JS null judgment operators | and? Usage of

1. Conversion rules

Other data types are converted to boolean Type of rules ：
null、undefined、‘’( The length is 0 String )、‘0’、0、NaN These are converted into false, Everything else is converted into true

2. The result of a logical operation

1. ！ The result is always true or false.
   First convert the data into true or fasle, Then reverse .

2. || Short circuit function . ( True is true )
   If The first 1 Number yes true Or it can be converted to true, The result is the first number .
   If The first 1 Number yes fasle Or it can be converted to false, The result is the second number .

3. && Short circuit function .( True is true )
   If The first 1 Number yes false Or it can be converted to false, The result is the first number .
   If The first 1 Number yes true Or it can be converted to true, The result is the second number .

3. Judgment operator || and ?? Usage of

1. || Operator ： When a property or a value is null、undefined、false、0、 An empty string "" when , The result is || Value after
2. ?? Operator ： When a property or a value is null or undefined when , take ?? Value after
   example ：

// 0,1 On behalf of Guan , open . The default value is 1  open 
    let open=0
    let open1 = open || 1		//0 Will be treated as false, The result is 1
    let open2 = open ?? 1		//0 It won't change , The result is 0

	console.log(0||1)   //1
    console.log(0??1)   //0    

    console.log(""||1)   //1
    console.log(""??1)   //""
    
    console.log(false||1)   //1
    console.log(false??1)   //false
    
    console.log(null||1)   //1 
    console.log(null??1)   //1     
    
    console.log(undefined||1)   //1
    console.log(undefined??1)   //1