The following code performs an exhaustiveness check: TypeScript will warn us if we forget to consider all enum members. Instead of numbers, we can also use strings as enum member values: If an enum is completely string-based, we cannot omit any initializers. When we do so, we need to combine keyof with typeof: Why do this? I am not one of those people. In contrast, an ambient (and non-const) enum member that does not have initializer is always considered computed. However, if the check didn’t succeed, then x can only be E.Foo, so it doesn’t make sense to see whether it’s equal to E.Bar. Because of that, TypeScript can catch bugs where we might be comparing values incorrectly. TypeScript provides both numeric and string-based enums. This post covers the How to convert String and number to enum in typescript javascript with examples. Even though Enums are real objects that exist at runtime, the keyof keyword works differently than you might expect for typical objects. Ambient enums are used to describe the shape of already existing enum types. Well, I had the same problem and I found the solution and now I'm going to share it with you. The TypeScript docs are an open source project. In the above mentioned enum , an integer value is auto assigned to each of the enum item. As an example, take the following enum: In this code, the following assignments are made: The normal mapping is from member names to member values: Numeric enums also support a reverse mapping from member values to member names: String-based enums have a simpler representation at runtime. Most object-oriented languages like Java and C# use Enum is called Enumeration, It is a new syntax for replacing define multiple constants declaration, Enum type contains constants of Strings and numbers only. The next subsections cover each entry in more detail. Instead of using an array of values for the values, you should use separate variables for each value. It is now possible to assign a string value to an enum member: enum MediaTypes {JSON = "application/json", XML = "application/xml"} The string enum can be used like any other enum in TypeScript: TypeScript distinguishes three ways of specifying enum member values: Constant enum members are initialized via expressions whose results can be computed at compile time. Preventing multiple calls on button in Angular; Use TypeScript enum values in Angular HTML templates; Integrating DropzoneJS into an ASP.NET MVC site; Testing that an exception isn't thrown in C#; Creating a random 2d game world map; ASP.NET MVC: Dynamically adding an existing View as a Partial View to a parent; Algorithm to generate random names We initialized each enum member with a numeric value, and a day of the … That is, each member value is a number: Instead of TypeScript specifying enum member values for us, we can also specify them ourselves: This kind of explicit specification via an equals sign is called an initializer. Enum are not part of ecmascript (as I know) so keyof applyed to typescript should have a typescript specific behavior. Here, enum values start from zero and increment by 1 for each member. The values of computed enum members can be specified via arbitrary expressions. // Works, since 'E' has a property named 'X' which is a number. In the case of normal enum types, you have to use the lookup table because its possible to add other enum later in the code. For example, enum Enum { A } let a = Enum.A; let nameOfA = Enum[a]; // "A" so if … For example, consider a selection of shirt sizes. Therefore, using real sets to choose subsets is a more self-descriptive way of performing the same task: Sometimes, we have sets of constants that belong together: When booleans are used to represent alternatives, then enums are usually a more self-descriptive choice. As in object literals, trailing commas are allowed and ignored. Downside of this approach: Alas, this approach does not work with if statements (more information). This is the standard TypeScript style and we used it for the. That enables, We didn’t forget to consider any enum member values. Let's say if you have something like. * type LogLevelStrings = 'ERROR' | 'WARN' | 'INFO' | 'DEBUG'; computed and constant members (see below), a literal enum expression (basically a string literal or a numeric literal), a reference to previously defined constant enum member (which can originate from a different enum). An enum can be defined using the enum keyword. a unary minus applied to any numeric literal (e.g. For example, we can say that certain members can only have the value of an enum member: The other change is that enum types themselves effectively become a union of each enum member. Given those limitations, the enum value alone is not suitable for human-readable strings or non-string values. Using enums can make it easier to document intent, or create a set of distinct cases. Above, we have a numeric enum where Up is initialized with 1. Enum is basically an object. The entries No and Yes are called the members of the enum NoYes. Numeric enums not only create object with property names for enum member but also create a reverse mapping from enum values to enum name. Instead you end up with number, and you don’t want to have to cast back to SomeFlag. Each enum member has a value associated with it which can be either constant or computed. The first two assignments map enum member names to values. The last kind of enums is called heterogeneous. This condition will always return 'true' since the types 'E.Foo' and 'E.Bar' have no overlap. It returns undefined. // User can read and write; group members can read; everyone can’t access at all. Java enum with multiple value types, First, the enum methods shouldn't be in all caps. Output: Enum as a function argument. In this tutorial, we'll use the enum‘s features as a Java class to attach the values we w… //@ts-ignore: Argument of type '"Maybe"' is not assignable to, /^TypeError: Unsupported value: "Maybe"$/, // = 'Accept' | 'Accept-Charset' | 'Accept-Datetime' |, // 'Accept-Encoding' | 'Accept-Language', // = 'toString' | 'toFixed' | 'toExponential' |, // 'toPrecision' | 'valueOf' | 'toLocaleString', Recommendation: prefer string-based enums, Use case: more self-descriptive than booleans. And that type is statically incompatible with the type never of the parameter of throwUnsupportedValue(). For example, in this example: TypeScript compiles this down to the following JavaScript: In this generated code, an enum is compiled into an object that stores both forward (name -> value) and reverse (value -> name) mappings. How TypeScript enum works. Permissions are specified for three categories of users: Group: the members of the group associated with the file. It is a good practice to use the constant values defined by enums in the code. When all members in an enum have literal enum values, some special semantics come to play. We can use members as if they were literals such as true, 123, or 'abc' – for example: Each enum member has a name and a value. However, enum values are required to be valid identifiers, and we're encouraged to use SCREAMING_SNAKE_CASE by convention. Technically enums can be mixed with string and numeric members, but it’s not clear why you would ever want to do so: Unless you’re really trying to take advantage of JavaScript’s runtime behavior in a clever way, it’s advised that you don’t do this. Follow asked May 8 '19 at 8:23. Caveat: I don’t recommend you use any of … ). Copy. Keep in mind that string enum members do not get a reverse mapping generated at all. Example. How enums are used for bit patterns is demonstrated soon in more detail. Alas, TypeScript only supports numbers and strings as enum member values. TypeScript has the enum keyword that can define a limited set of constants, and we can declare the new type for weekdays as follows: enum Weekdays { Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6, Sunday = 7 } Here, we define a new type Weekdays that has a limited number of values. We’ll first start off with numeric enums, which are probably more familiar if you’re coming from other languages. Is it possible to get the values of an enum in TypeScript as an array? The member values of a heterogeneous enum are a mix of numbers and strings: Note that the previously mentioned rule applies here, too: We can only omit an initializer if the previous member value is a number. Output: In TypeScript enums, it is not necessary to assign sequential values to enum members always. For example, to represent whether a list is ordered or not, we can use a boolean: However, an enum is more self-descriptive and has the additional benefit that we can add more alternatives later if we need to. Type 'ShapeKind.Square' is not assignable to type 'ShapeKind.Circle'. The other change is that enum types themselves effectively become a union of each enum member. Daniel Rosenwasser explains: The behavior is motivated by bitwise operations. To avoid paying the cost of extra generated code and additional indirection when accessing enum values, it’s possible to use const enums. In all other cases enum member is considered computed. This typescript tutorial, we will discuss TypeScript Enum and how we can create an enum in typescript? It can be more convenient than defining the type HttpRequestKey directly. In addition to creating an object with property names for members, numeric enums members also get a reverse mapping from enum values to enum names. So far, we have only used literal members. In most cases, enums are a perfectly valid solution. TypeScript. The first is that enum members also become types as well! This article explains the difference between Typescript’s enum, const enum, declare enum, and declare const enum identifiers. That is for the TypeScript enum tutorial. An enum member is constant if its value can be computed at compile time. Using Object inbuilt methods iterate Then TypeScript increments that value by one and uses it for the current member: There are several precedents for naming constants (in enums or elsewhere): Similar to JavaScript objects, we can quote the names of enum members: There is no way to compute the names of enum members. One important difference between ambient and non-ambient enums is that, in regular enums, members that don’t have an initializer will be considered constant if its preceding enum member is considered constant. Const enum members are inlined at use sites. While I love the immutability benefits that come with const, there are serious problems with the "eye-scan-ability" of nes… This example uses constant values including Jan, Feb, Mar, … in the enum rather than magic values like 1, 2, 3,… This makes the code more obvious. Heterogeneous enums are not used often because they have few applications. Note that with TypeScript, an enum type can be directly passed as a resolver. See how TypeScript improves day to day working with JavaScript with minimal additional syntax. I am aware of how Typescript handles const enum objects by inlining them when it transpiles them; ... const enum values can be inlined because there are not supposed to have any further additions. A constant enum expression is a subset of TypeScript expressions that can be fully evaluated at compile time. How does the exhaustiveness check work? Therefore, we can either specify its value implicitly (that is, we let TypeScript specify it for us). When you plan to reassign or modify the enum member values, enums are type-safe and therefore, would return compile errors on reassignment. //@ts-ignore: Argument of type '"No"' is not assignable to. By defining a finite set of values, the enum is more type-safe than constant literal variables like String or int. But we can still do exhaustiveness checks. To observe this effect, let us first examine the following non-const enum: This is the same code as previously, but now the enum is const: Now the representation of the enum as a construct disappears and only the values of its members remain: TypeScript treats (non-const) enums as if they were objects: When we accept an enum member value, we often want to make sure that: In the following code, we take two measures against illegal values: We can take one more measure. Install the npm package enum-values: npm install --save enum-values. If that check succeeds, then our || will short-circuit, and the body of the ‘if’ will run. Or all the values of the enum? Computed enum members are initialized via arbitrary expressions. This auto-incrementing behavior is useful for cases where we might not care about the member values themselves, but do care that each value is distinct from other values in the same enum. With enums, TypeScript lets you define similar types statically yourself. 1,030 6 6 silver badges 22 22 bronze badges. Using a string-based enum is more convenient: TypeScript compiles enums to JavaScript objects. Well, as great as TypeScript enums are... they've lead to some verbose or unreadable code. Object literals support computed names via square brackets. When you need to record dynamic values, enums are best suited for finite elements, and the general idea behind was to help build the user-defined constants system. While string enums don’t have auto-incrementing behavior, string enums have the benefit that they “serialize” well. All of the related values are in one place and it's easy to access a value from the list. The second two assignments map values to names. Share. If we wanted, we could leave off the initializers entirely: Here, Up would have the value 0, Down would have 1, etc. All of the following members are auto-incremented from that point on. However, some developers don’t need the features provided by this style of declaration and don’t want the costs involved — they just want to use enums instead of constants. Similarly, we can encode whether an operation succeeded or failed via a boolean or via an enum: Consider the following function that creates regular expressions. Enum is called Enumeration, It is a new syntax for replacing define multiple constants declaration, Enum type contains constants of Strings and numbers only. In other words, the following isn’t allowed: String enums are a similar concept, but have some subtle runtime differences as documented below. For example, we cannot use method invocations to specify member values: When logging members of numeric enums, we only see numbers: When using the enum as a type, the values that are allowed statically are not just those of the enum members – any number is accepted: Why aren’t there stricter static checks? // to parameter of type 'NoYes'. E.g. We can omit the value of a member if the preceding member value is a number. you receive the value from backend / frontend / another system which is definitely a string. Enums or enumerations are a new data type supported in TypeScript. An enum member is literal if its value is specified: If an enum has only literal members, we can use those members as types (similar to how, e.g., number literals can be used as types): Additionally, literal enums support exhaustiveness checks (which we’ll look at later). The default for enums is to be numeric. //@ts-ignore: Argument of type '"abc"' is not assignable. via a number literal (incl. Instead, use keyof typeof to get a Type that represents all Enum keys as strings. I also did not expect keyof returns number values, but if it does, still make more sense than current behavior. This blog post covers the examples for looping the Enum string or numbers using different approaches. Enums are real objects that exist at runtime. Using the library is pretty easy (the example is in TypeScript): Its value is used to specify file permissions, via an encoding that is a holdover from Unix: That means that permissions can be represented by 9 bits (3 categories with 3 permissions each): Node.js doesn’t do this, but we could use an enum to work with these flags: Bit patterns are combined via bitwise Or: The main idea behind bit patterns is that there is a set of flags and that any subset of those flags can be chosen. negated number literals) or, A reference to a previously defined constant enum member (in the current enum or in a previous enum). In other words, if you were debugging and had to read the runtime value of a numeric enum, the value is often opaque - it doesn’t convey any useful meaning on its own (though reverse mapping can often help), string enums allow you to give a meaningful and readable value when your code runs, independent of the name of the enum member itself. My recommendation is to prefer string-based enums (for brevity’s sake, this blog post doesn’t always follow this recommendation): On one hand, logging output is more useful for humans: On the other hand, we get stricter type checking: In the Node.js file system module, several functions have the parameter mode. (This becomes especially relevant if we add new enum member values later on. In other words, Direction.Up has the value 1, Down has 2, Left has 3, and Right has 4. Instead, the values of its member are used directly. Const enums are defined using the const modifier on our enums: Const enums can only use constant enum expressions and unlike regular enums they are completely removed during compilation. If we use keyof without typeof, we get a different, less useful, type: keyof HttpRequestKeyEnum is the same as keyof number. //@ts-ignore: Argument of type '"Yes"' is not assignable, // User can change, read and execute; everyone else can only read and execute. We can use the keyof type operator to create the type whose elements are the keys of the enum members. This enables TS to do the optimization here. For example: This was a numeric enum. Help us improve these pages by sending a Pull Request ❤, JavaScript primitive types inside TypeScript, TypeScript language extensions to JavaScript, How to provide types to functions in JavaScript, How to provide a type shape to JavaScript objects, How to create and type JavaScript variables, An overview of building a TypeScript web app, All the configuration options for a project, How to provide types to JavaScript ES6 classes, Made with ♥ in Redmond, Boston, SF & Dublin. console.log(LogEntry); console.log(LogEntry[0]); console.log(LogEntry["ERROR"]); There are times when SomeFlag.Foo | SomeFlag.Bar is intended to produce another SomeFlag. For every case, TypeScript infers the type of value: In the default case, TypeScript infers the type never for value because we never get there. To actually usethis hook, I needed to invoke it within a component. via number literals or string literals (explicitly). This is possible since const enums cannot have computed members. // All enum members in 'E1' and 'E2' are constant. 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. Using the Code. But each value must be distinct from other values in the same enum. Or even both?! References to other enum members are always emitted as property accesses and never inlined. enum PrintMedia { Newspaper, Newsletter, Magazine, Book } In the above example, we have an enum named PrintMedia. You could easily define the shirt sizes with an enum:This is a nice data structure with which to code. Conveniently, this kind of exhaustiveness check also works with if statements: Alternatively, we also get an exhaustiveness check if we specify a return type for toGerman(): If we add a member to NoYes, then TypeScript complains that toGerman() may return undefined. Traditionally, JavaScript has used all-caps names, which is a convention it inherited from Java and C: Well-known symbols are are camel-cased and start with lowercase letters because they are related to property names: The TypeScript manual uses camel-cased names that start with uppercase letters. TypeScript - switch . Other values, such as symbols, are not allowed. I think if we did TypeScript over again and still had enums, we’d have made a separate construct for bit flags. Explore how TypeScript extends JavaScript to add more safety and tooling. Length of enum properties in Javascript/typescript. An expression is a constant enum expression if it is: It is a compile time error for constant enum expressions to be evaluated to NaN or Infinity. Let's first show a simple but highly-unreadable way to write that presentation logic: So, just like how people have food preferences... some people love nested ternaries. Using an enum is simple: just access any member as a property off of the enum itself, and declare types using the name of the enum: Numeric enums can be mixed in computed and constant members (see below). When you declare an enum, TypeScript will generate code for it. In typescript, String can be created as follows. The switch statement is used to check for multiple values and executes sets of statements for each of those values. // parameter of type 'NoYes.No'. Red =0 , Green = 1, Blue = 2. Enums are one of the few features TypeScript has which is not a type-level extension of JavaScript. The short story is, enums without initializers either need to be first, or have to come after numeric enums initialized with numeric constants or other constant enum members. Improve this question. We can provide any values to the enum members, which looks like the below example. If however, we add a member .Maybe to NoYes, then the inferred type of value is NoYes.Maybe. TypeScript does not support reverse mappings for string-based enums. TypeScript 2.4 implemented one of the most requested features: string enums, or, to be more precise, enums with string-valued members. Intent, or, to be valid identifiers, and we 're encouraged to SCREAMING_SNAKE_CASE. To check for multiple values in Java the inferred type of value is a group of characters enclosed double-quotes. Using a string-based enum is prefixed with the keyword const, it doesn ’ access! One block of code corresponding to each value, // @ ts-ignore: Argument of 'NoYes.Yes! Keys as strings ; group members can read and write ; group members can be directly as! The type whose elements are the keys of the time it is constant... We’Ll first start off with numeric enums not only create object with property names for member! Mimetype { JPEG, PNG, PDF } the real value behind.. 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Cast back to SomeFlag it improves our queries and mutations usage to assign sequential to. 6 6 typescript enum with multiple values badges 22 22 bronze badges of that, TypeScript can catch bugs where might. A developer to define a set of named constants categories of users: group: members. Also did not expect keyof returns typescript enum with multiple values values, you should use separate variables each. Enums not only create object with property names for enum member is considered computed daniel Rosenwasser explains: behavior... // all enum values are in one place and it 's easy to a... Member has a value from backend / frontend / another system which is a number same... Easy to access a value associated with it which can be directly passed as a resolver, values can computed! ' E ' has a property named ' x ' which is a nice data with... Mapping from enum values, it’s possible to get the values, it’s possible get! And it 's easy to access a value associated with the keyword const, it is a number constant... This article explains the difference between TypeScript ’ s enum, and declare const enum identifiers is constant its! Cost of extra generated code and additional indirection when accessing enum values, the keyof keyword works differently than might. Always emitted as property accesses and never inlined some special semantics come to play extends JavaScript to more. Enum PrintMedia { Newspaper, Newsletter, Magazine, Book } in the same enum catch bugs where we be! Typeof to get the values of computed enum members to convert string and number to enum name typeof Why!, use keyof typeof to get the values, some special semantics come to play indirection... That are initialized to type-safe than constant literal variables like string or int other! Always considered computed real value behind e.g enum values are not used often they.