Record some pits encountered by JS

1.typeof： Undefined variables typeof return "undefined".

2. data type ：

1）js in 0 == '' The return is true

2）1 == '1' The return is true, But in Map in 1 and '1' It's different

3） When obtaining input box data for calculation , Please convert to number first

4）undefined+ Any numerical result is NaN

5） character string +undefined The result is a string +'undefined' character string

3. Judge ：

1）if(data), When data by 0 Or empty string ('') perhaps undefined when , Will not enter the judgment

2）if(!!data), For judgment true perhaps false however data It doesn't have to be true perhaps false

4. Easy to make mistakes ：

1） Example 1： Because the function is not in console.info() Print it when you need it , Instead, save the function outside ,10 One function has the same scope , And are 10

var a = [];
for(var i=0; i<10; i++){
    a[i] = function (){
        console.info(i);
    }
}
for(var i=0; i<a.length; i++){
    //  Output is normal , But after the loop is completed, the data is all 10
    a[i]();
}
for(var k=0; k<a.length; k++){
    //  The output results are all 10
    a[k]();
}

2） Example 2： because num Undefined ,num by undefined,undefined+ Any number is NaN

var num;
for(var i=1; i<=10; i++){
    num = num+i;
}
//  The output is NaN
console.info(num);

3） Example 3：return When the line of the statement has nothing else , Will be in return Automatically insert semicolon after statement

var f1 = function (){
    return
    {
        test: 'test'
    }
}
//  The printout is undefined
console.info(f1());