Enum or Enumeration allowed us to declare set of named constants. Javascript does not support enums, hence typescript creates runtime artifact to support them (Except, const enum which do not have a runtime artifact). TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. In typescript enum is available from typescript version 2.4. TypeScript emits some mapping code for each enum which constructs a mapping object. enum CompassDirection { … Defining a type for object properties that are strings. 0. String Enums in TypeScript October 27, 2017. Instead, it will inline the value for each enum member at all use sites, potentially saving a few bytes and the overhead of the property access indirection: But what if, for some reason, we need access to the mapping object at runtime? Here is an example Enum values are zero-based auto-incrementing numbers by default. For example in Java you can write: enum Colors {RED("Red"), GREEN("Green"), BLUE("Blue")} and you can get the enum key by the value and reverse. 0. It is now possible to assign a string value to an enum member: Specifying enum member values # TypeScript distinguishes three ways of specifying enum member values: Literal enum members are initialized: implicitly or; via number literals or string literals (explicitly). Compare Number in enum type in Typescript. TypeScript Evolution String enums are useful when the meaning of string value isn’t apparent because it can be given a meaningful name to help the readability of the code. String Enums. I had previously used them in C# and felt a reassuring familiarity. There are many ways we can iterate enum … TypeScript provides both numeric and string-based enums. Here is the generated code when TypeScript transpiles the Theme enum: So, when declaring your types, you'll need to export the string literal type and use it the same way you would use an enum. Differences between generic and explicit types In a string enum, each enum values are constant-initialized with a string literal, or with another string enum member rather than numeric values. 列挙型をさらに詳しく調べる前に、生成するJavaScriptを見てみましょう。ここにはサンプルのTypeScriptがあります: 次のJavaScriptを生成します。 Tristate[Tristate["False"] = 0] = "False";という行に焦点を当てましょう。Tristate["False"] = 0は自己説明的でなければなりません。つまり、Tristate変数の"False"メンバを0に設定します。JavaScriptでは、代入演算子は割り当てられた値(この場合は0)を返します。したがって、JavaScriptランタイムによって次に実行されるのは、Tristate[0] = "False"です。こ … enum Day { BeforeNoon = "AM", AfterNoon = "PM" } In this case we can directly get names of enum by looping string enum object. Extend a `Record` with a different type of property. Enums allow a developer to define a set of named constants. Numeric enums are not strongly-typed to the values in the enum, but string enums are. Most object-oriented languages like Java and C# use enums. As a result, the generated code will have a smaller size. String enums are useful when the meaning of string value isn’t apparent because it can be given a meaningful name to help the readability of the code. In typescript, Enum properties are strongly typed, In the first approach, Month enum accepts strings values and returns Enum object. All of the related values are in one place and it's easy to access a value from the list. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. For string-valued enum members, this mapping object defines mappings from key to value, but not vice versa: This means we can resolve a value by its key, but we cannot resolve a key by its value: Compare this to an enum with number-valued members: In this case, the compiler additionally emits a reverse mapping from value to key: This reverse mapping allows use to resolve both a key by its value and a value by its key: To avoid paying the cost of the generated enum mapping code, we can turn our MediaTypes enum into a const enum by adding the const modifier to the declaration: With the const modifier in place, the compiler will not emit any mapping code for our MediaTypes enum. If the strings are meaningful and don’t need to be mapped to something more meaningful, then a string literal union is a concise way of creating the type. TypeScript 2.4 now allows enum members to contain string initializers. TypeScript provides both … That’s perhaps not what we expected. Use string enum value in TypeScript interface as a computed property key. If the meaning of the constant’s value is not apparent, it can make code easier to understand. Using enums can make it easier to document intent, or create a set of distinct cases. For example, if we had the value 2 but weren’t sure what that mapped to in the Color enum above, we could look up the corresponding name: An enum is short for enumeration and is a type that represents named constants. Enums are also a feature from “the old days” of TypeScript where the JavaScript landscape was a lot different than it is now. Why Enums are used? This will also prevent you from assigning a traditional "string" type to the string literal. In simple words, enums allow us to declare a set of named constants i.e. However, it is recommended not to create an enum … This separates the enum options from strings or integers, for example. The type name follows the enum keyword. Interface incorrectly extends interface. How TypeScript enum works. The above array contains enum values as string type. TypeScript has a discrete enum type that allows various compile-time checks and constraints to be enforced when using such types. It is arguably easier to understand than the first if statement. Convert string to number in typescript. You can see this in the resulting Javascript code in the TypeScript Playground. To convert string to Enum in Typescript or angular follow the below steps. You might find some of my other posts interesting: // type error - type '"VH"' is not assignable to type 'Level', Controlling Type Checking Strictness in TypeScript, Inferring Object and Function Types in TypeScript, Type-safe Data Fetching with unknown in TypeScript. enum Colors { Red = "RED", Green = "GREEN", Blue = "BLUE" } The caveat is that string-initialized enums can’t be reverse-mapped to get the original enum member name. So, generally, a numeric enum isn’t a great choice for a type. For example, consider a selection of shirt sizes. TypeScript does not generate code for the union of string literals. source from TypeScript readme. Wrap up You’ll see why later When we run this code, we see: So that’s a special thing to note! Basically, you can provide a finite list of strings a variable can be assigned. For example: What if we assign the enum variable to a string that is isn’t "H", "M" or "L": What if we set level to a string within the Level type: So, string enum values are strongly-typed to the named values declared in the enum. This can be a set of string or number values. Enum in TypeScript allows us to define a set of named constants. Enum pattern # The following example demonstrates a Java-inspired enum pattern that works in plain JavaScript and TypeScript: Take this enum: Now add this code to log the values: Note: I’m using a separate log function to show what type the value is. It is now possible to assign a string value to an enum member: The string enum can be used like any other enum in TypeScript: Here's the ES3/ES5 output that the compiler generates for the above code: This output almost looks like the output that the compiler would generate for enums with numeric members, except that there's no reverse mapping for string-valued members. In typescript, String can be created as follows. And it works. We can validate that the Level enum represents a zero-based auto-incrementing number by outputting the values to the console: What if we assign the enum variable to a number that is isn’t 0, 1 or 2: No type error occurs! The string is a group of characters enclosed in double-quotes. Other values, such as symbols, are not allowed. 8. In other words, you can't write Colors["RED"] to get the string "Red". One of the first things I liked about the brave new world of TypeScript was the TypeScript enum. By default an enum is number based, starting at zero, and each option is assigned an increment by one. 3. It would be extremely useful to allow generic constraints to be limited to enum types - currently the only way to do this is via T extends string | number which neither conveys the intent of the programmer, nor imposes the requisite type enforcement. in the enum rather than magic values like 1, 2, 3,… This makes the code more obvious. One of Javascript’s most glaring omissions is first-class support for enums. If you think about inputs such as dropdowns or radio buttons where the user must select a single value from multiple choices, the underlying values oftentimes map nicely to an enum data structure. This will not work if --noImplicitAny is enabled and throws an error // To Enum / number var month : Month = Month ["JAN"]; Other approach when - … Enums emit code # My most prefered way of writing TypeScript is to. 2.If the string is part of the enum name entry the value will be returned. Enums are a type that can make code more readable, where the meaning of the value of a variable is not apparent. A handy feature of enums is that you can also go from a numeric value to the name of that value in the enum. However, the following example passes a number instead of an enum to the isItSummer() function. Numeric enum values are not strongly-typed to the values defined in the 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. Alas, TypeScript only supports numbers and strings as enum member values. Let’s create an enum in TypeScript to represent the days of the week: 1 2 3 4 5 6 7 8 9 The value names are then listed inside curly brackets. String enums do not have auto-incrementing behavior. You can see from the informative gif above that autocomplete works as well! In the below example, Chars Enum is declared with constants but not its value, Numbers are assigned (A=0, B=1, C=2, D=3) Here the number is compared against Enum numeric values. 1.Pass the given string to Enum object as a key. How to iterate strings,values of an Enum type in javascript/typescript Enum is an enumeration of names and values replacing multiple constants with a single namespace. series. In typescript we can have string enums as well. Calculate union of strings from enum keys. You could easily define the shirt sizes with an enum:This is a nice data structure with which to code. This is now available in TypeScript too. 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