In simple words, enums allow us to declare a set of named constants i.e. ... typescript best practices - redefine type vs use existing types' parameters. I find that I use it for unit testing, but there’s other applications, too. Typescript requires that enums have number value types (hopefully soon, this will also include string value types). Object.keys. This is somewhat connected to Angular perhaps. String Enums in TypeScript October 27, 2017. ... (Sunday being 0, Saturday being 6), but the object also has number keys with a string value representing the named constant. TypeScript Data Type - Enum. Sometimes it is still useful to just loop through each property on an object in order to do something. This is now available in TypeScript too. Conditionally show content using an Enum and *ngIf; Get all the values of an enum as an Array Intro You can skip the next block of text if you just want to get the actual details (I respect that) I love using Enums in TypeScript as I am a terrible developer with a terrible memory and let my editor's auto complete do 80% of my job. A TypeScript string enum for compile-time safety when working with event.key - nfriend/ts-key-enum ... ts-key-enum. Especially const enums, they don't have an object at all. Here’s a good example: an object declaring a set of buttons for a modal dialog. Therefore using keyof myEnum results in generic string keys. Instead, we'll require that the key actually exists on the type of the object that is passed in: function prop < T, K extends keyof T >(obj: T, key: K) {return obj[key];} TypeScript now infers the prop function to have a return type of T[K], a so-called indexed access type or lookup type. Enums in ts don't have keys in the usual sense that js objects have keys. Enums or enumerations are a new data type supported in TypeScript. Using `enum` values as strictly typed object keys 25 Feb 2020, typescript, enum, mapped types, improve this page In TypeScript, it’s often useful to define interfaces or complex (structured) types whose properties (or keys) may only be values of a previously defined enum type. a collection of related values that can be numeric or string values. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. Most object-oriented languages like Java and C# use enums. A TypeScript string enum for compile-time safety when working with event.key. myEnum can be used as a type but it references the values of the enum. We’ll first see how typescript create an object for Heterogeneous enum. It is now possible to assign a string value to an enum member: enum MediaTypes {JSON = "application/json", XML = "application/xml"} Object.keys allows you to enumerate the properties of an object, returning you an array of string which represent the property names. If you’ll log above enum, it’ll show the following output: { 1: "Up" 3: "Left" Down: "DownValue" Left: 3 Right: "RightValue" Up: 1 } Here you can see that for the numeric member, typescript created object with reverse mapping as well. It represents the type of the property K of the type T. A few tips on how to use enums in TypeScript, and some gotcha's to watch out for. 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