The non-null assertion operator symbol is an Exclamatory symbol(!) ), use a non-null assertion operator, the symbol is Exclamatory Mark(! For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can and appended to a variable to avoid null or undefined. However, what if the value of myStr is 0, would the check 0 == '' return false too? To do that we assign the literal null to the variable. 2021 Copyrights. See how TypeScript improves day to day working with JavaScript with minimal additional syntax. This tells the typescript compiler that the variable should never be null or undefined. Contributed on Mar 02 2020 . In TypeScript, optional chaining is defined as the ability to immediately stop running an expression if a part of it evaluates to either null or undefined.It was introduced in TypeScript 3.7 with the ?. Below is a TypeScript Optional<T> class modeled loosely after Java's Optional class. So for the checking, if the variable is undefined or not at the root level, typeof is suggested. thanks a lot. There are several ways to check for an empty string. Fact is you will need to deal with both. declare const loggedInUsername: string; Typescript component, Written a function for checking null or undefined, or empty using the typeOf operator It is check for undefined values and returns true if it is null isNameCheck (): boolean { if (typeof this.stringValue != 'undefined' && this.stringValue) { return false; } return true; } Here is a complete Angular component example TypeScript is super set of JavaScript. In JavaScript undefined and nullvalues are hard to check. The problem is with the typing of the isNullOrUndefined function. To avoid this, Use one of the approaches before accessing the variableif(typeof ez_ad_units!='undefined'){ez_ad_units.push([[320,50],'cloudhadoop_com-medrectangle-3','ezslot_1',117,'0','0'])};__ez_fad_position('div-gpt-ad-cloudhadoop_com-medrectangle-3-0');if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[320,50],'cloudhadoop_com-medrectangle-3','ezslot_2',117,'0','1'])};__ez_fad_position('div-gpt-ad-cloudhadoop_com-medrectangle-3-0_1');.medrectangle-3-multi-117{border:none!important;display:block!important;float:none!important;line-height:0;margin-bottom:7px!important;margin-left:0!important;margin-right:0!important;margin-top:7px!important;max-width:100%!important;min-height:50px;padding:0;text-align:center!important}. Method #5: Use ?? . Hi there! In the case of dealing with an empty string, JavaScript will convert it into 0 when checking against another number. Typescript automatically constrains the type of value in the if branch to T. In the if statement, we check if the variable is not null and is not undefined. There are different solutions presented online for such a simple task, yet they might fail to only check whether a variable is an empty string or not. The rest of this page applies for when strictNullChecks is enabled. Typescript doesn't understand the concept of: "If this variable is undefined, it means that the other one is not." To Typescript this just means the both of the variables can be undefined. The second example failed because the arrays find function looks a bit like this simplification: How to provide types to functions in JavaScript, How to provide a type shape to JavaScript objects, How TypeScript infers types based on runtime behavior, How to create and type JavaScript variables, An overview of building a TypeScript web app, All the configuration options for a project, How to provide types to JavaScript ES6 classes, Made with in Redmond, Boston, SF & Dublin. We need to do set it explicitly. We use two if-else statements to check whether those variables equal the undefined value and print a message accordingly. If the string is empty, the boolean returned will be false. The learned object is possibly null or undefined in typescript with solutions. It all has to do with double equals == being an abstract equality comparison. The following example declares the variable foo. There is a way to shorten this comparison, thanks to the fact that the undefined value is falsy in TypeScript. When strictNullChecks is false, null and undefined are effectively ignored by the language. Another issue you might encounter is that process.env[variable] will always return string | undefined even though you checked in on a previous line. Twitter, We have not given it any initial value. Because null == undefined is true, the above code will catch both null and undefined. By default, it gets the value undefined. Hacker News. What causes error TS2533: Object is possibly null or undefined"? Whenever we declare a variable without initializing it with a value, TypeScript initializes it as undefined. When we use an if statement (or equivalent) to check its existence, product no longer has the undefined type. to Cast to Boolean, Use Double Equals (==) to Compare Against an Empty String, Use Triple Equals (===) to Compare Against an Empty String, comparison with the double equal, I recommend reading this article. you . Douglas Crockford said that writing could be seen as a mistake is not allowed. Typescript converted to Javascript, Before compiling, It found that the object or property value is null or undefined, Typescript compiler throws an error when an object or its property is possibly null. When you use the condition like this if (value !== null && value !== undefined) { results.push (value); // value is only T here it is used as a type constrain. 2. I'd highly appreciate that. TypeScript has two special types, Null and Undefined, that have the values null and undefined respectively. At this point, the only thing to be aware of is the possible presence of an empty space for the string variable myStr, a common problem faced in all the alternative solutions previously mentioned. It is a compile-time error for an object possibly is null after setting the strictNullChecks=true compiler configuration flag. This error caused due to an object property might result in null or undefined. How to Check for Empty String in JavaScript/TypeScript April 12, 2022 by Andres Reales Verifying whether a variable is an empty string is a common task that at some point developers have to do. Therefore, getting a false value from converting a string to a boolean can be a quick way to tell whether a string is empty. You generally don't want to make a distinction between the two. Andrs Reales is the founder of Become a Better Programmer blogs and tutorials and Senior Full-Stack Software Engineer. class Example { doSomething(): void { const username = 'David' as string | undefined; // add ! But TypeScript didnt gave any type error because in TypeScript, null and undefined can be assigned to any type. TypeScript 3.7 Optional Chaining Playground Optional chaining is issue #16 on our issue tracker. By using simple if condition. We have to handle null or undefined checks using a variable in if the expressionif(typeof ez_ad_units!='undefined'){ez_ad_units.push([[320,50],'cloudhadoop_com-large-leaderboard-2','ezslot_11',123,'0','0'])};__ez_fad_position('div-gpt-ad-cloudhadoop_com-large-leaderboard-2-0'); This checks for string objects that exist or null and undefined values. Checking if a variable has an empty string by using the triple equals === (strict equality operator) to compare against an empty string is also a similar solution when compared to using double equals ==. However, in the case 0 == '' we are referring to 0 == 0 after the empty string is converted into a number. By default null and undefined handling is disabled, and can be enabled by setting strictNullChecks to true. Our website specializes in programming languages. The value null represents the intentional absence of any object value. The value undefined means value is not assigned & you don't know its value. As long as you are dealing only with checks between the same type, in this case, comparing a string with another string, the solution of using double equals should be ok if you are looking to check for empty strings, unless the variable myStr could have empty spaces and that is categorized as an empty string as part of your business logic. I hope this article helped you find a definite answer to such a common and simple task as checking for empty strings. In TypeScript, you assign types to variables and function parameters to catch type related errors early, even before you run the code. To check array is null or not in typeScript, use if.else statement it will handle all falsy value like undefined, null, empty and etc. What is Null Value in TypeScript. typescript null and undefined check. 3 ## String. Method 1: Use Simple If Condition. A Computer Science portal for geeks. Refresh the page, check Medium 's site status, or find something interesting to read. Whether you use JavaScript or TypeScript to verify a variable is an empty string ensure the type of the variable is a string and verify the string has a length of zero after removing any white spaces the string could have. The value null represents the intentional absence of any object value. We and our partners use cookies toStore and/or access information on a device.We and our partners use data forPersonalised ads and content, ad and content measurement, audience insights and product development.An example of data being processed may be a unique identifier stored in a cookie. So, the following is also perfectly fine as per TypeScripts type checker. @andy-ms Thanks for the quick reply! at the end, you let the TypeScript compiler that there is no way this variable will be undefined or null. In undefined, you can't tell the difference between "not defined yet" and "not explicitly"! So, the only way to really validate number type to check if they are null or undefined is to use 1) == null or 2) == undefined. Hence, the output from this check is true. Also, if myStr has empty spaces, casting the string to a boolean will have an output of false. Another alternative solution is to use double exclamation marks to cast a string value to a boolean. When "strictNullChecks" is set to false, TypeScript will ignore the undefined and null values. | by Aleksei Jegorov | Dev Genius 500 Apologies, but something went wrong on our end. The result from comparing 0 == '' would betrue. While the conversion still returns false, we are not truly checking if the variable has an empty string. As TypeScript is a statically typed language, it allows users in disabling type control by using unknown and any types. Remember, TypeScript is a superset of JavaScript. Upon adding this option, TypeScripts compiler will start showing typing errors because null and undefined are no longer valid values for any type. It represents nothing or no value. The problem is if the value of the string is "0" or "true" than it will goes in the if . It is an intentional absence of value. may be used to assert that its operand cannot be null or undefined during runtime. You'll get a notification every time a post gets published here. The value null indicates that you know that the field does not have a value. For context, there have been over 23,000 issues on the TypeScript issue tracker since then. null means we know that the variable does not have any value. We used it right after the address property, so we are telling TypeScript that emp.address will never have a value of null or undefined. By using this solution, we dont have to worry about when myStr value is undefined or null. 2. This is the problem: "any = can be anything including null or undefined but will NEVER checks for it" Consider this code: get_db_entry(query, parseEntry); function processEntry(err: any, entry: any) { entry['someKey'] = []; // ^ entry could very well be undefined here, typescript don't care } This is how we would like it to look: By using Array.include() function. Since this check can be a repetitive task, it is recommended, although not required, to generate a utility function. I think it should be baked into the language service itself being that it's less of a coding pattern and more of a coding contract feature like their example TypeScript won't let you compare '1 === 2'. Note: In this article, we are going to constantly refer to JavaScript rather than TypeScript. We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript. Popularity 10/10 Helpfulness 3/10 Source: stackoverflow.com. Once again, depending on the business logic, casting a string to a boolean to check for an empty string can be an ok solution as long as you understand all the possible scenarios that can happen when using this solution. Explore how TypeScript extends JavaScript to add more safety and tooling. How To Install (Upgrade or Downgrade) a TypeScript Version? Other Solutions (Be Aware Of Unexpected Behaviors), Use Double Exclamation Marks (!!) But TypeScript doesnt warn you when you assign null and undefined to the variables of common types. Ive written this simple function in TypeScript that accepts its only parameter url which is of type string. an expression evaluated to false for the below values In other words, it is JavaScript with type definition. the purpose of answering questions, errors, examples in the programming process. But TypeScript never assigns null to any variable. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Having said that, if the value of myStr variable is either undefined or null , the result of casting the string to a value will be false. nullish coalescing operator validateToken (token ?? Facebook, used for variables that tell the compiler that variables are optional. ), use nullish coalescing operators in the latest javascript. When you use this approach, you basically tell TypeScript that this value will never be null or undefined. But TypeScript still complains about the last statement. 1. Previously it was not possible to explicitly name these types, but null and undefined may now be used as type names regardless of type checking mode.. It is an unintentional absence of value. The condition uses to check null or undefined values in javascript. Copy 1arr.length This will return 0, as there are 0 items in the array. If both conditions are met, TypeScript knows that the variable is a string and allows us to use string-specific methods like toLowerCase (). Interestingly in JavaScript with ==, null and undefined are only equal to each other: Recommend == null to check for both undefined or null. - > What do you want to do about it? Example Fix for Object is possibly null or undefined, disabling Compiler Options strictNullChecks i.e false to avoid this error, Manually check for undefined using the if condition type guard, use Optional Chaining Operator(Question Mark(? This new behavior of non-nullable types is actually opt-in TypeScript version 2 and above, so we need to go into our tsconfig.json file to enable it. 3 . a === b , as the triple equals wont convert the values to a common type. Another situation that can cause issues is when there are empty spaces in the string. Comment . Once again, make sure to remove all empty spaces to verify if there is a character in myStr or rather it is an empty string. Angular 14 innerHtml example with property binding and event handler, Angular 15 Decimal Pipe examples | rounded number examples, Angular 15 material divider tutorial| mat-divider examples. Therefore, if in the comparison a == b , a is a number and b is a string, JavaScript will attempt to convert the string b to a numeric value. But the next expect already ensure it is not null. At its core, optional chaining lets us write code where TypeScript can immediately stop running some expressions if we run into a null or undefined . For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will: ts. if(expression) { console.log ('Not null')} } the expression must be evaluated to be true or false. 4. In this case, You can use and if conditional expression to avoid a null or undefined check. How to Split a String in Rust? Example Solution 5: Use logical OR (||) Summary This operator can be used where the compiler is unable to check that a variable cannot be null/undefined. All rights reserved. Optional chaining is often used together with nullish coalescing, which is the ability to fall back to a default value when the primary expression evaluates to null or undefined. Doing the comparison using double equals == wouldnt work at all times. If you like what I write and want me to continue doing the same, I would like you buy me some coffees. . Otherwise, if myStr is undefined or null, attempting to convert the string using the Boolean object will lead to a false value. // arg must be a string as `!=` rules out both null and undefined. Hence, all JavaScript concepts will also apply when using TypeScript. // Object is possibly 'null' or 'undefined'. Share it on Social Media. [Last Updated: Oct 27, 2018] Previous Page Next Page The post-fix expression operator ! With this method, we will . The consent submitted will only be used for data processing originating from this website. Using a juggling-check, you can test both null and undefined in one hit: if (x == null) { If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables: if (x === null) { You can try this with various values using this example: Otherwise, if there is an empty string, the boolean generated will be false. I write articles about all things web development. For instance, if the value of myStr is 1 , then 1 == '' is false. We use cookies to ensure that we give you the best experience on our website. Typescript compiler throws an error when accessing an object or its property values and possibly these values are undefined. Using TypeScript, 3 places where the exclamation mark operator appears. Typescript is a superset of javascript with an extra feature, typing, and assertions, Every piece of code works in typescripts. Share: Copy The function can now gracefully accepts null and undefined and returns values accordingly. In this solution, we wont have to worry about JavaScript converting the type of the operands a and b, i.e. Join 6,000 subscribers and get a daily digest of full stack tutorials delivered to your inbox directly.No spam ever. By using TypeScript Nullish Coalescing & Optional chaining. The Elvis Operator The Elvis operator is a shorthand way to check for null or undefined without requiring a separate if statement. It represents nothing or no value. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Booleans can be either true or false. The first is when you declare a variable without giving it a value. (Read a sample. Now, if I change provide null instead of a valid string, compile and run the generated JS code, I would get the following error. I didn't see in the description where it warns for this behavior, but if it does I can see it being quite useful. Thats why 1 == '' is false , because what is actually happening is checking 1 == 0 , which are not the same numeric value. If the strictNullChecks flag is on will not allow the user to define types null and undefined. write code as though it will: Setting strictNullChecks to true will raise an error that you have not made a guarantee that the loggedInUser exists before trying to use it. Manage SettingsContinue with Recommended Cookies. NEW, Take full-size screenshots of websites without any tools in Chrome, PHP type declarations A guide for beginners, Handling images with inconsistent height in CSS, Convert any value to a boolean in JavaScript, Singleton routes for one-off resources in Laravel 9.x. With the purpose of helping others succeed in the always-evolving world of programming, Andrs gives back to the community by sharing his experiences and teaching his programming skillset gained over his years as a professional programmer. To fix our appendDomain function, we can add more types to the arguments to include null and undefined like so and checking if the provided value in the function is of type string explicitly like so. To make the conversion, pass a string parameter when calling theBoolean() object. (Explained with Examples). For example, in Angular applications, add this to angularCompilerOptions in tsconfig.json, In react typescript applications, open tsconfig.jsonand change strictNullChecks to false in compilerOptions. // When strictNullChecks: false the undefined is removed from the type system, // allowing you to write code which assumes it always found a result. It means that a variable has been declared but has not yet been assigned a value. Comment . In the below code, object str is defined with string array or undefined or null. Angular 14 How to load data before rendering the component? str property possibility of null or undefined values.if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[250,250],'cloudhadoop_com-box-4','ezslot_4',121,'0','0'])};__ez_fad_position('div-gpt-ad-cloudhadoop_com-box-4-0'); In the above code line (str.length), accessing the length of a string object causes this error. typescript string or undefined javascript if not null or empty js check null or empty string javascript check if object is empty or undefined check for null or undefined in javascript javascript if undefined then empty string check if object is empty or undefined javascript typescript non null assertion null and undefined in typescript console.log(obj?.hello) // logs null if the obj is null, and does not reference hello Popularity 10/10 Helpfulness 1/10 Source: . There are different ways we can check both null or undefined in TypeScript or Angular. Every time we pass truthy values to convert to a boolean, the output will be true . Object is possibly null or undefined at compile time as well as linting if configured. Gorgeous Gnat. ), nullish coalescing operators in the latest javascript, skip this error by disabling strictNullChecks i.e false, Manually check for null using the if condition typeguard, use Optional Chaining Operator, Symbol is Question Mark(? Get the latest articles delivered right to your inbox! When using !variablecheck then!undefined === true!null === true!false === true . if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[250,250],'cloudhadoop_com-medrectangle-4','ezslot_5',137,'0','0'])};__ez_fad_position('div-gpt-ad-cloudhadoop_com-medrectangle-4-0');To fix this, Use one of the approaches before accessing the variable. TypeScript. Depending on the business logic, empty spaces can be seen as an empty string leading to a flaw in the check if (!Boolean(myStr)) {}. In JavaScript, there are two main ways that a variable can be ' undefined '. The type checker previously considered null and undefined assignable to anything. The nullish coalescing operator symbol is ?? For instance, this can be Boolean("Hello World") or Boolean(myStr) where the variable myStr contains a string value. It removes null and undefined from a type without doing any explicit type checking. How do you fix the error object that is possibly undefined? What is the difference between section and div tag in html, VScode Solution for Code runner not supported or defined, Top 3 Input placeholder style examples in css/html, fix multiple formats for file Select a default formatted, gitignore git files in flutter and dart project, 5 ways to use Input blur event in Angular| Angular blur event tutorials. Lets look at each of the solutions and the things you must take into account if you decide to use any of the following: To convert a string to a boolean, we use the Boolean object. TypeScript is smart enough to know that by doing the simple existence check that it no longer undefined. There are different solutions presented online for such a simple task, yet they might fail to only check whether a variable is an empty string or not. Below is the code: const result: User|null = getResult (); expect (result).not.toBeNull (); expect (result.name).toBe ('Joey'); // typescript compiles `result` could be null here. 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Thankfully, in Typescript, there are several shorthand ways to deal with nulls. Typescript check if string is null or empty code example // Test whether strValue is empty or is None if (strValue) { //do something } // Test wheter strValue is empty, but not None if (strValue === "") { //do something } if (typeof propertyValue!='undefined' && propertyValue) { //execute code here } Thats because we passed an argument (null) for which JavaScript can not find concat method on it as its not a valid string. How can I determine if a variable is 'undefined' or 'null'? as below. index.ts console.log(null == undefined); 2 Answers Avg Quality 4/10 typescript null and undefined check. The third example uses loose equals ==, instead of strict equals ===, to check if the variable is equal to null or undefined. Sometimes the TypeScript compiler isn't able to determine what type of value it may at a certain point. By using typescript compiler tcs we transpile typescript code to javascript and then run the javascript file. Is there any merit in using null in the first place? However, depending on the business logic, the following solutions might or not be the best alternative to do the string check. Null- and undefined-aware types. undefined is a variable that refers to something that doesn't exist, and the variable isn't defined to be anything. The non-null assertion operator symbol is an Exclamatory symbol (!) So, for the above example, it will show the following error. This means that the logical OR (||) operator will return the value to the right if the value to the left is any of the aforementioned 6 falsy values. When strictNullChecks is false, null and undefined are effectively ignored by the language. When strictNullChecks is true, null and undefined have their own distinct types and youll get a type error if you try to use them where a concrete value is expected. The second example shows how to check if the variable is NOT null. Although a string made out of just empty spaces seems to be an empty string, technically empty spaces represent a truthy value in JavaScript. In strictNullChecks configuration, a parameter configured in tsconfig.json, if set to false, then null types are always a subtype of each other defined type. And we don't even have to specifically check for undefined. 'default-token'); This solution works in a similar way we used the Boolean object to convert a string into a boolean. Therefore, "strictNullChecks" should always be turned on (set to true). This means it could either be a number or even a NaN. 5 Comparing a string is producing the same value when undefined and null than the boolean and number. TypeScript does not set the value of a variable to null. What is Null Value in TypeScript . Whenever we convert a string to a boolean, it will return true as long as there is a value in the string. for check array is defined, we are able to use typeof operator or for check empty array we have to just use array.length syntax. It's only a Product type within the if statement. 0. To check more about the type conversions when doing a comparison with the double equal, I recommend reading this article. It is released in Typescript and used to return the right-hand operand when the left-hand operand is null or undefined. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page. Solution 2: Use an if expression to check. For instance, take the following example. and appended to a variable(variable?) The result can be either User type or null. It only calls the length method if a string object exists. Cheers! For example, Let's see short example to use if.else with array to check a array is null in typeScript. typescript check if null or undefined or empty string ts string is null or empty typescript string is not null or empty string is null or empty ts check null empty "" in javacript typescript check if string is not null or undefined or empty how to check whether a string is empty or not in typescript angular check null or empty But TypeScript doesn't warn you when you assign null and undefined to the variables of common types.. For instance, take the following example. null is a variable that is defined but is missing a value. However, there are a few points you must be aware of when using this solution. The second is when you try to access a variable that does not it is undefined . It looks something like this in practice. introduced in the latest Javascript ES2021. To make a variable null we must assign null value to it as by default in typescript unassigned values are termed undefined. How do you fix the possibly null error object? Now, this will work fine if you compile it down to the JavaScript file providing I have the following tsconfig.json file setup. Types null and undefined are primitive types and can be used like other types, such as string. Copy 1const arr: Array<Object> = []; Now we can check if the array is empty by using .length. To do that we assign the literal null to the variable. ), Auto implemented properties in TypeScript, Avoid relative import paths in TypeScript, Constructor assignment of field variables in TypeScript, Type annotate arguments for subset of a given type in TypeScript, Difference between Export and Export Default in TypeScript, Singleton routes for one-off resources in Laravel 9.x We need to do set it explicitly. There may be many shortcomings, please advise. This can lead to unexpected errors at runtime. Verifying whether a variable is an empty string is a common task that at some point developers have to do. when you add an exclamation mark after variable/property name, you're telling to TypeScript that you're certain that value is not null or undefined. The reason for the error "Type "undefined" is not assignable to type" in TypeScript Some solutions for this error Solution 1: Make them in sync type Solution 2: Use type assertion Solution 3: Use a conditional (ternary) operator Solution 4: Use nullish coalescing operator (??) This solution is not fixing an error, but skipping an error at compile time by setting the compiler flag. No Need Of Null Checks Anymore In Typescript | by Aakash Garg | Dev Genius Sign In Get started 500 Apologies, but something went wrong on our end. The Name type can be null, undefined or a string. The falsy values in JavaScript (and TypeScript) are undefined, null, 0 , false, "" (empty string), NaN (not a number). This means it will try to convert the two operands into the same type. Refresh the page, check Medium 's site status, or find something interesting to read. I'm Amit. Copyright Cloudhadoop.com 2022. Unsubscribe any time. and appended to a variable to avoid null or undefined. We have to assign Null to variable to make it null. Like this article? null means we know that the variable does not have any value. This solution has the same scenarios as the solution of converting a string to a boolean using the Boolean object that you must take into account, whether you are exclusively checking for a string or not. In TypeScript, you assign types to variables and function parameters to catch type related errors early, even before you run the code. The same code above works in Vue Typescript, and node typescript applications. TypeScript has a powerful system to deal with null or undefined values. However, this solution is not highly recommended since it can lead to other runtime errors after. If you want to, shouldn't you be more explicit and not in such a vague way? Unary Operator const a = !isNumber(input); Optional Parameter The language feature is called Non-null assertion operator. ), non-null assertion operator, the symbol is Exclamatory Mark(! In typescript applications, open tsconfig.json and set strictNullChecks to false. Therefore, attempting to cast a string to a boolean will lead to a true boolean value. All Rights Reserved. This behavior can be fixed by adding a compiler option called strictNullChecks and set it to true in tsconfig.json like so. How do I check for an empty/undefined/null string in JavaScript? The third argument is called when both first two arguments are not null, undefined nor . What if for any reason the variable myStr can have any value besides a regular string? It is common to find JavaScript developers implementing solutions, such as checking whether a variable is equal to a string, that is either partially correct or completely correct as it all comes down to what the business logic is. First, let's create a new array with no elements. You can use the qualities of the abstract equality operator to do this: if (variable == null) { // your code here. } Setting strictNullChecks to true, separates null type and other types. For instance, if we pass a variable myStr that is inferred to be a string to convert it to a Boolean, we must make sure there is a string value. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. In that scenario, there should be additional logic to remove all empty spaces from the string variable prior to doing the comparison. This checks for both null and undefined because when using loose equals ( == ), null is equal to undefined. Typescript: when to use null, undefined or empty array? This tells the typescript compiler that the variable should never be null or undefined let str: string [] | undefined|null let len: number = str!.length ; Fix with nullish coalescing operators . This feature is called "Non-null assertion operator", basically it means that when you add the exclamation mark after a property/value, you are telling TypeScript that you are certain that value is not null or undefined. Many people, including myself, have made logical mistakes as JavaScript might behave completely differently when compared to other programming languages, and it becomes challenging to determine what the error could be when the solution can be correct at the naked eye of a typical programmer. nullStop. This can lead to unexpected errors at runtime. We provide programming data of 20 most popular languages, hope to help you! operator.. By adding the exclamation mark (!) There are multiple solutions to avoid this. Checking if a variable has an empty string by using the double equals == (abstract equality operator) to compare against an empty string is a straightforward solution. TypeScript does not set the value of a variable to null . Share your thoughts by replying on Twitter of Become A Better Programmer or to my personal Twitter account. Learn more about typescript-nullsafe: package health score, popularity, security, maintenance, versions and more. If you continue to use this site we will assume that you are happy with it. Use the strictNullChecks flag to check the type of property. TypeScript - Non-Null Assertion Operator ! Optional Chaining is introduced in the typescript 3.7 version. The difference between the two is perhaps a bit more clear through code: let a; console .log (a); // undefined let b = null ; console .log (b); // null This can cause some serious issues if not handled properly. So we can simply use if condition to do null or undefined . The optional Chaining operator is ? Check Undefined in Typescript at Root Level If you use == at the root level for the undefined checking in TypeScript, and the variable is undefined, you get a ReferenceError exception and the whole call stack unwinds. Like values like 0, NaN, null, and false, undefined always gets coerced and evaluated like the false value. All we have to do to enable this behavior is to set "strictNullChecks": true (or rely on it being set via the umbrella "strict": true flag) in our tsconfig.json compilerOptions. 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