Front-end Interview Preparation 2----undefined and null summary in JavaScript

There are two values in JavaScript that denote "null", respectively, undefined and null;

　　(1) Undefined　　

Defines a global variable whose value is undefined that this variable is also called undefined . But this variable is not a constant, nor is it a keyword. This means that its value can be easily overwritten.

ES5 Tip: in the strict mode of ECMAScript 5, it undefined is no longer writable. But its name can still be hidden, such as defining a function named undefined .

The following conditions return a undefined value:

• Access to unmodified global variables undefined .
• A function that does not have an return expression defined implicitly returns.
• returnAn expression does not explicitly return anything.
• Accesses a property that does not exist.
• The function parameter has not been explicitly passed the value.
• Any variable that is set to a undefined value.

　　nullThe use of

The undefined usage scenario in JavaScript is similar to nullin other languages, which is actually a null different type of data in JavaScript.

It has some usage scenarios inside JavaScript, such as declaring the end of a prototype chainFoo.prototype = null，又比如在删除DOM0级方法指定的事件处理程序时，

　　将事件处理程序属性的值设置为null即可，btn.onclick=null), but in most cases it can be used undefined instead.

　　JS Judgment Undefinde

typeof returns a string with six possibilities: "Number", "string", "Boolean", "Object", "function", "undefined"

The following are the correct uses:

var exp = undefined;
if (typeof (exp) = = "undefined")
{
Alert ("undefined");
}

　　JS to judge Null

This function is not isNull in JavaScript.

　　The following are the correct uses:

var exp = null;
if (!exp && typeof (exp)! = "undefined" && exp!=0)
{
Alert ("is null");
}

Front-end Interview Preparation 2----undefined and null summary in JavaScript