Detailed explanation of JavaScript data types and their conversion

This article brings you a detailed explanation of JavaScript data types and their conversion. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. Introduction

Every value in JavaScript language belongs to a certain data type. There are seven data types in JavaScript:

1. Number: integer and decimal (such as 1 and 3.14).

2. String (string): text (such as Hello World).

3. Boolean: Two special values ​​that represent authenticity, namely true (true) and false (false).

4. undefined: means "undefined" or does not exist.

5. null: Indicates a null value, that is, the value here is empty.

6. Object: A collection of various values.

7. Symbol

Symbol is a newly added basic data type in ES6. This article only introduces the first six types.

2. Detailed explanation of data types

1. Numeric value (number):

1.1 Integers and floating point numbers

JavaScript internal, All numbers are stored as 64-bit floating point numbers, even integers.

1 === 1.0 // true

1.2 Precision

In the international standard IEEE 754, the 64 binary bits of JavaScript floating point numbers, starting from the leftmost, the first digital sign bit determines the sign of a number ( 0 is a positive number, 1 is a negative number), the exponent part from 2 to 12 digits determines the size of the value, and the decimal part from 13 to 64 determines the accuracy of the value.
1.3 The base of numerical values

JavaScript provides four base representation methods for integers: decimal, hexadecimal, octal, and binary.

• Decimal: Numeric value without leading 0.

• Octal: A value with a prefix of 0o or 0O.

• Hexadecimal: A value with a prefix 0x or 0X.

• Binary: A value with prefix 0b or 0B.

1.4 NaN

NaN means Not a Number, which mainly occurs when there is an error in parsing a string into a number, such as:

618 - 'x' // NaN

NaN不等于任何值，包括它本身。：

NaN === NaN // false

2 . String:

is quoted with '' or "". If it does not contain any content, it is an empty string with a length of 0. The one with spaces in it is called a space string, with a length of 1, and the two are different.

2.1 Multi-line string

If the string is divided into multiple lines, you can use a backslash at the end of each line, otherwise an error will be reported. In addition to using backslashes, you can also use the concatenation operator ( ) to concatenate multiple single-line strings. The results of the two methods are consistent. It is recommended to use the concatenation operator ( ).

var longString = 'Long \
long \
long \
string';

longString
// "Long long long string"

var longString = 'Long '
  + 'long '
  + 'long '
  + 'string';

longString
// "Long long long string"

In ES6, backticks (`) are added to write multi-line strings, and the length of this multi-line string will include the length of the carriage return.

2.2 Escape

Backslash () has special meaning in the string and is used to represent some special characters, so it is also called an escape character. Special characters that often need to be escaped with backslashes are:

• 0: null (u0000)

• n: newline character (u000A)

• r: Enter key (u000D)

• t: Tab character (u0009)

• ': Single quotation mark (u0027)

• ": Double quotation mark (u0022)

  \: Backslash (u005C)

2.3 length attribute

The length attribute returns the length of the string. This attribute is determined by the string itself and cannot be changed through assignment.

var s = 'mamamoo';
s.length // 7
s.length = 5;
s.length // 7

3. boolean ):

Boolean values ​​only have two values: "true" and "false". "True" is represented by the keyword true, and "false" is represented by the keyword false. If JavaScript expects A certain position should be a Boolean value, and the existing value at that position will be automatically converted to a Boolean value. The conversion rules are six values ​​except undefined, null, false, 0, NaN, "" or '' (empty string) is converted to false, and other values ​​are regarded as true.

4. undefined and null:

Both null and undefined can mean "none", meaning Very similar. The difference between the two is:

• null is an object that represents "empty" and is 0 when converted to a numerical value; undefined is a primitive that represents "no definition here" Value, it is NaN when converted to a numerical value.

• If a variable has no value, undefined is returned.

• When there is an object object but you do not want to assign a value yet It is recommended to use null; when there is a non-object but you do not want to assign a value, it is recommended to use undefined.

5. Object:

5.1 Generation method

The object is simply a set of "key-value pairs", which is an unordered composite data collection.
The generation method is to use a large Parentheses wrap the key-value pair and assign it to the variable. The two key-value pairs are separated by commas. ":" is preceded by the key name, and ":" is followed by the key value. For example:

var obj = {
  foo: 'Hello',
  bar: 'World'
};

5.2 Key Name and key value

对象的所有键名都是字符串，要加引号，不加也会自动转为字符串。如果键名不符合标识名的条件（比如第一个字符为数字，或者含有空格或运算符），且也不是数字，则必须加上引号，否则会报错。而键值是什么类型就用该类型的格式。

5.3 属性

对象的每一个键名又称为“属性”（property），它的“键值”可以是任何数据类型。

属性的读取

读取对象的属性，有两种方法，一种是使用点运算符，还有一种是使用方括号运算符。

var obj = {
  p: 'Hello World'
};

obj.p // "Hello World"
obj['p'] // "Hello World",[]内''一定要加

属性的删除delete(无value无key)

var obj = { p: 1 };
Object.keys(obj) // ["p"]

delete obj.p // true
obj.p // undefined
Object.keys(obj) // []

属性的查看Object.keys

var obj = {
  key1: 1,
  key2: 2
};

Object.keys(obj);
// ['key1', 'key2']

属性是否存在：in

in运算符用于检查对象是否包含某个属性（注意，检查的是键名，不是键值），如果包含就返回true，否则返回false。

属性的遍历：for...in 循环

var obj = {a: 1, b: 2, c: 3};

for (var i in obj) {
  console.log('键名：', i);
  console.log('键值：', obj[i]);
}
// 键名： a
// 键值： 1
// 键名： b
// 键值： 2
// 键名： c
// 键值： 3

三、如何知道变量类型？

使用type of可得变量的数据类型，如：

var t = 619;
type of t;//"number"

特别注意的是：

• 当数据类型为null时，用type of打出的数据类型却是'object'。

• 当定义了一个函数时，用type of打出的数据类型却是'function'。

四、数据类型的转换

4.1 转为string

• 使用toString()

var t = 619;
t.toString();//"619"

null没有toString这个API，不能使用toString，会报错

var n = null;
n.toString;
//Uncaught TypeError: Cannot read property 'toString' of null

undefined也会报错

var n = undefined;
n.toString();
//Uncaught TypeError: Cannot read property 'toString' of undefined

object使用toString会得到"[object Object]"。

var object = {name:"po"};
object.toString()
//"[object Object]"

• 通过与空字符串相加(+"")也能转化为字符串类型，且null、undefined也适用。

• 使用window.String()

window.String(null)//"null"

为什么1 + '1' = '11'?
这是因为当两个不同数据类型相加时，会优先选择转化为字符串，所以1 + '1'相当于1.toString() + '1'，于是结果为两个字符串1相加，即字符串11。

4.2 转为布尔

使用Boolean()

Boolean(0)//false
Boolean('')//false
Boolean(' ')//true

使用!!

!! ''//false
!! ' '//true

五个falsy值：0、NaN、空字符串、null、undefined

4.3 转为Number

使用Number()

Number('1')//1

使用parseInt()

//第二位参数要写，是表示进制

使用parseFloat()

parseFloat('1.23')//1.23

使用 -0

'1'-0//1

使用 +

+ null//0

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