Arithmetic expression

Arithmetic expression

Arithmetic operator

1. +： Both operators are numbers to add , The rest are hyphens
4. /
5. %

Implicit type conversion
If an operand involved in a mathematical operation is not of numeric type , This operand is automatically converted to numeric type （ The essence is to call... Internally Number Function converts the corresponding value to a numeric type ）

of IEEE754
terms of settlement ： When performing decimal operations , To call the number toFixed Method , Keep the specified number of decimal places
Negative numbers have no open root , So for NaN

Relationship expression

Relational operator ： The result of relational operator operation is Boolean value

2. <

3. =

4. <=
5. ==
6. !==
7. ===
8. !=

Equality and congruence

• equal ： Two equal signs do not compare the type of value , It compares the values after implicit conversion
• Congruence ： The three equal signs compare not only the same value but also the same type
• null and undefined use == Compare （ Involving implicit cast ,ES5 The specification States ）
  If x by null,y by undefined, The result is true; If x by null,y by undefined, The result is true
• null and undefined use === When comparing, the result is false Because of different types

typeof null  //object
typeof undefined //undefined
NaN==NaN //false
NaN===NaN //false

How to judge whether it is NaN：isNaN()

Logical expression

Logical operators

1. ！
2. &&
3. ||

Short circuit calculation
a&&b Operation ：a really , The value of the expression is b;a false , The value of the expression is a
a||b Operation ：a really , The value of the expression is a;a false , The value of the expression is b
Logical operation sequence ： Not ---- And ---- or

Assignment expression

Assignment operator

1. =
2. +=
3. -=
4. *=
5. /=
6. %=
7. ++
8. –

Synthetic expression

The order of operations ： Non operation ---- Mathematical operations ---- Relationship between operation ---- Logical operations

The range of variables represents

// 5<=a<=12
if(a>=5&&a<=12){
    }