JS learning-operators (arithmetic operators, relational operators, logical operators, unary operators, assignment operators)

1. Arithmetic operator +-* /% (remainder sign)

The computer performs automatic data type conversion: there is no way to perform operations between different data types, convert the data to the same data type, and then perform calculations.

1. One of the operands is a string, and the operator is a + sign. When other data types are converted into strings, the two strings will be spliced ​​together.

    alert('hello' + 100);//hello100
    alert('hello' + true);//hellotrue
    alert('hello' + NaN);//helloNaN
    alert('hello' + undefined);//helloundefined
    alert('hello' + 'de');//hellode

2. Any data and string do operations other than +, then the string must be converted into a number before performing the operation. (100-"20" results in 80)

• If the string is a string composed of pure numeric characters, convert it to the corresponding number;

• If the string contains characters other than numbers, it is converted into NaN, NaN and any data operation are NaN;

alert(100 - '20');//相当于100-20  结果为80
alert(100 -"20a");//'20a'转换成数字为NaN,所以结果为NaN

3. When performing arithmetic operations on data other than character strings, first convert them to numbers, and then perform operations

     alert(10 + true);//true=>1
     alert(10 +false); //false=>0
     alert(100 - NaN); //结果NaN
     alert(100 -undefined);//结果NaN
     alert(100 -null); //结果100  null=>0

note:

<1>The computer will not perform decimal operations.
<2>The divisor can be 0 in js

 alert(0.8-0.1)

   alert(10 / 0);//Infinity
   alert(-10 / 0);//-Infinity

Infinity是一个数字，可以跟数字运算 ，结果总是无穷大。

Second, the relational operator <> <= >= == != === !==

1. The result of the judgment is absolutely a Boolean value
2. The rules that different data types follow when comparing:
   (1) If both operands are numeric values, then the numeric values ​​are compared.
   (2) If the two operators are both character strings, compare
   the character code values ​​corresponding to the two character strings. The ASCII code table is a table of codes corresponding to each character in the computer.
   ① If two single characters are compared, directly compare the ASCII code value.
   ②Compare bit by bit until the size is compared and the result is obtained.
   "Abcd" <"ad": a is the same, b is 98, d is 100, so the result is true.
   
   (3) If one of the two operands is a value, the other is converted to a value, and then the value is compared.

1 == true //true转成数字是1,false转成数字是0，比较结果是true 
10 == "10" //结果是true

(4) NaN, == returns false, != returns true, and NaN is not equal to itself.

alert(10 == "10a" );//false
alert(10 != "10a");//true
alert("10a" == NaN);//false

3. === Identities must be of the same number and data type before returning true

   Note: Number(null) is 0; Number(undefined) is NaN; but null == undefined is true.

Third, the logical operator && ||!

1. AND operation:

       &&  （表达式1 && 表达式2 ） 只有当两个表达式都为true时，整个表达式才为真。
   
       注意短路操作：只要左边一个为假，整个表达式就为假，不会再运行右边的代码。比如 10<5 && alert(num);    不会报未声明变量num的错误，因为右边不执行。
   

2. Or operation:

       || （表达式1 || 表达式2）  只有当两个表达式都为false时，整个表达式为假。
       （同样表达式1为真的话就会不在运行表达式二，直接得出整个表达式为true）
   

3. Non-operation:

       ！表达式   将表达式的数据类型转换为布尔值(非0即真，非空即真)，然后再取反。
   

Four, unary operator ++--

1. a++:

Add 1 to the original variable, first take the value of a as the value of a++ expression, and then add one to a

2. ++a:

To add 1 to the original variable, first add one to a, and then use the value of a as the expression of ++a.

  var a = 5;
alert(a++);//5
alert(++a);//7 上一语句先将a的值作为a++的值输出，再对a加一变成6，此句将a加一变成7，再讲a得值7作为++a的值输出。

3. a- - 、 - -a

Decrement the original variable by 1

Five, assignment operator basic = compound += -= *

   “=” 将等号右边的值赋值给等号左边的变量：

   var num = 10 + 20;
   alert(num); //结果是30

   “+=” : num = num + 9  =>  num += 9