Types which are globally included in TypeScript. This one's applicable and useful in some cases and could possiblty be of some help. Want to check how much you know TypeScript? If you are starting a new project, you should definitely enable Strict Type Check Mode from the beginning. Array.some will return true is the conditional check is satisfied at least once. In TypeScript, every parameter is assumed to be required by the function. key local storage ReferenceError: localStorage is undefined. We're a place where coders share, stay up-to-date and grow their careers. It's speed, asynchronous capabilities and a set of amazing other features makes it an unbeatable candidate among all other languages out there. However, it's definitely good to know the basics of the JavaScript engine and see how it handles our human-friendly JS code, and turns it into something machines understand! SyntaxKind is a TypeScript enum which describes the kind of node. null means no value. Angular . For example, if you try to get the length of the result of translateNumber(0), which is undefined, JavaScript will throw this TypeError at runtime: Cannot read properties of undefined (reading 'length'). But how can you match the return type to all possible return values? It has confusing and contradictory usage: const a = new Array(2); // [undefined, undefined] const b = new Array(2, 3); // [2, 3]; Instead, always use bracket notation to initialize arrays, or from to initialize an Array with a Great blog post, strictNullChecks is a must if you want to avoid headaches in the future. I have categorized the possible solutions in sections for a clear and precise explanation. How to check for undefined in Typescript? of use and privacy policy. As you can see in the above example, speech is undefined, because the sayHi function internally returns undefined even if return type is void. As much as I like TypeScript, it sometime inherits of the bad side of JavaScript. That means anything declared with the type number could be null or undefined. For example, you write a function which will not return to its end point or always throws an exception. If you get the error type undefined is not assignable to type in ts, dont worry, it is a common error in TypeScript. code of conduct because it is harassing, offensive or spammy. This is the TypeScripts configuration file and you can read more about it here. As these methods return promises, they can be chained. What Gatsby v4 brings to your static site? You can either add a return statement so the function always returns a string (solution #1), or change the return type from string to string | undefined (solution #2). There is a way to shorten this comparison, TypeScript is now aware the return type does not match all possible return values, and this could lead to problems at runtime! Your experience on this site will be improved by allowing cookies. If no tsconfig.json is loaded from disk, ts-node will use the newest recommended defaults from @tsconfig/bases compatible with your node and typescript versions. Code This tutorial will help you to check an array is empty or not in typescript, here we will use array length to check array has value or not but before checking the array, we have to check the array is defined or not. For instance, we can assign null to a value of type string: let str: string = "test"; str = null; And you see the too classic cannot read property 'alpha' of undefined coming 'I always does something and never ends. That error message is telling youthe function is returning undefined implicitly, but its return type does not include undefined in it. Convert Existing JavaScript to TypeScript. TypeScript code must not use the Array() constructor, with or without new. toPromise ())?. Finally, to check for the existence of global variables, you can go with a simpler approach. In the if statement, we check if the variable is not null and is not undefined. Contents Code Examples ; how to check if object is undefined in typescript; Related Problems ; Strict Type Check Mode changes how TypeScript interprets undefined and null values. Angular TypeScript localStorage. operator, since the property can be undefined. Thus, never type is used to indicate the value that will never occur or return from a function. If the response doesn't have the expected format, we will get null instead of undefined error. // Uncaught TypeError: Cannot read properties of undefined (reading 'length'), // Solution #1: add a return statement so it always returns a string, // Solution #2: return type as string | undefined, Always Know When to Use Share vs. ShareReplay, How to Create Custom ESLint Rules (It's Not as Hard as You Think). This means null and undefined values are valid values for all types. A more succinct way to write this validation is to use the double bang (!!). Updated on Feb 28 This latest will also evaluate for null as well as undefined and also true/false. When type checking, take into account null and undefined.--strictPropertyInitialization: boolean. Online one-click setup. It doesn't look like it compiles, but it does. Improve this answer. When you choose a solution like Solution #2, TypeScript expects you to write code that handles possible nullish values. @ng-web-apis/common LOCAL_STORAGE AppServerModule providers @ng-web-apis/universal UNIVERSAL_LOCAL_STORAGE LOCAL_STORAGE localStorage , Angular Component PLATFORM_ID ngIf , Component structure Directive , Component , Angular Create better web applications. I will provide screenshots + code snippets. . You can use it to graphically design domain models, to leverage those models at design time by creating and editing dynamic instances, to collaborate via Eclipse's team support with facilities for comparing and merging models and model instances @ViewChild() supports names of template variables as string: @ViewChild('keywordsInput') keywordsInput; The type guard is definitively wrong, and yet, Typescript is OK. While using this site, you agree to have read and accepted our terms Here's the final solution you can try out in case no other solution was helpful to you. How To Check Form Is Dirty Before Leaving Page/Route In React Router v6? If you were to compile the following code again using Solution #2, TypeScript would throw the error Object is possibly 'undefined'. For instance, we write if (email === undefined) { } to check if the According to the data from caniuse , it should be supported by around 85% of the browsers(as of January 2021). Built on Forem the open source software that powers DEV and other inclusive communities. index.ts type Country = string | undefined; let country: Country; I It returns true or it returns false; it cannot return undefined. This is an idiomatic pattern in JavaScript, but it gets verbose when the chain is long, and it's not safe. Use type when you need specific features. With TypeScript, you can create complex types by combining simple ones. There are two popular ways to do so: with unions, and with generics. With a union, you can declare that a type could be one of many types. These were a few of many solutions that were found helpful for your issue. Short-circuiting, and saving a parse operation: Here's the fiddle: http://jsfiddle.net/opfyrqwp/28/. You can check if it's is undefined first. Angular TypeScript localStorage. Ill show you how Regular Type Check handles undefined (the same applies to null) and how Strict Type Check prevents you from introducing unwanted behavior in our code, like that infamous TypeError Cannot read properties of undefined. true if strict, false otherwise. Conclusion To check for TypeScript gives you two ways of interpreting null and undefined types, also known as Type Check Modes, and one of them can avoid this easily overlooked TypeError. However, by default, TypeScripts type checker considers null and undefined as legal values for any type. Most upvoted and relevant comments will be first. 1. code of conduct because it is harassing, offensive or spammy. Modified today. What is the difference between enum and object in typescriptEnum. Importantly, you can't assign to the enum members and types are checked to the enum members, rather than string.ObjectUnion. I don't aggre with @Fenton. Objects are the type safe. If FieldNames is an object, this means this function expects an instance which has the properties FirstField and SecondField. Templates let you quickly answer FAQs or store snippets for re-use. If bitovi is not suspended, they can still re-publish their posts from their dashboard. Technical Problem Cluster First Answered On January 5, 2022 Popularity 7/10 Helpfulness 3/10 Contributions From The Grepper Developer Community. Eclipse Modeling Tools. With you every step of your journey. notation that allows us to unwind the response and select just the data we need. Here, we will take string variable with some data and use ifelsestatement to check array is undefined. Now that we have switched to Strict Type Check Mode, TypeScript throws this error for translatePowerLevel function: Function lacks ending return statement and return type does not include 'undefined'. Unflagging kontent_ai will restore default visibility to their posts. Please consider going through all the sections to better understand the solutions. Let start today tutorial How to check an array is empty in typescript? Each global variable is stored as a property on the global object (window in a How to download XLSX file from a server response in javascript? Now you understand how TypeScript interprets null and undefined types and how you can migrate your project to Strict Mode. Javascript is the most used language on the web. Strict Type Check differs from Regular Type Check because it considers null and undefined types of their own. If you are making a comparison in an if statement, use the logical AND (&&) operator to make sure the property is of the correct type. It will become hidden in your post, but will still be visible via the comment's permalink. And as far as the compiler knows, the type of document.getElementById(elementId) expression does not have value property. When processing JSON responses in TypeScript, how do you safely check if a variable is an array and is not empty? . At the end, you should always avoid to simply rely that an undefined value will return false in a chain of condition. Once suspended, kontent_ai will not be able to comment or publish posts until their suspension is removed. For a reference check HTMLElement and HTMLInputElement in MDN. But at the same time, TypeScript infers the return type to be only of type, Cannot read properties of undefined (reading 'length'), Strict Type Check Mode changes how TypeScript interprets, Function lacks ending return statement and return type does not include 'undefined'. It will be done Interpreting undefined or null as subtypes of all other types can lead to runtime problems. Related Posts. The function translatePowerLevel below takes a number as argument and returns strings one, two, many or it's over 9000!. To check for undefined in Typescript, we can use the === operator. TutorialsTeacher.com is optimized for learning web technologies step by step. They can still re-publish the post if they are not suspended. Typescript is not relevant to this at all. 554 MB ; 69 DOWNLOADS; The Modeling package provides tools and runtimes for building model-based applications. Of course, you can always initialize an array like shown below, but you will not get the advantage of TypeScript's type system. react/typescript 'string | undefined' is not assignable to type 'string' even I already checked the type. And in case you will migrate from Regular to Strict Type Check, our team can help with strategies to do so in a less painful way. That error message is telling you the function is returning undefined implicitly, but its return type does not include undefined in it. check if object is undefined typescript; typescript check undefined; check if object is undefined javascript; check if type is undefined javascript; typescript string or There are a number of reasons why you might need to check if a string is empty or not. If you use never type, speech:never will give a compile time error, as void is not assignable to never. Subscribe to TutorialsTeacher email list and get latest updates, tips & In Regular Type Check Mode, TypeScript is aware that a function might return undefined. TypeScript is telling us that the value we are passing to the function might be undefined, which is not compatible with the type of the function's parameter, which only expects a string. However, this code doesn't handle 0, a valid input - yes, looking at you, Yamcha. If you fancy a shorter, obtuse form of short circuiting: Of course, I'd suggest letting the minifier take care of that. That depends, do you also want to cast strings as potential integers as well? In this article, I'll demonstrate a common example that cause problem. Find centralized, trusted content and collaborate around the technologies you use most. Awesome! If you were to compile the following code again using Solution #2, TypeScript would throw the error Object is possibly 'undefined'. MDN has also provided the following polyfill for browsers not supporting Number.isInteger(), mainly all versions of IE. Then, we check if the array contains any items via .length property: You can also wrap the code in try/catch block to make sure you also catch errors from network communication. Inside compilerOptions property, all we need to do is add the property "strictNullChecks": true. In typescript (null == undefined) is true. Inside compilerOptions property, all we need to do is add the property "strictNullChecks": true. Type guards enable you to instruct the TypeScript compiler to infer a specific type for a variable in a particular context, ensuring that the type of an argument is what you say it is. I have tried to cover all the aspects as briefly as possible covering topics such as Javascript and a few others. This one's applicable and useful in some cases and could possiblty be of some help. Share. With the optional chaining operator (?. If you are starting a new project, you should definitely enable Strict Type Check Mode from the beginning. Note that null field values do not match != clauses, because x != null evaluates to undefined. When you choose a solution like Solution #2, TypeScript expects you to write code that handles possible nullish values. This means null and undefined values are valid values for all types. Examples might be simplified to improve reading and basic understanding. TypeScript 2.0 introduced Strict Type Check Mode (also referred to as strict nullchecking mode). To examine a generic type and its type parametersGet an instance of Type that represents the generic type. Use the IsGenericType property to determine whether the type is generic, and use the IsGenericTypeDefinition property to determine whether the type is a generic type definition. Get an array that contains the generic type arguments, using the GetGenericArguments method. More items Lets go into We're a place where coders share, stay up-to-date and grow their careers. Strict Type Check Mode changes how TypeScript interprets undefined and null values. Unflagging bitovi will restore default visibility to their posts. When processing JSON responses in TypeScript, how do you safely check if a variable is an array and is not empty? However, this code doesn't handle 0, a valid input - yes, looking at you, Yamcha. If kontent_ai is not suspended, they can still re-publish their posts from their dashboard. Unfortunately, TypeScripts Regular Type Check Mode is not able to alert you to when youmay have made that mistake. If you check by value, you will get that variable is assigned a value or not. The translatePowerLevel function return value is typed explicitly as string, but it is possibly also returning undefined when the argument powerLevel has the value 0. Typescript: undefined is not assignable, but the variable cannot be undefined. Make the undefined return type explicit so wherever translatePowerLevel is used, you have to handle nullish values as well. Adding a return statement so it is always explicitly returning a value - in the code below, it is now returning the string zero. Interpreting undefined or null as subtypes of all other types can lead to runtime problems. example: W3Schools offers free online tutorials, references and exercises in all the major languages of the web. tricks on C#, .Net, JavaScript, jQuery, AngularJS, Node.js to your inbox. TypeScript is not smart enough to catch this possibility and will let the function returning undefined instead of a boolean. Ask the community on the Kontent.ai Discord server or sign up for a full-featured trial at https://kontent.ai. Once unsuspended, bitovi will be able to comment and publish posts again. Strict Type Check differs from Regular Type Check because it considers nulland undefined types of their own. typeof returns a string that tells the type of the operand. Well help. Schedule your free consultation call to get started. to check for undefined or null javascript != null typescript "strictNullChecks": true, 0 undefined if typescript == null typescript ts es check null or undefined or empty string if condition for undefined and null in typescript if type is null angular indicate its not undefined in typescript TypeScripts type system defines two very special types, null and undefined, to represent the values null and undefined, respectively. So if you declare a variable as: let uemail : string | undefined; Then you can check if the variable z is let nothing: never = null; // Error: Type 'null' is not assignable to type 'never' In TypeScript, a function that does not return a value, actually returns undefined. true if strict, false otherwise. To check for undefined in Typescript, we can use the === operator. By default, TypeScript assumes that null and undefined are in the domain of every type. Once unsuspended, kontent_ai will be able to comment and publish posts again. DEV Community 2016 - 2022. However, people write: The alert in that case will be undefined if the "prop" is not defined or if the myObject is not defined. With TypeScript, there are two ways of interpreting null and undefined types and one of them can avoid the 'Cannot read properties of undefined' runtime TypeError. Note that camel case should be used, since -is not allowed in template reference names. In this case, you cannot use the bang(!) For example, if youtry to get the length of the result of translateNumber(0), which is undefined, JavaScript will throw this TypeError at runtime: Cannot read properties of undefined (reading 'length'). Templates let you quickly answer FAQs or store snippets for re-use. You can also use the if (trr.propObject) and if (! DEV Community A constructive and inclusive social network for software developers. Solution 5: Use logical OR (||) Summary The reason for the error Type undefined is not assignable to type in 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. To make a variable null we must assign null value to it as by default in typescript unassigned values Even though its clear that there must be some strings not present in movieWatchCount, previous versions of TypeScript treated optional In the root of your project, there should be a tsconfig.json file. Ask Question Asked today. Most upvoted and relevant comments will be first, Blazor, .NET and Vue.js dev, bus driver, 3D printing enthusiast, German Shepherd lover, dev evangelist @Kontent, Artisanal developer - coding with varying degrees of success since 1983, // as there is no need for checking the variable separately, JavaScript Visualized: the JavaScript Engine. Which mean that the following code also compile and return undefined as well. This is the most explicit and clear way to have exactly the result expected. isNullOrUndefined = (value: T | Here is a benchmark: Angular ReferenceError: localStorage is undefined , , ''. key local storage . Want to check how much you know TypeScript? The returned array will only contain objects that satisfy the condition. JavaScript return the last value that is not true in the chain, which we saw that can be undefined. TypeScript is not smart enough to catch this possibility and will let the function returning undefined instead of a boolean. !trr.propObject) without problem. posted in TypeScript on December 9, 2021 by Fbio Englert Moutinho. Allow cookies. In order to make it work, you need to type the isNullOrUndefined as a type contrain as well by specifying the return type as value is null | undefined. Default config. Once suspended, bitovi will not be able to comment or publish posts until their suspension is removed. To handle this case, you can do undefined / null check inside the template by using a *ngIf directive on the form which basically tells TypeScript that the developer is taking care of the possibly undefined property by performing a null check. our team can help with strategies to do so in a less painful way. Check if the property on the current object meets the condition. First, solve the problem. Awesome! 0 0. In the case of undefined, the assigned variable dont have any value but the variable exists. Before getting into the core of the issue. data ?. By using TypeScript Nullish Coalescing & Optional But you will say that you will never explicitly write undefined in an && condition. Compared to typeof approach, the try/catch is more precise because it determines solely if the variable is not defined, despite being initialized or uninitialized.. 4. In summary, I am getting undefined for property that is populated. This can be a little tricky because updatedTodo contains only the attributes of the todo that have been updated. Made with love and Ruby on Rails. With the latest node and typescript, this is @tsconfig/node16.. Other The API response is typed, so in case you get different type of data, it won't get mapped to the type and the items array will be empty which in the end helps you handle the problem. http://jsben.ch/#/htLVw. As expected, when undefined and null the result is the same. At the end, you should always avoid to simply rely that an undefined value will return false in a chain of condition. TypeScript gives youtwo ways of interpreting nulland undefined types, also known as Type Check Modes, and one of them can avoid this easily overlooked TypeError. And in case you will migrate from Regular to Strict Type Check, our team can help with strategies to do so in a less painful way. The methods Promise.prototype.then(), Promise.prototype.catch(), and Promise.prototype.finally() are used to associate further action with a promise that becomes settled. In Regular Type Check Mode, TypeScript is aware that a function might return undefined. API DI API. Here is what you can do to flag bitovi: bitovi consistently posts content that violates DEV Community 's 2 3 ## Object 4 5 The last test is to compare a custom object. let arr = [1, 3, 'Apple', 'Orange', 'Banana', true, false]; Arrays can contain elements of any data type, numbers, strings, or even objects. In the root of your project, there should be a tsconfig.json file. Let's say we get a response from an API (in this example, it's Kontent API ): const blogPosts : BlogPost [] = ( await deliveryClient . How to check if an element exists on the page in Playwright.js, Find solutions to your everyday coding challenges. Please upvote the solutions if it worked for you. If you use TypeScript to type check CJS JavaScript code, your only option is to use "moduleResolution": "node16". Check your email for updates. : list.includes(user.username as string) But at the same time, TypeScript infers the return type to be only of type string because TypeScript is widening the undefined type to string type. DOCUMENT Angular DI. 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 (??) If result is not equal to undefined, TypeScript knows that it's an object, so it allows us to access properties on the object. For example, if obj.first is a Falsy value that's not null or undefined, such as 0, it would still short-circuit and make nestedProp become 0, which may not be desirable. At Bitovi we highly recommend using - or migrating to - Strict Type Check Mode, as it can help you produce better, more reliable code. This is the TypeScripts configuration file and you can read more about it here. Unfortunately, TypeScripts Regular Type Check Mode is not able to alert you to when you may have made that mistake. I will explain the reason and the way to fix it. DEV Community A constructive and inclusive social network for software developers. The best part is that there are tons of libraries to make life easier. Of course, for any movie title not yet in the dictionary, movieWatchCount[title] will be undefined (TypeScript 4.1 added the option --noUncheckedIndexedAccess to include undefined when reading from an index signature like this). Ill show you how Regular Type Check handles undefined (the same applies to null) and how Strict Type Check prevents you from introducing unwanted behavior in our code, like that infamous TypeError Cannot read properties of undefined. Why is TypeScript not triggering an error? Until TypeScript 2.0, there was only one type check mode - regular - and it considersnulland undefined as subtypes of all other types. In TypeScript, you could also check undefined using null in the if condition in place of undefined; this will also return true if something is undefined plus will return true if null. Model object returns expected string but if I directly call the property, it returns undefined. But first, let's enable Strict Type Check Mode. TypeScript introduced a new type never, which indicates the values that will never occur. I have encountered very weird behavior using angular model when user selects from an option. If you need help with building amazing web apps feel free to reach us at bitovi.com. type ( " blog_post " ) . How to check for undefined in Typescript? Testing reveals that the short-circuiting solution has the best performance (ops/sec). No worries if you're unsure about it but I'd recommend going through it. Now that we have switched to Strict Type Check Mode, TypeScript throws this error for translatePowerLevel function: Function lacks ending return statement and return type does not include 'undefined'. Angular TypeScript localStorage. Not-equal (!=) and not-in queries exclude documents where the given field does not exist. They are basically in chronological order, subject to the uncertainty of multiprocessing. By using simple if condition. 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 Consider the following example. Are you sure you want to hide this comment? In this article, you will learn how to check if a sting is empty or null in JavaScript. A field exists when it's set to any value, including an empty string (""), null, and NaN (not a number). Why is TypeScript not triggering an error? Number.isInteger() seems to be the way to go. In Regular Type Check Mode, TypeScript is aware that a function might return undefined. When JavaScript reaches the end of a function that has no explicit return, it returns undefined. In TypeScript, a function that does not return a value, actually returns undefined. It will become hidden in your post, but will still be visible via the comment's permalink. Once unpublished, this post will become invisible to the public and only accessible to Fbio Englert Moutinho. TypeScript is telling us that the emp.name property might have a value of undefined, which is not compatible with the type of the name variable, which only expects a string. Let's be on the same page. Older versions of typescript are incompatible with @tsconfig/node16.In those cases we will use an older default configuration. To check undefined and null of any variable. if you have an array with either objects or primitives, printed using console.log, it always gives [object, object] that does not help debug. TypeScript gives you two ways of interpreting null and undefined types, also known as Type Check Modes, and one of them can avoid this easily overlooked TypeError. Here's the final solution you can try out in case no other solution was helpful to you. check if object is undefined typescript. Also, note the ?. How to fix Error: Not implemented: navigation (except hash changes). When you use this approach, you basically tell TypeScript that this value will never be null or undefined.. We used it right after the address property, so we are telling TypeScript that p1.address will never have a value of null or undefined.. One way to get around this is to use a non-null assertion. Once unpublished, this post will become invisible to the public and only accessible to Ondrej Polesny. TypeScript is now aware the return type does not match all possible return values, and this could lead to problems at runtime! Solution 3. TypeScript has a powerful system to deal with null or undefined values. However, the Array.isArray will work in pure JS too. class Example { doSomething(): void { const username = 'David' as string | undefined; // add ! Originally published at bitovi.com. Message function accepts ValidationArguments which contains the following information:. We use a set of checklists to keep us on track and ensure reproducible quality on every project. How to Avoid the Infamous "Cannot read properties of undefined" Error with TypeScript, In Regular Type Check Mode, TypeScript is aware that a function might return, . At Bitovi we highly recommend using - or migrating to - Strict Type Check Mode for. The first warning we get is TypeScript telling us to define the type of the update() function's updatedTodo variable. They can still re-publish the post if they are not suspended. typescript check if undefined or null or empty use ?? As another example, if you assign null or undefined to variables while in Regular Type Check Mode, TypeScript will infer these variables to be of type any. A newer way to check if a variable is null or undefined is to use the optional chaining (?.) Now you understand how TypeScript interprets null and undefined types and how you can migrate your project to Strict Mode. ('check is not null or not undefined'); } else { console.log('check is null or undefined'); } It will be very useful in Angular. Bitovi has expert Angular consultants eager to help support your project. For more information have a read of Basarat's AST tip.. And so on. items TypeScript uses some built-in JavaScript operators like typeof, instanceof, and the in operator, which is used to determine if an object contains a property. Let's say we get a response from an API (in this example, it's Kontent API): We expect that response to be an array of BlogPost objects. null and undefined is one of the main reasons to produce a runtime error in the JavaScript application. This happens if you don't check the value of unknown return variables before using it. If you are not sure that a variable will always have some value, the best practice is to check the value of variables for null or undefined before using them. But first, let's enable Strict Type Check Mode. An extra non-null check for the property inside the function is unnecessary eazy_g. , Not sure if headless CMS could help your next project? JavaScript return the last value that is not true in the chain, which we saw that can be undefined. DEV Community 2016 - 2022. Checking the type is done with the typeof operator. Add Answer . TypeScript Decorators overview. Type guards in typescript. But at runtime, undefined appear. As a JavaScript developer, Im sure youve encountered the frustrating runtime TypeError Cannot read properties of undefined. Actually, it is. If the purpose of the if statement is to check for null or undefined values before assigning a value to a variable, you can make use of the Nullish Coalescing Operator. When JavaScript reaches the end of a function that has no explicit return, it returns undefined. When such a type is an enum in TypeScript, it returns the original type of the enum where EnumClass is This doesnt mean that it cant be given null or undefined, but rather, when the function is called, the compiler will check that the user has provided a value for each parameter. It won't be false. Using a juggling-check, you can test both null and undefined in one hit: if (object.property == 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 (object.property === null) { Typescript does Javascript is awesome in many ways. There is two ways to solve that issue. If incomingValue is not declared, TypeScript should return an exception. But how can you match the return type to all possible return values? Typescript property returning undefined. It works in TypeScript 4.2.4, but I have not tested earlier versions. But at the same time, TypeScript infers the return type to be only of type string because TypeScript is widening the undefined type to string type. Until TypeScript 2.0, there was only one type check mode - regular - and it considers null and undefined as subtypes of all other types. Made with love and Ruby on Rails. But the compiler can't know that, it can only base on static types, not dynamic behaviour of your code. Hope it turns out helpful for you. Consider the following example. In the above example, the throwError() function throws an error and keepProcessing() function is always executing and never reaches an end point because the while loop never ends. To determine the type of a variable after a conditional statement you can use type guards. Then, write the code. You can either add a return statement so the function always returns a string (solution #1), or change the return type from string to string | undefined (solution #2). We can help you create amazing applications. This was the only solution I found to check if an attribute is undefined that does not generate warnings. typescript check string is not null or undefined; typescript check for not null; typescript check if has a value; value be object or null in typescript; typescript check for null If both conditions are met, TypeScript knows that the variable is a string and allows us to use string In a JavaScript program, the correct way to check if an object property is undefined is to use the typeof operator. Using window.hasOwnProperty(). The .then() method takes up to two arguments; the first argument is a callback function for the fulfilled case of the promise, and Make the undefined return type explicit so wherever translatePowerLevel is used, you have to handle nullish values as well. 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