Typescript switch enum

March 17, 5 min read Enum, short for Enumerated Type, is a common language feature of many statically typed languages such as C, CJava, and Swift.

The enum is denoted using the enum keyword followed by the name of the enum DayOfWeek. Then we define the constant values that we want to make available for the enum. This is a nice way to remove the use of magic values within a codebase, since we have type-safe representation options that are all related to each other. It might surprise you to know that this is valid TypeScriptand the compiler will happily take it for you.

It has the named properties we defined, and they are assigned a number representing the position in the enum that they exist Sunday being 0, Saturday being 6but the object also has number keys with a string value representing the named constant.

Therefore, we can pass in numbers to a function that expects an enum. The enum itself is both a number and a defined constant. So why is it useful? In your database, you may have this value stored as a number. By defining it as a TypeScript enum, we can cast it properly:. If we were to receive the enum value E or 5is that the result of a bitwise operation of Foo.

A Foo. D or Foo. B Foo. Initially, the values are defined using their position in the index with Sunday through Tuesday being 0 to 2.

Then, when we reset the order at Wednesday, everything after that is incremented from the new starting position. Instead, we have to do an explicit cast:. But why stop at strings? In fact, we can mix and match the values of enums within an enum itself:. Provided that all assignable values are of the same type numeric in this case, we can generate those numbers in a bunch of different ways, including computed values.

If they are all constants, we can mix types to make a heterogeneous enum:. This is quite confusing and can make it difficult to understand how the data works behind the enum. LogRocket is a frontend application monitoring solution that lets you replay problems as if they happened in your own browser. Instead of guessing why errors happen, or asking users for screenshots and log dumps, LogRocket lets you replay the session to quickly understand what went wrong. Reply 1. My area of specialty is front-end web development, focusing on architecture around SPA and other UI-heavy web applications.

We made a custom demo for. No really. Click here to check it out. Click here to see the full demo with network requests. Plug: LogRocketa DVR for web apps LogRocket is a frontend application monitoring solution that lets you replay problems as if they happened in your own browser.I ported my first nontrivial JS lib to typescriptlang and it was a pure joy.

What a lovely piece of technology. TypeScript is really awesome! I'm glad to see people are using it in some of Preact projects. FrontEnd Webpack Programming. I'm highly impressed with typescriptlang 2. Microsoft has really done great things with typescriptlang and code. TypeScript gets really nice features and VS Code is stable and fast. I don't want to work without them ever again.

But seriously, in every project I've converted, TypeScript has found bugs. And rationalizing the types leads to much clearer code. I just completed a huge refactoring in a node app. It took me two hours thanks to TypeScript. It would have taken me days without it Unhappy with 1. Take another look - 2. TypeScript starts from the same syntax and semantics that millions of JavaScript developers know today. TypeScript compiles to clean, simple JavaScript code which runs on any browser, in Node.

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Types enable JavaScript developers to use highly-productive development tools and practices like static checking and code refactoring when developing JavaScript applications. Types are optional, and type inference allows a few type annotations to make a big difference to the static verification of your code. Types let you define interfaces between software components and gain insights into the behavior of existing JavaScript libraries.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. It only takes a minute to sign up. I would like to know how I can use a switch statement with enum values for the following scenarios:. I am making a small program for a flight reservation system. The program is meant to enter certain details about the passenger.

The program also restricts the user to choose destinations other than the following 3 destinations. Once destination is chosen, the flight number is supposed to be automatically assigned.

The user will choose one of three destinations and program will assign an appropriate Flight No for the passenger.

typescript switch enum

So let's say I make an enum of destinations, then how can I use switch statement here? This isn't a homework. I am doing it just for the sake of exploring. Your scenario doesn't sound like an appropriate use for an enumeration.

Also, unless the values never change, this should be data driven. If the values never change, then a key-value pair is more appropriate where the destination is the key and the flight number is the value. This would eliminate any need for a switch statement, as the key can be used to directly and efficiently locate the value. I agree with Tim Medora, it is not a proper idea to use an enum in that case, because it forces you to have hardcode your destinations and your flight numbers.

Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. How to use a switch statement with enum efficiently? Ask Question. Asked 8 years, 3 months ago. Active 2 years, 4 months ago. Viewed 27k times. I would like to know how I can use a switch statement with enum values for the following scenarios: I am making a small program for a flight reservation system.

typescript switch enum

London Frankfurt Berlin So let's say I make an enum of destinations, then how can I use switch statement here?GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

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Already on GitHub? Sign in to your account. The main problem this suggestion solves, is it to be able to create an enum in a. In projects which use a module loader require. Currently, when defining an enum, one must import it on every. Expected behavior: Compiler will assume MyEnum is available at runtime from the global scope in addition to compiling the enum to the MyEnum.

Actual behavior: On. Oh but they can.

typescript switch enum

I assume you received the error MyEnum has members with initializers that are not literals That means that in order to be encoded in a. This only creates the declaration but not the definition, therefore I have to create another file which creates the enum and assigns it on the global scope, the problem here is that the compiler doesn't offer any feature that validates the actual enum definition with its declaration, meaning I can declare and define different enum member or order and still successfully compile my code.

This doesn't compile since MyEnum isn't declared globally, and again If I will declare it globally, it duplicates with the definition and there's no way I can enforce the compiler to match the definition and declaration. You are right, I misunderstood what you were having difficulty with. What you can do is create a.

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This file must not be a module. TypeScript will recognize this as a global because there are no top level import or export clauses in the file. If you have trouble with your loader or bundler, for example if it wraps the enum and thereby prevents it from being attached to the global object, you can add this line. I'd still suggest a more elegant way to do this, since I can see how this can be commonly used by many developers, but since it solves my problem I'll close the issue.

There are so many runtime issues for determining what the global scope actually is. For those running in a browser window is fine, but getting true isomorphic code is far more complex web workers and NodeJS are two major challenges.JavaScript has one type with a finite amount of values: boolean, which has the values true and false and no other values.

With enums, TypeScript lets you define similar types statically yourself. The entries No and Yes are called the members of the enum NoYes. As in object literals, trailing commas are allowed and ignored.

We can use members as if they were literals such as true, or 'abc' — for example:. Each enum member has a name and a value.

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The default for enums is to be numeric. That is, each member value is a number:. We can omit the value of a member if the preceding member value is a number. Then TypeScript increments that value by one and uses it for the current member:. There is no way to compute the names of enum members. Object literals support computed names via square brackets.

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The last kind of enums is called heterogeneous. The member values of a heterogeneous enum are a mix of numbers and strings:.

Why TypeScript enums suck

Note that the previously mentioned rule applies here, too: We can only omit an initializer if the previous member value is a number. Alas, TypeScript only supports numbers and strings as enum member values. Other values, such as symbols, are not allowed. Constant enum members are initialized via expressions whose results can be computed at compile time. In this list, earlier entries are less flexible, but support more features.

The next subsections cover each entry in more detail. If an enum has only literal members, we can use those members as types similar to how, e. An enum member is constant if its value can be computed at compile time. Therefore, we can either specify its value implicitly that is, we let TypeScript specify it for us. Or we can specify it explicitly and are only allowed to use the following syntax:. But we can still do exhaustiveness checks. The values of computed enum members can be specified via arbitrary expressions.

For example:. This was a numeric enum. String-based enums and heterogeneous enums are more limited. For example, we cannot use method invocations to specify member values:. 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:.

Daniel Rosenwasser explains :. The behavior is motivated by bitwise operations. There are times when SomeFlag. Foo SomeFlag.In this article, we are explaining how we can use the switch statement in TypeScript with an example. Specifically, it can be used in place of an if statement with multiple else if just one expression is tested for equality with several values.

The switch statement implements a control structure that is often referred to as the case structure. The switch statement starts with the word switch followed by a switch expression inside of parentheses.

This expression is not a logical expression.

JAVA - Зачем нужен Enum? Фрукты по списку.

It is an expression that returns a single value that will be used to determine which case to execute. The expression is often as simple as a single variable. Once the value for the expression is found, the switch statement checks each of the values in the case labels.

Then, it begins executing the code that follows the first case label that is equal to the result of the expression. It continues executing until it reaches either a break statement or the end of the switch statement. If no case labels match the value in the switch expression, the switch statement starts executing the code that follows the default label.

But this default case is optional. If it is omitted and no case labels match the expression, the switch statement won't execute any code. In this example the switch statement is used to display the marks range against a particular grade.

TypeScript Data Type - Enum

Let's see how I implement the switch statement in TypeScript. Let's use the following steps. A window is opened. Give the name of your application as "switchcase" and then click ok.

This window is called the Solution Explorer. View All. Switch Statement In TypeScript. Nitin Bhardwaj Updated date, Oct 14 Switch Statement in TypeScript. A switch statement is a convenient way to express a certain form of condition. The following example shows how to use a switch statement in TypeScript.

Switch Statement In TypeScript

After this session the project has been created; a new window is opened on the right.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Since TypeScript 1. For more info about TypeScript 2. See for more information. In TypeScript 1. It mimics a string enum's expected behaviour.

Make sure to type all the strings in the object! If you don't then in the first example above the variable would not be implicitly typed to MyStringEnum. In TypeScript 0. The code works as we expected and Visual Studio can support for automatic code completion. In syntax, TypeScript doesn't allow us to create an enum with string values, but we can hack the compiler :p.

Lookup typesintroduced in TypeScript 2. With version 2. From the TS docs:. A slightly more robust way of making a set of strings that I use for React these days is like this:. Here's a fairly clean solution that allows inheritance, using TypeScript 2. I didn't try this on an earlier version. Update: Shortly after posting this I discovered another way, but forgot to post an update however, someone already did mentioned it above :. I just declare an interface and use a variable of that type access the enum.

Keeping the interface and enum in sync is actually easy, since TypeScript complains if something changes in the enum, like so. Flags were another necessity for me, so I created an NPM module that adds to this example, and includes tests.

Please look into my npm package, ts-transformer-enumerate. There a lot of answers, but I don't see any complete solutions. I don't really like the "update" either, it's complex and doesn't leverage types as well.

I think Michael Bromley's answer is most correct, but it's interface is a bit of a hassle and could do with a type. The draw back is you have to write the strings twice, but at least it's only in two places. I think you should try with this, in this case the value of the variable won't change and it works quite like enums,using like a class also works the only disadvantage is by mistake you can change the value of static variable and that's what we don't want in enums.

If what you want is mainly easy debug with fairly type check and don't need to specify special values for the enum, this is what I'm doing:. TypeScript Playground. Learn more.


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