How to obtain pure undefined in JavaScript _ javascript skills

This article describes how to obtain the pure undefined method in JavaScript. For more information, see section 1. Why do you want to obtain undefined?

Because undefined is not a reserved word in javascript, it can be assigned as a variable. If undefined is used to detect a variable in the future, the value of undefined is inaccurate;

Example:

Var undefined = 10; function sum (a, B) {if (a === undefined | B === undefined) {console. log ("Incorrect Parameter");} 18101130357 return a + B ;}

Sum ()-> If the parameter is correct, the console outputs "parameter error ";

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

2. How to obtain pure undefined?

1) void (0 ):

The ECMAScript 262 specification is described as follows:

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

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

2) input a parameter that is assigned a value.

[Case]

Function (_ undefined) {// The value of _ undefined is not assigned to the function body. The value of _ undefined is undefined. You can use _ undefined in this function}

3) unassigned variables

For example, var num // The truth is the same as 2 ).

Do not get bored with familiar things, make progress every day; Do not fear unfamiliar things, learn a little every day;

PS: js determines the undefined type

If (reValue = undefined) {alert ("undefined");} found that the judgment could not be found, and finally found the information to use the typeof method: if (typeof (reValue) = "undefined") {alert ("undefined ");}

Typeof returns a string, which may be "number", "string", "boolean", "object", "function", or "undefined"