Number type in JS (detailed tutorial)

This article mainly gives you a detailed analysis of the relevant knowledge points of the JavaScript numerical Number type

Number Question

Can you answer the following questions correctly?

• 0.1 0.2 == 0.3 Is it true?

• .e-5 means how much? How does

• represent octal?

• How to convert base?

• How to convert a string into a numerical value or integer? What about the other way around? How to deal with hexadecimal?

• What is the return value of parseInt(0x12, 16)? Is it 0x12?

• Number.MAX_VALUE is the maximum value. What is (new Number(12)).MAX_VALUE?

• How to round in JavaScript? How to preserve 3 decimal places of precision?

• How to get a random number? How to round? How to round up?

Number number representation method

The Number type represents a number, and JavaScript adopts the "double-precision 64-bit format defined by the IEEE 754 standard" IEEE 754 values") represent numbers.

Unlike other programming languages ​​(such as C and Java), JavaScript does not distinguish between integer values ​​and floating point values. All numbers are represented by floating point values ​​in JavaScript, so special attention should be paid when performing numerical operations. Missing progress problem.

0.1 + 0.2 = 0.30000000000000004;
0.1 + 0.2 == 0.3; // false
// 浮点运算判断相等
var ACCURACY = 1e-5; //定义精度精确到0.00001
var a = 0.1;
var b = 0.2;
var sum = 0.3;
// 判断相差小于精度就认为相等
if (a + b - sum < ACCURACY) {
 console.log(&#39;a+b == sum&#39;);
}

In specific implementations, integer values ​​are usually treated as 32-bit integer variables. In individual implementations (such as some browsers), they are also stored in the form of 32-bit integer variables until it is Used to perform certain operations that are not supported by 32-bit integers. This is to facilitate bit operations.

You can omit 0 to represent decimals, or you can use exponential form to represent numbers

.9; // 0.9
1E3 // 1000
2e6 // 2000000
0.1e2 // 10
1e-5 // 0.00001

Number numbers in different bases

Representation methods of different bases

Number can use four numeric bases: decimal, binary, octal and hexadecimal. Only use integers for non-decimal numbers.

• Binary representation: starting with zero, followed by a lowercase or uppercase Latin letter B (0b or 0B)

• Octal notation: starting with 0. If the number after 0 is not in the range 0 to 7, the number will be converted to a decimal number.

• The use of octal syntax is prohibited in ECMAScript 5 strict mode and will be treated as decimal

• When using octal numbers in ECMAScript 6, you need to give A number is prefixed with "0o"

• Hexadecimal notation: starting with zero, followed by a lowercase or uppercase Latin letter X (0x or 0X)

// 十进制
12345678
42
0777 // 在非严格格式下会被当做八进制处理 (用十进制表示就是511)
// 二进制
var a = 0b100000; // 32
var b = 0b0111111; // 63
var c = 0B0000111; // 7
// 八进制
var n = 0755; // 493
var m = 0644; // 420
var a = 0o10; // ES6 :八进制
// 十六进制
0xFFFFFFFFFFFFFFFFF // 295147905179352830000
0x123456789ABCDEF // 81985529216486900
0XA     // 10

Conversion of different bases

Conversion of bases mainly uses the toString() method or parseInt() method of Number:

• toString() method accepts an integer parameter with a value between 2 and 36 to specify the base. The default is decimal. Convert Number to String

• parseInt() second The parameter accepts an integer parameter with a value between 2 and 36. The parameter specifies the base system. The default is decimal. Convert the String to Number

// toString转换，输入为Number，返回为String
var n = 120;
n.toString(); // "120"
n.toString(2); // "1111000"
n.toString(8); // "170"
n.toString(16); // "78"
n.toString(20); // "60"
0x11.toString(); // "17"
0b111.toString(); // "7"
0x11.toString(12);// "15"
// parseInt转换，输入为String，返回为Number
parseInt('110'); // 110
parseInt('110', 2); // 6
parseInt('110', 8); // 72
parseInt('110', 16); // 272
parseInt('110', 26); // 702
// toString和parseInt结合使用可以在两两进制之间转换
// 将 a 从36进制转为12进制
var a = 'ra'; // 36进制表示的数
parseInt(a, 36).toString(12); // "960"

Number objectNumber

Number object It is a built-in number object that encapsulates a series of Number-related constants and methods.

var na = Number(8); // 8
na = Number('9.5'); // 9.5
na = new Number('9.3'); // Number 对象，仅可以使用原型方法

Number object properties:

Number对象方法

Number对象方法可以使用 Number. 的形式调用，也可以使用全局调用。

parseInt() 方法需要注意：

• parseInt() 参数可以有两个参数：数字字符串和进制

• 如果第一个参数为非指定进制的数字字符串，则结果为0

• 如果第一个参数为非字符串，会首先调用该参数的toString()方法转换为字符串

• 第一个参数中非指定进制可识别的字符会被忽略

• 如果给定的字符串不存在数值形式，函数会返回一个特殊的值 NaN

• 如果调用时没有提供第二个参数，则使用字符串表示的数字的进制

parseInt('123'); // 123
parseInt('123', 10); // 123
parseInt('123', 8); // 83
parseInt('123', 16); // 291
parseInt("11", 2); // 3
parseInt('0x123', 10); // 0
parseInt('0x123', 16); // 291
parseInt('0x123'); // 291
// 如果第一个参数不是字符串，会先把第一个参数转成字符串
parseInt('12', 16); // 18
parseInt(12, 16); // 18
// toString方法会将数字转换为10进制的字符串
parseInt(0xf, 16); // 21
0xf.toString(); // '15'
parseInt('15', 16); // 21
parseInt('23.56'); // 23
parseInt("hello", 16); // NaN
parseInt("aello", 16); // 174

Number类型原型方法

Number类型原型上还有一些方法来对数字进度进行取舍，这些方法可以被 Number 实例对象调用：

这些原型方法可以被Number实例对象调用：

var numObj = 12345.6789;
numObj.toExponential(); // "1.23456789e+4"
numObj.toExponential(2); // "1.23e+4"
numObj.toExponential(4); // "1.2346e+4"
numObj.toPrecision();  // "12345.6789"
numObj.toPrecision(2); // "1.2e+4"
numObj.toPrecision(4); // "1.235e+4"
numObj.toFixed();   // 返回 "12346"：进行四舍五入，不包括小数部分
numObj.toFixed(1);  // 返回 "12345.7"：进行四舍五入
numObj.toFixed(6);  // 返回 "12345.678900"：用0填充
(1.23e+20).toFixed(2); // 返回 "123000000000000000000.00"
(1.23e-10).toFixed(2); // 返回 "0.00"
2.34.toFixed(1);   // 返回 "2.3"
-2.34.toFixed(1);   // 返回 -2.3 （由于操作符优先级，负数不会返回字符串）
(-2.34).toFixed(1);  // 返回 "-2.3" （若用括号提高优先级，则返回字符串）

数学对象（Math）

和Number相关的是，JavaScript对象中内置的Math对象，提供了很多数学常数和函数的属性和方法。

属性列表：

The trigonometric function sin and other parameters in Math are radians. If it is an angle, you can use it (Math.PI / 180)

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