In this post, we will cover what TypeScript enums are and how to create them. TypeScript has a discrete enum type that allows various compile-time checks and constraints to be enforced when using such types. write regular, modern-day JavaScript. TypeScript … 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. This would get closer to a world where enums aren't necessary at all. However, the following example passes a number instead of an enum to the isItSummer() function. In other words, you can’t write Colors ["RED"] to … TypeScript does not generate code for the union of string literals. String Enums in TypeScript October 27, 2017. Differences between generic and explicit types Javascript does not support enums, hence typescript creates runtime artifact to support them (Except, const enum which do not have a runtime artifact). Most object-oriented languages like Java and C# use enums. (To me, enums have always felt like they go against the TS design goals since they're an expression-level syntax with non-trivial emit behavior and nominal by default.) source from TypeScript readme. The type name follows the enum keyword. 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. 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: The value names are then listed inside curly brackets. enum Day { BeforeNoon = "AM", AfterNoon = "PM" } In this case we can directly get names of enum by looping string enum object. 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. The string is a group of characters enclosed in double-quotes. You can see from the informative gif above that autocomplete works as well! It is now possible to assign a string value to an enum member: Defining a type for object properties that are strings. 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. Basically, you can provide a finite list of strings a variable can be assigned. 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. Otherwise, it should throw a type error. 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? Enums or enumerations are a new data type supported in TypeScript. Wrap up In this case, you can turn on the preserveConstEnums compiler option in your tsconfig.json file: If we compile our code again with the preserveConstEnums option set, the compiler will still inline the MediaTypes.JSON usage, but it will also emit the mapping code: This post is part of the Enum is called Enumeration, It is a new syntax for replacing define multiple constants declaration, Enum type contains constants of Strings and numbers only. By default, enum holds numeric values if a string value is defined. This is useful when the value is not important. Interface incorrectly extends interface. 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 - … 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. Sometimes, it might be necessary to emit the mapping code for a const enum, for instance when some piece of JavaScript code needs access to it. String Enums. 8. Luckily, TypeScript has a cleaner solution when you need those string 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 … TypeScript provides both numeric and string-based enums. Enums are a feature added to JavaScript in TypeScript which makes it easier to handle named sets of constants. 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). 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. Enum pattern # The following example demonstrates a Java-inspired enum pattern that works in plain JavaScript and TypeScript: 0. 1.Pass the given string to Enum object as a key. The second if statement uses an enum. Enum values are zero-based auto-incrementing numbers by default. Numeric enums are not strongly-typed to the values in the enum, but string enums are. TypeScript provides the enum keyword to define a set of labeled values. And it works. An enum is short for enumeration and is a type that represents named constants. In typescript, Enum properties are strongly typed, In the first approach, Month enum accepts strings values and returns Enum object. String enums are a similar concept to numeric enums, except that the enum has some subtle runtime differences. Enums emit code # My most prefered way of writing TypeScript is to. One of Javascript’s most glaring omissions is first-class support for enums. We can easily convert string to number in typescript. Other values, such as symbols, are not allowed. And you can see that, as enums work exceptionally different than any other type in TypeScript. My solution: String enums. Let’s create an enum in TypeScript to represent the days of the week: 1 2 3 4 5 6 7 8 9 TypeScript provides both … 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. 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. The enum actually contains both the numerical and the string value. 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. Enumerations or enums are a new data type supported by the TypeScript. The above array contains enum values as string type. Enum are predefined constants, can be created using the enum keyword. We’ll also discover the drawbacks of enums and use cases where they work well. 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. You’ll see why later When we run this code, we see: So that’s a special thing to note! Why Enums are used? Enum values can be string’s if we explicitly define a string value after the name. String Enums Const enums ... TypeScript enums are number based. As a result, the generated code will have a smaller size. If the meaning of the constant’s value is not apparent, it can make code easier to understand. String enums do not have auto-incrementing behavior. TypeScript Evolution You could easily define the shirt sizes with an enum:This is a nice data structure with which to code. In simple words, enums allow us to declare a set of named constants i.e. In typescript we can have string enums as well. Alas, TypeScript only supports numbers and strings as enum member values. One of the first things I liked about the brave new world of TypeScript was the TypeScript enum. Enums are a type that can make code more readable, where the meaning of the value of a variable is not apparent. Numeric enum values are not strongly-typed to the values defined in the enum. You can see this in the resulting Javascript code in the TypeScript Playground. 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. Non-enum values can be mistaken for enum values statically or at runtime (if we use string-valued properties). TypeScript emits some mapping code for each enum which constructs a mapping object. In typescript enum is available from typescript version 2.4. 3. When having a java-server and a REST API (JSON) with a typescript written frontend this is quite a … 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. I had previously used them in C# and felt a reassuring familiarity. The string enum in typescript differs from many other languages. If you are familiar with Typescript, up until now you may have been internally screaming “Typescript already has enums… 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. Enums are one of the few features TypeScript has which is not a type-level extension of JavaScript. Here is an example Enums are also a feature from “the old days” of TypeScript where the JavaScript landscape was a lot different than it is now. For example, consider a selection of shirt sizes. Enums are a set of named constants that can take either a numeric or string form. It is arguably easier to understand than the first if statement. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. 3.And then cast it to the enum object to get enum type of string. There are many ways we can iterate enum … Enum in TypeScript allows us to define a set of named constants. Enum is a collection of value the can be of type string or numeric as a named constants. enum Color {Red, Green, Blue} var col = Color. This will also prevent you from assigning a traditional "string" type to the string literal. I have always used the string-based enum, which I will use for all the examples in this post: That’s perhaps not what we expected. TypeScript 2.4 now allows enum members to contain string initializers. A handy feature of enums is that you can also go from a numeric value to the name of that value in the enum. 2.If the string is part of the enum name entry the value will be returned. However, it is recommended not to create an enum … All of the related values are in one place and it's easy to access a value from the list. series. 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! This means that numbers can be assigned to an instance of the enum, and so can anything else that is compatible with number. In a string enum, each enum values are constant-initialized with a string literal, or with another string enum member rather than numeric values. Compare Number in enum type in Typescript. The caveat is that string-initialized enums can't be reverse-mapped to get the original enum member name. This is now available in TypeScript too. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. Using enums make our life easier to document intent or create a set of distinct cases. So, generally, a numeric enum isn’t a great choice for a type. If you to learn more about TypeScript, you may find my free TypeScript course useful: Subscribe to receive notifications on new blog posts and courses. 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. enum CompassDirection { … To convert string to Enum in Typescript or angular follow the below steps. 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. Enum or Enumeration allowed us to declare set of named constants. By default an enum is number based, starting at zero, and each option is assigned an increment by one. In other words, you can't write Colors["RED"] to get the string "Red". It is a good practice to use the constant values defined by enums in the code. Use string enum value in TypeScript interface as a computed property key. Using enums can make it easier to document intent, or create a set of distinct cases. Enum, short for Enumerated Type, is a common language feature of many statically types languages such as C, C#, Java, Swift any many others, is a group of named constant values that you can use within your code. String literal unions are like string enums in that they create a narrow type of specific strings. Calculate union of strings from enum keys. How TypeScript enum works. Convert string to number in typescript. This can be a set of string or number values. In typescript, String can be created as follows. 0. 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. in the enum rather than magic values like 1, 2, 3,… This makes the code more obvious. Enums allow a developer to define a set of named constants. Starting from TypeScript 2.4, the enum would not contain the key as a member anymore. This separates the enum options from strings or integers, for example. Extend a `Record` with a different type of property. a collection of related values that can be numeric or string values. The const type declaration syntax would also let you create a structurally-typed string enum like this: