How to get pure undefined in JavaScript_javascript tips

1. Why should we get undefined?

Because undefined is not a reserved word in JavaScript, it can be assigned a value by the user as a variable. If we need to use undefined to detect a variable later, the detected value will be inaccurate;

Give me an example:

var undefined=10;
function sum(a,b){
 if(a===undefined||b===undefined){
  console.log("参数不正确");
 }18101130357
 return a+b;
}

sum(10,10)->Originally correct parameters, the console output is indeed "parameter error";

At this time, in order to be compatible with all browsers, we need to obtain a pure undefinde

2. How to get pure undefined?

1) void (0):

In the ECMAScript 262 specification, it is described as follows:

The void Operator
The production UnaryExpression : void UnaryExpression is evaluated as follows:
Let expr be the result of evaluating UnaryExpression.
Call GetValue(expr).
Return undefined.

In short, just remember that no matter what the expression after void is, the void operator will return undefined

2) Pass in a formal parameter for assignment

[Case]

function(_undefined){
//函数体中不给_undefined赋值，形参_undefined的值就是undefined，在这个函数用就可以使用_undefined了
}

3) Unassigned variable

For example: var num //The principle is the same as 2)

Don’t be bored with familiar things, make a little progress every day; don’t be afraid of unfamiliar things, learn a little every day;

PS: js determines undefined type

 if (reValue== undefined){
 alert("undefined");
 }
 发现判断不出来，最后查了下资料要用typeof
方法：
if (typeof(reValue) == "undefined") { 
 alert("undefined"); 
}

typeof returns a string, there are six possibilities: "number", "string", "boolean", "object", "function", "undefined"