javascript check undefined or null

Null and undefined comparisonsNull is a JavaScript keyword that represents a non-object.Undefined indicates that there is no value, indicating that the value does not exist or is not initialized. Same point Different points Own type unique value Null is an object,

When discussing the raw data types in JavaScript, most people know the basics, from String,number to Boolean. These primitive types are fairly simple and behave in a common sense. However, this article will focus more on the unique original data types null and undefined, what makes them so similar, but specious. I. Understanding

value instead of undefined.In general, if you need to specify a constant value for a variable or attribute, pass it to a function, or return null from a function, null is almost always the best choice. In short, JavaScript uses undefined and programmers should use null.Another feasible use case of

make the page does not jump. In href= "javascript:void (0)", a URI that begins with a javascript: protocol is used, and the browser defaults to the code after the colon, and then displays the result on the new page, except in one case, if the result is undefined, The browser will not refresh the page rendering new values. In fact, there are a number of ways to achieve the effect of a Hack, a collection t

"; null indicates that it does not exist.Console.log (NaN);//print "NaN"; it is a concept within the number type. When two numbers are not actually numbers. That is, the non-numeric not a number. Nan. For example, 0/0. This is meaningless. So it's Nan .Console.log (undefined);//print "undefined"; indicates that the variable is declared in. But I don't know what t

JavaScript, null and undefined were once confusing. The following analysis helps you to recognize it more clearly (or to make you more confused):-NULL is a keyword; undefined is a property of the global object-NULL is an object (

This article mainly introduces undefined in JavaScript. If you want to use a variable that is not declared, you can also use a variable that has been declared but has no value, or use an object attribute that does not exist, the returned value is this value. Although undefined and null are not the same, the = Operat

Objective I'm sure you all know that when you declare a variable and do not assign a value, its initial value is undefined. But in JavaScript, how do you check if a value is undefined? In short, in modern browsers, you can safely compare whether a variable is undefined

TypeOf is just parsing the variable age, which is equivalent to the JavaScript engine parsing when it finds that there is not a time, but JavaScript also returns undefined. So when typeof is; When the JavaScript engine returns undefined;2. UNLL Understanding2.1UNLL//My unde