javascript check undefined or null

JavaScript itself does not determine whether a variable is a null function, because the variable may be string,object,number,boolean and other types, different types, and the judging method is different. So in the article wrote a function, to determine whether the JS variable null value, if it is undefined,

Null represents an object, or the value of a variable is null, undifined means that an object (variable) is declared, not initialized to him, or the object (variable) is not declared at all.1, NULL is the JavaScript keyword undifined not.2, typeof return string, there are six possible: "Number", "string", "Boolean", "O

This article describes how to use JavaScript to check whether a pop-up window is blocked. whether the pop-up window is blocked or not. if you need it, you can refer to the pop-up window below. a pop-up window is required. Programmers who develop such websites have a difficult problem. they do not know whether these pop-up windows have been blocked by browsers or various browser plug-ins. Of course, the brow

This article describes how to use JavaScript to check whether a pop-up window is blocked. whether the pop-up window is blocked or not. If you need it, you can refer to the pop-up window below. A pop-up window is required. Programmers who develop such websites have a difficult problem. They do not know whether these pop-up windows have been blocked by browsers or various browser plug-ins. Of course, the brow

JavaScript checks whether the textarea value is null and provides the corresponding prompt, javascripttextarea In webpage design, JavaScript is often used to determine whether a user inputs valid data. If the user does not input data, a prompt is displayed. How can this problem be determined? This section describes how to use

This article introduces a blog written by a foreign user about how to select attributes of the check box, I hope my friends will like it. Anyone familiar with web front-end development knows that checking whether the check box is selected is a frequent task and there are many judgment methods. However, the compatibility of these methods is often ignored during development, but the effect is good. Many metho

This article introduces a blog written by a foreign user about how to select attributes of the check box, I hope my friends will like it. Anyone familiar with web Front-end development knows that checking whether the check box is selected is a frequent task and there are many judgment methods. However, the compatibility of these methods is often ignored during development, but the effect is good. Many metho

This article describes how to check the existence of object property in JavaScript. This article describes four methods to check whether an object o has propertyx. For more information, see in JavaScript, you can use four methods to check whether an object o has property x:

field In this process, a series of events are fired, using these events to find the time to validate the data 1 when the input field is selected – Fires the onfocus event (Focus) 2 when leaving the input field – fires the onblur event (leaving the focus) 3 Leave the field and the input content changes – fires the onchange event The most correct choice is to validate the data when the onblur event is fired First step of validation: Check that th

This article describes how to check the existence of Object property in JavaScript. This article describes four methods to check whether an object o has propertyx. For more information, see in JavaScript, you can use four methods to check whether an object o has property x:

Observe the following PHP code: If (''= false) echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL;If ('') echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL; If ('0' = false) echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL;If ('0') echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL; If (array () = false) echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL;If (array () echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL; If (array (0) = false) echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL;If (array (0) echo 'OK'. PHP_EOL;Else echo 'no'. PHP_EOL;

When js obtains null objects, many of my friends may have encountered this problem. As long as JavaScript parses strings differently from the log strings, this is the dynamic output data on the page. This is the data obtained by ajax. I want to display it on the page. The Code is as follows: Success: function (data ){Var arr = data. split ('_');Var numArr = arr [0]. split (",");Var numStr = "";If (numArr.

The following small series will bring you an article to check the brackets in the javascript regular expression ]. I think the summary is good. I want to share it with you. I hope to give you a reference. When using regular expressions in javascript, you need to pay attention to a pitfall in brackets, that is, the metacharacters in brackets. I stepped on the trap

In Javascript, the common way to check whether a variable exists is if (variable ){.... // variable exists} else {.... // variable does not exist}. When variable is undefined | null, that is, the variable does not exist. However, if variable is 0, this code will run... syntaxHighli In

　 Function overload and type check A common feature of other object-oriented languages (such as Java) is the ability to "overload" functions: pass them to different numbers or types of parameters, and the functions perform different operations. Although this capability is not directly available in JavaScript, the provision of some tools makes this exploration completely possible.Each function in

Check box for JavaScript operation checkbox, javascriptcheckbox Similar to radio, JavaScript uses the checked attribute of element items. First, get the checkbox Element Set, traverse the set, and perform operations on each item in the set. Here are some common examples of commonly used checkbox check boxes. Value

This article mainly introduces the JavaScript to check whether pop-up windows are blocked method tips, this article is used to detect window.open pop-up window is blocked, the need for friends can refer to the The exit box is useful and requires a pop-up window. For programmers who develop such a site, they have a tricky problem, and they don't know if the pop-ups are blocked by browsers or various browse

The exit box is useful and requires a pop-up window. For programmers who develop such a site, they have a tricky problem, and they don't know if the pop-ups are blocked by browsers or various browser plug-ins, not popping out. Of course, the browser notifies the user, but these rarely cause the user's attention. Here's an easy way to test whether your pop-up window is blocked. The JavaScript Copy Code code as follows: var windowname = ' Us

This article mainly introduces the JavaScript static type check tool FLOW introduction. if you need it, you can refer to the static type check tool developed by Facebook for JavaScript. Its code is hosted on github and complies with the BSD open-source protocol. About Flow It helps us capture common errors in

Introduction to the JavaScript static type check tool FLOW, javascriptflow Flow is a static type check tool developed by Facebook for JavaScript. Its code is hosted on github and complies with the BSD open-source protocol. About Flow It helps us capture Common Errors in JavaScript