Enums are also a feature from “the old days” of TypeScript where the JavaScript landscape was a lot different than it is now. It is arguably easier to understand than the first if statement. series. For example, consider a selection of shirt sizes. The enum actually contains both the numerical and the string value. There are many ways we can iterate enum … 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. TypeScript has a discrete enum type that allows various compile-time checks and constraints to be enforced when using such types. Enum values can be string’s if we explicitly define a string value after the name. This will not work if --noImplicitAny is enabled and throws an error // To Enum / number var month : Month = Month ["JAN"]; Other approach when - … 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: One of the first things I liked about the brave new world of TypeScript was the TypeScript enum. The second if statement uses an enum. Here is the generated code when TypeScript transpiles the Theme enum: Basically, you can provide a finite list of strings a variable can be assigned. 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). By default, enum holds numeric values if a string value is defined. One of Javascript’s most glaring omissions is first-class support for enums. The caveat is that string-initialized enums can't be reverse-mapped to get the original enum member name. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. 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. (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.) 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 A handy feature of enums is that you can also go from a numeric value to the name of that value in the enum. You’ll see why later When we run this code, we see: So that’s a special thing to note! This will also prevent you from assigning a traditional "string" type to the string literal. enum Color {Red, Green, Blue} var col = Color. To convert string to Enum in Typescript or angular follow the below steps. How TypeScript enum works. TypeScript provides both … This is useful when the value is not important. Javascript does not support enums, hence typescript creates runtime artifact to support them (Except, const enum which do not have a runtime artifact). In simple words, enums allow us to declare a set of named constants i.e. Enum are predefined constants, can be created using the enum keyword. 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. TypeScript Evolution Why Enums are used? Enums are a feature added to JavaScript in TypeScript which makes it easier to handle named sets of constants. String enums do not have auto-incrementing behavior. Otherwise, it should throw a type error. Non-enum values can be mistaken for enum values statically or at runtime (if we use string-valued properties). String literal unions are like string enums in that they create a narrow type of specific strings. enum Day { BeforeNoon = "AM", AfterNoon = "PM" } In this case we can directly get names of enum by looping string enum object. When having a java-server and a REST API (JSON) with a typescript written frontend this is quite a … Luckily, TypeScript has a cleaner solution when you need those string values. 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. The const type declaration syntax would also let you create a structurally-typed string enum like this: However, it is recommended not to create an enum … 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. The above array contains enum values as string type. Enum or Enumeration allowed us to declare set of named constants. Extend a `Record` with a different type of property. You can see from the informative gif above that autocomplete works as well! Let’s create an enum in TypeScript to represent the days of the week: 1 2 3 4 5 6 7 8 9 If the meaning of the constant’s value is not apparent, it can make code easier to understand. The type name follows the enum keyword. 3. It is a good practice to use the constant values defined by enums in the code. This can be a set of string or number values. source from TypeScript readme. 1.Pass the given string to Enum object as a key. Enums are a type that can make code more readable, where the meaning of the value of a variable is not apparent. 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. However, the following example passes a number instead of an enum to the isItSummer() function. 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. Calculate union of strings from enum keys. 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. And you can see that, as enums work exceptionally different than any other type in TypeScript. The string enum in typescript differs from many other languages. String Enums in TypeScript October 27, 2017. TypeScript … 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. Enum pattern # The following example demonstrates a Java-inspired enum pattern that works in plain JavaScript and TypeScript: Using enums can make it easier to document intent, or create a set of distinct cases. 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. I had previously used them in C# and felt a reassuring familiarity. 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. 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. You can see this in the resulting Javascript code in the TypeScript Playground. String enums are a similar concept to numeric enums, except that the enum has some subtle runtime differences. An enum is short for enumeration and is a type that represents named constants. 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. In typescript we can have string enums as well. The string is a group of characters enclosed in double-quotes. That’s perhaps not what we expected. My solution: 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? In a string enum, each enum values are constant-initialized with a string literal, or with another string enum member rather than numeric values. 0. As a result, the generated code will have a smaller size. We can easily convert string to number in typescript. Other values, such as symbols, are not allowed. Wrap up enum CompassDirection { … Using enums make our life easier to document intent or create a set of distinct cases. By default an enum is number based, starting at zero, and each option is assigned an increment by one. in the enum rather than magic values like 1, 2, 3,… This makes the code more obvious. We’ll also discover the drawbacks of enums and use cases where they work well. Enumerations or enums are a new data type supported by the TypeScript. Enums emit code # My most prefered way of writing TypeScript is to. You could easily define the shirt sizes with an enum:This is a nice data structure with which to code. 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. In typescript, Enum properties are strongly typed, In the first approach, Month enum accepts strings values and returns Enum object. 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. 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. Enum values are zero-based auto-incrementing numbers by default. In other words, you can't write Colors["RED"] to get the string "Red". 0. The value names are then listed inside curly brackets. 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