Let's say that we want to create an enum called … This blog post covers the examples for looping the Enum string or numbers using different approaches. 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. The constructor is a special type of method which is called when creating an object. Enums are a new data type supported in TypeScript. This gives us everything we need to start using our new enum class just like we would a Typescript enum: This looks great! 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 This quickly leads to situations where string values taken directly from an enum won’t be accepted anywhere we expect an enum. Some code philosophers argue this goes against the ethos of enums, which are meant to be static lists and nothing more. Classes may have fallen out of vogue in the shift to functional-reactive styles over the last few years, but this is a situation where classes could offer the more declarative approach. Note that they should be in uppercase letters: In my experience, however, enums don’t live in isolation from changes in the application, and they’re rarely static. Typescript supports costant enumerables, declared through const enum. 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 in 5 minutes. Enums in TypeScript are named constants which are used to create unique classes. As with most things, I think the answer is a rock-solid “it depends”. With union enums, the type system is able to leverage the fact that it knows the exact set of values that exist in the enum itself. DEV Community – A constructive and inclusive social network for software developers. Problem 2: Typescript enums can’t be extended Enums in Python or Java are classes, which allows for custom attributes and methods directly on the enum. Still, the decision mostly comes down to your application’s needs. 1, 100) a unary minus applied to any numeric literal (e.g. Some of these improvements might properly address scalability / DRY issues. Union enums and enum member types any string literal (e.g. This transformation issue isn’t just isolated to file read/writes or API requests. This transformation issue isn’t just isolated to file read/writes or API requests. In my experience, however, enums don’t live in isolation from changes in the application, and they’re rarely static. First, we define the static collection asArray, as this needs to be instantiated before any values can be added. As soon we start creating more enums, we’ll find ourselves attempting to factor out the common operations — but this proves to be challenging. Some of these improvements might properly address scalability / DRY issues. But if you just want easy iteration and don’t need any custom methods, class enums are probably overkill. 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. Still, the decision mostly comes down to your application’s needs. Creating enum. Next, we create our Priority enums. Even if we try to annotate upstream, the problem persists. Many other programming languages (C/C#/Java) have an enum data type but JavaScript does not. The end result is that even with some clever abstraction, there will still be a lot of duplicated code with each new enum. This creates a gap between the original data and the data used by the application. Java enums are a special kind of Java class used to define collections of constants. Advanced Types. Then, define constant values for the enum. If we look ahead to the constructor, we see that deprecated is defaulted to false for other enums, and each new Priority is getting added to the static asArray collection. But Typescript’s implementation is quite basic — under the hood, they’re just objects, and this presents two significant pain points. Use TypeScript enum values in Angular HTML templates You can use your enum values in your HTML templates. This gives us everything we need to start using our new enum class just like we would a Typescript enum: This looks great! Functions. 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. Problem 1: Iteration over Typescript enums requires converting to Array. This is useful when the value is not important. Enums are a feature added to JavaScript in TypeScript which makes it easier to handle named sets of constants. Recreating enums in Typescript Typescript has enums. We would first have to transform this data to ensure that error.priority gets instantiated with our Priority enum. But has this been worth all the effort? Variable Declarations. It is not necessary for a class to have a constructor. 1) String-based Enums. The end result is that even with some clever abstraction, there will still be a lot of duplicated code with each new enum. So, are class-based enums worth the effort? For example, this TypeScript snippet: will compile to this JavaScript: The reasons for this are explained in the documentation. enum CompassDirection { … With you every step of your journey. 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