This comes in handy for cases like ngSwitch where it’s more readable to use the enum value than it’s underlying value. With you every step of your journey. Enum is called Enumeration, It is a new syntax for replacing define multiple constants declaration, Enum type contains constants of Strings and numbers only. String Enum. One of Javascript’s most glaring omissions is first-class support for enums. After the individual values are created, we can create arbitrary subsets of values manually or by using other properties of the enum. write regular, modern-day JavaScript. This might seem like a nitpick since ES6 gave us Object.values— but if we consider the most common use cases for enums, there’s a constant need for iteration. Next, we create our Priority enums. GraphQL also has enum type support, so TypeGraphQL allows us to use TypeScript enums in our GraphQL schema. enum MimeType { JPEG, PNG, PDF } the real value behind e.g. The constructor is a special type of method which is called when creating an object. It can be a numeric or string enum - the internal values of enums are taken from the enum definition values and the public names taken from the enum … Developer and designer with a focus on complex UX problem-solving. Let’s start with the code, and then walk through its features. Even if we try to annotate upstream, the problem persists. If you find that some of your enums tend to come with tightly-coupled business logic or you need a structure that flexibly supports additional properties and metadata, this may be a useful pattern. MimeType.PDF will be 2. How might we accomplish this in Typescript? This introduces additional layers in a pipeline that might otherwise have been seamless. Babel has the plugin plugin-proposal-class-properties for public static fields. this.empCode or this.name. If we’re ingesting raw data from an external source — say, some JSON with a property we want to annotate: We can’t annotate this object with our enum as-is. As soon we start creating more enums, we’ll find ourselves attempting to factor out the common operations — but this proves to be challenging. To create an enum, use the enum keyword (instead of class or interface), and separate the constants with a comma. We can define the enums by using the enum keyword. It is not necessary for a class to have a constructor. Typescript supports costant enumerables, declared through const enum. This creates a gap between the original data and the data used by the application. If you find that some of your enums tend to come with tightly-coupled business logic or you need a structure that flexibly supports additional properties and metadata, this may be a useful pattern. This gives us everything we need to start using our new enum class just like we would a Typescript enum: This looks great! After the individual values are created, we can create arbitrary subsets of values manually or by using other properties of the enum. Lastly, we have our accessors. It provides a way to create symbolic constants, especially for a set of related constants, such as the day of the week (Monday, Tuesday and so on...) or say an enum stores special values. As with most things, I think the answer is a rock-solid “it depends”. String enums are similar to numeric enums, except that the enum values are initialized with string values... Heterogeneous Enum. var something = 0; Use TypeScript enum values in Angular HTML templates You can use your enum values in your HTML templates. // Error: Type 'string' is not assignable to type 'Blah', // Error: Type 'string' is not assignable to type 'Bah'. Built on Forem — the open source software that powers DEV and other inclusive communities. If we’re ingesting raw data from an external source — say, some JSON with a property we want to annotate: We can’t annotate errorData with our PrioritizedError interface as-is. TypeScript Enums. DEV Community © 2016 - 2021. Then, define constant values for the enum. Problem 1: Iteration over Typescript enums requires converting to Array. This might seem like a nitpick since ES6 gave us Object.values — but if we consider the most common use cases for enums, there’s a constant need for iteration. This quickly leads to situations where string values taken directly from an enum won’t be accepted anywhere we expect an enum. For example, this TypeScript snippet: will compile to this JavaScript: The reasons for this are explained in the documentation. Enums allow us to define or declare a collection of related values that can be numbers or strings as a set of named constants. TypeScript language extensions to JavaScript. Lastly, we have our accessors. An enum is also an object in javascript, Object class provides the following methods. But some of the time it is important to have the enum resolve to a different type. Variable Declarations. Recreating enums in Typescript Typescript has enums. Some code philosophers argue this goes against the ethos of enums, which are meant to be static lists and nothing more. We face the reverse issue anywhere we might be sending data to an external source, requiring another transformation back into string format. TypeScript has supported static fields in classes for a very long time. const criticalMessage = new ErrorMessage(Priority.CRITICAL); const originalData = require('error.json'); React: Functional Component and Class Component, Speed up your coding using Emmet — a really powerful tool, Understanding JavaScript Automatic Semicolon Insertion, ReactHelmet | Enhance the SEO of your React App, How I Built a Full-Featured Social Media Application With React Native and NodeJS, How to get TypeScript type completion by defining process.env types, Define a static sort order for iteration/display. This is useful when the value is not important. Anyone who’s spent time in other languages knows the value of these simple structures, so it’s no surprise that one of Typescript’s few language additions is the enum. But if you just want easy iteration and don’t need any custom methods, class enums are probably overkill. However, all of our static members are specific to each enum, and can’t be abstracted away. JavaScript primitive types inside TypeScript. Problem 1: Iteration over Typescript enums requires converting to Array. In the constructor, members of the class can be accessed using this keyword e.g. Migrating to Typescript: Keeping it smooth & steady, Define a static sort order for iteration/display. As soon we start creating more enums, we’ll find ourselves attempting to factor out the common operations — but this proves to be challenging. TypeScript is a superset of JavaScript, so whatever possible to do in JavaScript is also possible in TypeScript. Enums An enum is a special "class" that represents a group of constants (unchangeable variables, like final variables). ... we actually modified a field in the parent enum}; // The TypeScript compiler should refuse to compile this code // But after removing the "Finish2" line, // the TypeScript compiler should successfully handle it as one would normally expect with the spread operator. Another problem is that these enums require instantiation. This transformation issue isn’t just isolated to file read/writes or API requests. Our only option is to cast or assert, which defeats the purpose of working with strong types and creates unhelpful noise in our code. It’s these handoffs that are particularly dangerous, as external dependencies might not warn us when we’ve failed to provide data in the expected format. But Typescript’s implementation is quite basic — under the hood, they’re just objects, and this presents two significant pain points. The string is a group of characters enclosed in double-quotes. Some of these improvements might properly address scalability / DRY issues. But has this been worth all the effort? There are many ways we can iterate enum data. How to provide a type shape to JavaScript objects. This gives us everything we need to start using our new enum class just like we would a Typescript enum: class ErrorMessage { constructor(public priority: Priority) {} } const criticalMessage = new ErrorMessage(Priority.CRITICAL); const allErrors = Priority.asArray.map(ErrorMessage); const warnings = Priority.GENERATES_WARNINGS.map(ErrorMessage); These implementations are certainly not optimal — I’m sure there’s plenty that could be improved on, leveraging some more advanced features in Typescript. Note that they should be in uppercase letters: Enum is used to give a unique name to a constant and if we want to use it, we can access it via its name. The end result is that even with some clever abstraction, there will still be a lot of duplicated code with each new enum. Enums are also a feature from “the old days” of TypeScript where the JavaScript landscape was a lot different than it is now. An enum is a way to organize a collection of related values. Since TypeScript 2.4 it's possible to declare string enums: enum MimeType { JPEG = 'image/jpeg', PNG = 'image/png', PDF = 'application/pdf', } You can explicitly provide numeric values using the same method Interfaces. Anyone who’s spent time in other languages knows the value of these simple structures, so it’s no surprise that one of Typescript’s few language additions is the enum. TypeScript Data Type - Enum Numeric Enum. An enum is a special "class" that represents a group of constants (unchangeable variables, like final variables).. To create an enum, use the enum keyword (instead of class or interface), and separate the constants with a comma. -1, -100) E.g. 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