javascript check if not null or undefined

supported down to like ie5, why is this not more well known!? You might want to check if a particular global variable representing a piece of functionality has already been defined. With the July 2021 Chrome for Windows (Version 92.0.4515.107), I tried: if ( myVar === undefined ), if ( myVar === 'undefined' ), if ( myVar === void 0), or if ( !myVar ) All failed! Could you please explain? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. An example of the operator is shown below: This will ensure that the variable will be assigned to an empty string (or any other default value) if some_variable is null or undefined. And the meme just sums it all . With this we can now use the datatype to check undefined for all types of data as we saw above. In order to understand, Let's analyze what will be the value return by the Javascript Engine when converting undefined , null and ''(An empty string also). I think the most efficient way to test for "value is null or undefined" is. There is no simple whiplash solution in native core JavaScript it has to be adopted. It becomes defined only when you assign some value to it. Mathias: using strict or non-strict comparison here is a matter of personal taste. We are frequently using the following code pattern in our JavaScript code. In JavaScript, one of the everyday tasks while validating data is to ensure that a variable, meant to be string, obtains a valid value. Note: We cannot use the typeof operator for null as it returns object. console.log("String is null"); This would return a false while bleh has not been declared AND assigned a value. Test for null. Very obvious and easy to maintain code. It's also pretty much the shortest. 0 is a reasonable value in a lot of cases, that's why the word reasonable is wrapped with quotes :). How do you check for an empty string in JavaScript? Testing nullity (if (value == null)) or non-nullity (if (value != null)) is less verbose than testing the definition status of a variable. Those two are the following. How do I check if an element is hidden in jQuery? The . Consider this example: But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. How can I check if a variable exist in JavaScript? b is defined as a null-value. We have seen the nullish coalescing operator is really useful when you only care about the null or undefined value for any variable. Although it does require you to put your code inside a function. We need edge case solution. But we would replace undefined with void(0) or void 0 as seen below: In this article, we learned how to check if a variable is undefined and what causes a variable to be undefined. We cannot use the typeof operator for null as it returns an object. How to create an image element dynamically using JavaScript ? How do you check for an empty string in JavaScript? jsperf.com/type-of-undefined-vs-undefined/6, developer.mozilla.org/en/Core_Javascript_1.5_Reference/, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined, How Intuit democratizes AI development across teams through reusability. This Is Why. You can see all are converting to false , means All these three are assuming lack of existence by javascript. In practice, most of the null and undefined values arise from human error during programming, and these two go together in most cases. When checking for one - we typically check for the other as well. In the code given below, a variable is checked if it is equivalent to null. How to add an object to an array in JavaScript ? Both will always work, and neither is more correct. Is it correct to use "the" before "materials used in making buildings are"? Similar to what you have, you could do something like, if (some_variable === undefined || some_variable === null) { Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, good point ;) But let's say I know it can't be false or 0 and I just want to check whether I can use it in some logic (as a string, array, etc.). It's been nearly five years since this post was first made, and JavaScript has come a long way. About Us. Get started, freeCodeCamp is a donor-supported tax-exempt 501(c)(3) charity organization (United States Federal Tax Identification Number: 82-0779546). In the following example, we have one global variable and upon click of a button, we will check if the variable is not null or undefined and display the result on the screen. It looks like this: Also, when you try accessing values in, for example, an array or object that doesnt exist, it will throw undefined. This uses the ?? Cool. How do you access the matched groups in a JavaScript regular expression? We've had a silent failure and might spend time on a false trail. How can I determine if a variable is 'undefined' or 'null'? typeof is a language statement, it cannot be redefined any more than if/else/while/for/function etc. Use the in operator for a more robust check. How to Replace a value if null or undefined in JavaScript? Like it. This way will evaluate to true when myVar is null, but it will also get executed when myVar is any of these:. javascript; npm; phaser-framework; Share. Recovering from a blunder I made while emailing a professor. So sad. STEP 1 We declare a variable called q and set it to null. [], but will work for false and "". This is compatible with newer and older browsers, alike, and cannot be overwritten like window.undefined can in some cases. So, if we use ===, we have to check for both undefined and null. this answer being circa 2019 has a dearth of upvotes but it's the KISS one for this use case. What is the most appropriate way to test if a variable is undefined in JavaScript? How to react to a students panic attack in an oral exam? How do I include a JavaScript file in another JavaScript file? A difference of 3.4 nanoseconds is nothing. Topological invariance of rational Pontrjagin classes for non-compact spaces. If you pass any non-empty string then it will pass the test which is wrong, so for that we have to use Method 2 but to understand Method 2, you should try & test Method 1. let devices = [ 'keyboard', 'laptop', 'desktop', 'speakers', 'mouse' ]; On the above example, we have . Ltd. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. Another vital thing to know is that string presents zero or more characters written in quotes. At present, it seems that the simple val == null test gives the best performance. == '' does not work for, e.g. This is because null == undefined evaluates to true. Be careful with nullables, depending on your JavaScript version. Moreover, testing if (value) (or if( obj.property)) to ensure the existence of your variable (or object property) fails if it is defined with a boolean false value. ), How to handle a hobby that makes income in US. In our example, we will describe more than one example to understand them easily. Running another early check through include makes early exit if not met. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2. how to check document.getElementbyId contains null or undefined value in it? It could depend on whether your default position is to always use strict comparison unless specifically requiring type coercion (as recommended by Crockford, for example) or whether you prefer to use non-strict comparison except when strictness is required. ; This is a bit contentious, but I would generally advise typeof undefined over "undefined". We also have an interactive JavaScript course where you can learn JavaScript from basics to advanced and get certified. In repeating the tests in the original post, I found no consistent difference between the following test methods: Even when I modified the tests to prevent Chrome from optimizing them away, the differences were insignificant. I like it anyway. It's redundant in most cases (and less readable). You'd have to do, It doesn't work! [duplicate], How to check a not-defined variable in JavaScript, How to handle 'undefined' in JavaScript [duplicate]. Type checking and string comparison are much slower in some browsers, so if you do it a lot in a tight loop you will lose some performance. optional chaining operator will short-circuit and return undefined if data is nullish (null or undefined) which will evaluate to false in the if expression. @matt Its shorthand. Since a is undefined, this results in: Though, we don't really know which one of these is it. For instance - imagine you made a typo, and accidentally input somevariable instead of someVariable in the if clause: Here, we're trying to check whether someVariable is null or undefined, and it isn't. Why are trials on "Law & Order" in the New York Supreme Court? Below simple || covers the edge case if first condition is not satisfied. You'll typically assign a value to a variable after you declare it, but this is not always the case. In this article, you will learn the various methods and approaches you can use to know if a variable is undefined in JavaScript. Difference between var and let in JavaScript, Convert a string to an integer in JavaScript. In this article, we will discuss how to determine if a JavaScript variable is undefined or null. How to Use the JavaScript Fetch API to Get Data? (In many cases, you can simply read the code O_o). In this short guide, we've taken a look at how to check if a variable is null, undefined or nil in JavaScript, using the ==, === and typeof operators, noting the pros and cons of each approach. It this is attribute - then it works, example: How can I check for "undefined" in JavaScript? The typeof operator determines the type of variables and values. How to get value of selected radio button using JavaScript ? Therefore. Super expression must either be null or a function, not undefined. Test whether a variable is null. if (!emptyStr) { conditions = [predefined set] - [to be excluded set] I've a variable name, but I am not sure if it is declared somewhere or not. Hence, you can check the undefined value using typeof operator. From Mozilla's documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined, I see this: One reason to use typeof() is that it does not throw an error if the variable has not been defined. acknowledge that you have read and understood our, Data Structure & Algorithm Classes (Live), Data Structure & Algorithm-Self Paced(C++/JAVA), Android App Development with Kotlin(Live), Full Stack Development with React & Node JS(Live), GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam. When we code empty in essence could mean any one of the following given the circumstances; In real life situation as OP stated we may wish to test them all or at times we may only wish to test for limited set of conditions. Open the Developer tools in your browser and just try the code shown in the below image. As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable. In this article, we discussed how to use a hook and the typeof operator to determine whether a JavaScript variable is undefined or null. How is Jesus " " (Luke 1:32 NAS28) different from a prophet (, Luke 1:76 NAS28)? Follow Up: struct sockaddr storage initialization by network format-string. As such, I'd now recommend abc === undefined for clarity. If you really care, you can read about this subject on its own.). Sometimes you don't even have to check the type. Properties of objects aren't subject to the same conditions. As you might know, the variables that you define globally tend to get added to the windows object. @Anurag, you're welcome, since you talk about ES5, maybe is worth mentioning that. Lodash and other helper libraries have the same function. I independently thought of this solution, however yours has a bug: Due to the operator precedence it is actually equivalent to. Complete Data Science Program(Live) Mastering Data Analytics; School Courses. It becomes defined only when you assign some value to it. However, due to the typo, somevariable is checked instead, and the result is: In a more complex scenario - it might be harder to spot this typo than in this one. @J-P: The semicolon after the closing brace is just an empty statement. This condition will check the exact value of the variable whether it is null or not. Oh, now I got what you mean; your comment is misleading because was looking like related to the correctness of the code. I often test for truthy value and also for empty spaces in the string: If you have a string consisting of one or more empty spaces it will evaluate to true. How do I check for an empty/undefined/null string in JavaScript? However, Lodash is already present in many projects - it's a widely used library, and when it's already present, there's no efficiency loss with using a couple of the methods it provides. The . I know this. JavaScript in Plain English. In this case, if someObject.someMember doesn't hold a value, then it will be assigned the value of null. If it is, then we step inside the code block. ReferenceError: value is not defined. There's a common idiom based on this fact: which means "if obj has the property prop, assign it to value, otherwise assign the default value defautValue". 'xyz' in window or 'xyz' in self are much better, @JamiePate: Just to be clear, I disagree that. Parewa Labs Pvt. (typeof value === "string" && value.length > 0); } This checks if the type of the value is "string" (and thus non-null and not undefined), and if it is not empty. Also. No assignment was done and it's fully unclear what it should or could be. How to check whether a string contains a substring in JavaScript? This will create a bug in your code when 0 is a valid value in your expected result. By using typescript compiler tcs we transpile typescript code to javascript and then run the javascript file. Why is this sentence from The Great Gatsby grammatical? In ES5 or ES6 if you need check it several times you cand do: for example I copy vladernn's answer to test, u can just click button "Copy snippets to answer" to test too . You will miss NaN, 0, "" and false cause they are not null nor undefined but false as well. You can add more checks, like empty string for example. Thanks. To learn more, see our tips on writing great answers. 2968 Is there a standard function to check for null, undefined, or blank variables in JavaScript? There are two approaches you can opt for when checking whether a variable is undefined or null in vanilla JavaScript. Null is often retrieved in a place where an object can be expected, but no object is relevant. Has 90% of ice around Antarctica disappeared in less than a decade? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Both values can be easily distinguished by using the strict comparison operator. On the other hand, this can make typeof silently fail and signal that the incoming value might have an issue, instead of raising an error that makes it clear you're dealing with a non-existing variable. Also, like the typeof approach, this technique can "detect" undeclared variables: But both these techniques leak in their abstraction. On the other hand, a is quite literally nothing. Unlike null, you cannot do this. }, How to Check for Empty/Undefined/Null String in JavaScript. whatever yyy is undefined or null, it will return true. If my_var is " (empty string) then the condition will execute. Coordinating state and keeping components in sync can be tricky. There are numerous ways to check if a variable is not null or undefined. We accomplish this by creating thousands of videos, articles, and interactive coding lessons - all freely available to the public. Good to see you added the quotes now. For JavaScript, check null or undefined values can be pointed out by using == but not for falsy values. The typeof operator for undefined value returns undefined. Our website specializes in programming languages. @Andreas Kberle is right. Whenever you create a variable and do not assign any value to it, by default it is always undefined. Loose equality may lead to unexpected results, and behaves differently in this context from the strict equality operator: Note: This shouldn't be taken as proof that null and undefined are the same. could be. }, let emptyStr = ""; What is the point of Thrower's Bandolier? Why do many companies reject expired SSL certificates as bugs in bug bounties? #LearnByDoing Strict equality checks (===) should be used in favor of ==. How to Check if Variable is Not Null or Undefined in Javascript, How to Check if a Variable is Null or Undefined in Javascript, How to Check if Variable is Null or Empty or Undefined in Javascript, How to Check if Variable is Undefined or Null in Javascript, How to Check if Variable is Defined and Not Null in Javascript, How to Check if a Variable is Undefined in Javascript, How to Insert Dash After Every Character in Input in Javascript, How to Insert Dash After Every 2nd Character in Input in Javascript, How to Insert Dash After Every 3rd character in Input in Javascript, How to Add Space After Every 4th Character in Input in Javascript, How to Insert Space After Every 4th Character in Input in Javascript, We have done some basic styling using CSS and added the link to our, We have also included our javascript file, In the event handler function, we are using. What is a word for the arcane equivalent of a monastery? In JavaScript, null is treated as an object. NaN is a value representing Not-A-Number and results from an invalid mathematical operation.