javascript check undefined or null

Undefined means "missing value", that is, there should be a value here, but it has not been assigned a value. Typical uses are: (1) The variable is declared, but when it is not assigned, it is equal to undefined. (2) when calling a function, the argument that should be supplied is not provided, which equals undefined. (3) The object does not have

First, prefaceBecause the null and undefined in JavaScript are somewhat unclear, a summary is made after the experiment.Second, the source codehtml> head> meta charset="Utf-8"> meta name="viewport" content="Width=device-width" > title>title> script type="Text/javascript" charset="Utf-8"> var A; var b

Functiondemo () {alert (typeofa== "undefined");//true}function demo () {alert (a==null);//consoleerroraisnot defined}functiondemo (a) {alert (typeofa== "undefined");// Truealert (A==null);//true}functiondemo () {var a;alert (typeofa== "undefined");//true alert (a==

In the various data types defined in the programming language, we will define a null value or a false value for it, such as a null value for the object type null,.net the null value of the database field in the framework Dbnull,boolean false value of the type, and so on. There are also many kinds of "

A deep understanding of javascript null and undefined Javascript has two easily overlooked problems: null = undefined and null! = Undefined

Examples of differences between null and undefined in JavaScript Let's talk about undefined: Variables in Javascript are of a weak type. Therefore, when declaring a variable, you only need to use the var keyword. If a strong-type language like C is used to declare a variable

/*** Undefined==null Returns True* undefined = = = NULL returns FALSE* Undefined indicates that the variable is defined but not assigned* NULL indicates a reference to an empty object*/function typeExt.onready (function () {functi

1. Undefined and null are automatically converted to false in the Boolean unary count.2. Null = = Undefined;//trueNull = = Undefined;//false2. typeof Null;//objecttypeof undefined;//und

first talk about undefined:The variables in JavaScript are weakly typed, so just use the var keyword when declaring a variable. If it is a strongly typed language like C, if you do not specify the initial value when declaring a variable, then give him a default value, such as the default value of the int variable is 0. But in a weakly typed language like JavaScript, there is no way to determine what the def

Let's talk about undefined: The variables in JavaScript are weakly typed, so you need to use the VAR keyword when declaring variables. If it is a strongly typed language like C, declaring the variable without specifying an initial value will give him a default value, such as the default value of the int variable is 0. But in a weak type language such as JavaScript