A simple method to determine whether the JavaScript Object is null or the property is null

A simple method to determine whether the JavaScript Object is null or the property is null

First, let's talk about the difference between null and undefined:

If typeof is executed on declared but uninitialized and undeclared variables, "undefined" is returned ".

Null indicates a null object Pointer. The typeof operation returns "object ".

Generally, the value of a variable is not explicitly set to undefined, but the opposite is null. For a variable that will save the object, the variable should be explicitly set to save the null value.

 

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Var bj; Alert (bj); // "undefined" Bj = null; Alert (typeof bj); // "object" Alert (bj = null); // true Bj = {}; Alert (bj = null); // false

The following two functions are provided by brother Deng. Thank you.

 

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/* * Check whether the object is a null object (excluding any readable attributes ). * This method detects both attributes of an object and attributes inherited from the prototype (hasOwnProperty is not used ). */ Function isEmpty (obj) { For (var name in obj) { Return false; } Return true; };

Whether the empty object is {} or null. I wrote a test case.

 

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Var a = {}; A. name = 'realwall '; Console. log (isEmpty (a); // false Console. log (isEmpty ({}); // true Console. log (isEmpty (null); // true // When the parameter is null, there is no syntax error. That is, although the null pointer object cannot be added, you can use the for in statement. 　　 ? /* * Check whether the object is a null object (excluding any readable attributes ). * The method only detects the attributes of the object and does not detect the attributes inherited from the prototype. */ Function isOwnEmpty (obj) { For (var name in obj) { If (obj. hasOwnProperty (name )) { Return false; } } Return true; };

Difference between {} and null:

This is very important.

 

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Var a = {}; Var B = null; A. name = 'realwall '; B. name = 'Jim '; // an error is reported here. B is a null pointer object and cannot be directly added as a normal object. B =; B. name = 'Jim '; // At this time, a and B point to the same object. A. name and B. name are both 'jam'