javascript check undefined or null

The following is transferred from: http://blog.csdn.net/hongweigg/article/details/380900931. Question: In JavaScript, typeof (undefined) = = undefined established?Answer: Not true, the global function typeof () returns a value type of string type , six possible values: "Number", "string", "Boolean", "Object", "function", and "

This article mainly introduces how to determine the undefined type of JavaScript code. For more information, see The Code is as follows: If (reValue = undefined ){Alert ("undefined ");}The system found that the data could not be determined, and finally found the information to use the typeof method:If (typeof (reVal

The data types in the Javascrip are divided into the original data type (primitive type) and the object data type (objects type).Raw data typeThe original data types are: Number, string, Boolean, null, undefined.Object data typeAn object data type is an object type in which data types in JavaScript, except numbers, strings, and Boolean values, are objects.A brief introduction to the data types in

The first thing to know about JavaScript's automatic conversion is what type it needs, so JavaScript automatically passes the current type to the constructor of the required type to generate the required typeGuess 1:undefined is a class estimate in JavaScript (does not inherit object), and Null is a subclass of object,

In JavaScript development, people ask: what is the difference between null and undefined. A bad time to answer, especially undefined, because this involves the realization of the principle of undefined. So, after thinking carefully, write down this article, please heroes to

OverviewnullAnd undefined both can mean "no", meaning very similar. Assign a variable to undefined or, to be null honest, the grammatical effect is almost indistinguishable.var a = undefined;// 或者var a = null;In the above code, the a variables are assigned to

To say Null first, it represents a special value that is commonly used to describe "null values." Performs a typeof operation on NULL, and the result returns the string "Object", which means that null can be considered a special object value, meaning "Non-object" (It feels strange). In fact, it is generally assumed tha

Original addressJavaScript has a lot of quirks and incomprehensible places. Among them null undefined is more interesting. Now that you have it, why does JavaScript have to get a null?Equality ComparisonLet's start by having a look at the following comparison to explore null

Web page effects Null object detailed Null This is an object, but is empty. Because it is an object, typeof null returns ' object '. Null is a JavaScript reserved keyword. Null is automatically converted to 0 when it participa

This article will share with you the method of JSJquery to determine whether an object is null. it is very simple and practical. if you need it, you can refer to it. A clever implementation is found: check whether an Object is null, that is, it does not contain any element. Objects in Javascript are a dictionary that c

Check whether the checkbox of the Javascript check box is selected Assume that there is a form named form1 on the webpage, and there is a checkbox named checkbox1 in the form. The Code is as follows: Var f = document. form1;Var sum = 0;If (f. checkbox1.length ){For (var I = 0; I If (f. checkbox1 [I]. checked = true)Sum = sum + 1;}} E

var A1;var a2 = True;var a3 = 1;var a4 = "Hello"; var a5 = new Object (); var a6 = null;var a7 = nan;var A8 = Undefined;alert(typeof a);Show "undefined" alert (typeof A1);Show "undefined" alert (typeof A2);Displays "boolean" Alert (typeof A3);Show "number" alert (typeof A4);Displays "string" alert (typeof A5);Show "Obj

Null is a JavaScript literal that represents a null value, that is, no object is rendered. He is one of the original JavaScript values. A null value is often placed on an object that is expected, but does not refer to the parameter location of any object, that is, the initia

JS array to filter out false, NULL, 0, "", Undefined, and Nan values methodFor False,null,0,undefiend,nan Direct fetch ! is all true , so here only need to judge the empty string "" ;Look directly at the code:function bouncer(arr) { // Don‘t show a false ID to this bouncer. return arr.filter(function(val){ return !(!val || val === ""); });}bouncer([7, "ate", "",

http://blog.csdn.net/yiluoak_47/article/details/7766760First, the difference between null and undefined:Executes typeof on declared but uninitialized and undeclared variables, returning "undefined".Null represents an empty object pointer, and the typeof operation returns "Object".Generally do not explicitly set the value of the variable to

Transferred from: http://blog.csdn.net/yiluoak_47/article/details/7766760First, the difference between null and undefined:Executes typeof on declared but uninitialized and undeclared variables, returning "undefined".Null represents an empty object pointer, and the typeof operation returns "Object".Generally do not explicitly set the value of the variable to

How javascript jquery judges an object as null There are many null pointers in java, which need to be prevented and judged frequently. Otherwise, the console suffers annoying exceptions, causing confidence to be compromised and there is no experience in writing programs in the early days, you cannot find the root cause of the problem based on the exception inform

First, the difference between null and undefined:Executes typeof on declared but uninitialized and undeclared variables, returning "undefined".Null represents an empty object pointer, and the typeof operation returns "Object".Generally do not explicitly set the value of the variable to undefined, but