In simple words, enums allow us to declare a set of named constants i.e. myEnum can be used as a type but it references the values of the enum. Most object-oriented languages like Java and C# use enums. Typescript requires that enums have number value types (hopefully soon, this will also include string value types). This is somewhat connected to Angular perhaps. The last step is very important: TypeScript has limited reflection ability, so this is a case where we have to explicitly provide the enum type for object type fields, input type fields, args, and the return type of queries and mutations: @ InputType class JourneyInput {@ Field (type => Direction) // it's very important direction: Direction;} We’ll first see how typescript create an object for Heterogeneous enum. Especially const enums, they don't have an object at all. It represents the type of the property K of the type T. Object.keys allows you to enumerate the properties of an object, returning you an array of string which represent the property names. It is now possible to assign a string value to an enum member: enum MediaTypes {JSON = "application/json", XML = "application/xml"} String Enums in TypeScript October 27, 2017. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. The end result is a JavaScript object that contains every enum value. 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. ... typescript best practices - redefine type vs use existing types' parameters. a collection of related values that can be numeric or string values. 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. This is now available in TypeScript too. Sometimes it is still useful to just loop through each property on an object in order to do something. Yes, keyof typeof is correct. A TypeScript string enum for compile-time safety when working with event.key - nfriend/ts-key-enum ... ts-key-enum. Here’s a good example: an object declaring a set of buttons for a modal dialog. A TypeScript string enum for compile-time safety when working with event.key. A few tips on how to use enums in TypeScript, and some gotcha's to watch out for. ... (Sunday being 0, Saturday being 6), but the object also has number keys with a string value representing the named constant. Therefore using keyof myEnum results in generic string keys. Enums or enumerations are a new data type supported in TypeScript. I find that I use it for unit testing, but there’s other applications, too. Object.keys. TypeScript Data Type - Enum. Enums in ts don't have keys in the usual sense that js objects have keys. 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. 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. String keys redefine type vs use existing types ' parameters used as a type but references... 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