Detailed analysis of JavaScript variable types_javascript skills

Variable type                                   

typeof,instanceof

    var obj = null;
    console.info(typeof obj);    //Object
    var arr = [];
    console.info(arr instanceof Object);  //true
    console.info(arr instanceof Array);  //true

Wapper Object of primitive data type

Data type conversion

console.log(parseInt("34", 10)); //34
console.log(parseInt("34s5b", 10)); //34
console.log(parseInt("s", 10)); //NaN
console.log(parseInt(3.14, 10)); //3

//number
var value = 100;
//string
value = "qiu";
//object
value = [1, 'two', 3];

"="Various patterns of numbers:

var x = 42;
var y = "42";
console.log(x == y) //true;
console.log(x === y) //false

Udefine: means the variable is undefined and does not have a valid value (there is no variable yet, so how can we talk about the value)                     null:       nothing, a variable does not refer to any object. null is an object, type object (has variables, but does not reference values)

var obj = null;
if (obj === null) {
alert("obj === null"); //此句将执行
}
else {
alert("obj!=null");
}
alert(typeof obj); //object

var myVar;
//true
console.log(typeof myVar === "undefined");
console.log(myVar === undefined);
var myVar2 = null;
console.log(typeof myVar2); //object
//true;
console.log(myVar2 == null);
console.log(myVar2 === null);
//true
console.info(myVar == myVar2); //undefine == null; 是true
//false
console.info(myVar === myVar2); //undefine === null; 是false

undefined, null,NaN,"",0 Except for these values, other values ​​are true;

Operator: ! ! with ||

                                !! Convert the following expression into a boolean value and return true or false                   !! "qiu"                                                                                                                                                                                                                                                                                                                                                                                                                                             var ns = ns || {}
If ns is undefined, return {}, otherwise return ns

Note: You must use var to define variables, otherwise you will get stuck! If you don’t write var, it will become a global variable

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