// foo不存在，它从来没有被定义过或者是初始化过：
foo;
"ReferenceError: foo is not defined"
// foo现在已经是知存在的，但是它没有类型或者是值：
var foo = null; 
foo;
null

The difference between undefined and null

// typeof返回数据的类型
typeof null        // "object" (因为一些以前的原因而不是'null')
typeof undefined   // "undefined"

null means "no object", that is, there should be no value there

undefined means "missing value" , that is, there should be a value here, but it has not been defined yet

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