Code should fix name collisions by using a module import or renaming the For example, you might use a Type to represent the response from an API: In this example, the ApiResponse Type defines the expected properties and their types for the response from an API. They are not changed by the spread. Use arrow functions with expressions or blocks as their body as appropriate. Code must not use unary plus (+) to coerce strings to numbers. sometimes lets you drop the constructor entirely. While Enum and Type share some similarities, they have distinct differences that make them useful for different situations. formatting and placement of the comment is not prescribed. be converted to booleans with Boolean() or ! cases may require special handling. It is lacking in JavaScript, though. For example, you cannot do this: Without const, you would get the value 'UP', but because this is a const enum, we don't have the ability to get the actual value of the north direction. Here well elaborate more on these new declarations and why theyre preferable to var. after the slash, and is less obvious to readers. the form import {foo} from 'bar'; Your code must not use the namespace Foo { } construct. One problem they exacerbate is the fact that it is not an error to declare the same variable multiple times: Maybe it was easy to spot out for some experienced JavaScript developers, but the inner for-loop will accidentally overwrite the variable i because i refers to the same function-scoped variable. separate, non-exported class. One of the most important properties of unions is that they can contain all different kinds of types, not just strings and numbers. One of the main differences between Enum and Type is their respective structures. TypeScript supports two methods to organize code: namespaces and modules, Never use var. Enums are a set of named constants that can take either a numeric or string form. and debug code, in particular with re-exports across multiple modules. a pure function (i.e., result is includes several type operators based on these (Record, Partial, Readonly TypeScript code must not use obj['foo'] to bypass the visibility of a For example, if you are using @ts-ignore to suppress a type error, then it's In addition, TypeScript supports a special construct for optional parameters and That page quotes the TypeScript team lead: Honestly, my take is that it should Zero + Numbers. all type expressions (variables, fields, return types, etc). Fortunately, TypeScript allows you to specify that members of an object are readonly. Function expressions must not use this unless they specifically exist to Testing Blog We to There though: This is equivalent to spelling out the properties on FoodPreferences: To reduce duplication, User could extend FoodPreferences, or (possibly To document these fields, use JSDoc's @param annotation. Consider limiting the number of parent steps (../../../) as those can make Fundamentally ambient const enums (values -- types are fine) are incompatible with isolatedModules. requires them, Only to import libraries for code. If needed, document parameters at call sites inline using block comments. For non-ASCII characters, use the actual Unicode character (e.g. If the Some libraries might commonly use a namespace import prefix that violates this The snippet above is an example of type inference, explained earlier in the handbook. Avoid overly defensive programming. be used with no recommendations of their usage. Rather than just introducing a new environment to the loop itself, these declarations sort of create a new scope per iteration. document why it is legitimate. Notice that you can skip c if you dont need it. The main advantage that enums have over unions and constant objects is that the values are automatically mapped to their names, as well as the reverse: names can be mapped to values. types that are hard to understand. Announcing TypeScript 5.0 RC - TypeScript creates a temporary reference that can't be uninstalled. When using the spread operator on objects, later comments may not be necessary. Sometimes using any is legitimate, for example in tests to construct a mock Instead, code must only add |null or |undefined when the alias is actually conditional types this.listener(x); };), and should not obtain or pass references to instance most prone to create hard to understand and maintain programs. Leave out type annotations for trivially inferred types: variables or parameters If you click an affiliate link and subsequently make a purchase, we will earn a small commission at no additional cost to you (you pay nothing extra). They can only be downleveled to ES2015, not lower. Enums can be used to represent a set of options or choices in the application. Unless you take specific measures to avoid it, the internal state of a const variable is still modifiable. consistent and has no side effects). On the other hand, this does allow us to represent dynamic values to at least some degree. Reviewers may ask for annotations to clarify complex return When a class, method, or property have both decorators like @Component and // by invoking a function with its current value. Declaring a variable in JavaScript has always traditionally been done with the var keyword. Note: Number(''), Number(' '), and Number('\t') would return 0 instead both languages together. See also the So, if any values are repeated, then they will only appear once in the resulting type. and Treating Also, Maps can be frameworks may be structured with _ separators, e.g. to code that only runs server side (e.g. This should not be confused with the idea that the values they refer to are immutable. Type assertions (x as SomeType) and non-nullability assertions (y!) The direction is left-to-right, as if you had written: Confusingly, the colon here does not indicate the type. your IDE's find references (and thus rename property refactoring) will The spread operator is the opposite of destructuring. Explicitly terminate all is not specified. TypeScript enums already cannot be mutated; const enum is a separate language See advice on that parsing 12 Exception handlers must not defensively handle non-Error types unless the Since this is what we were doing anyway with our IIFE, we can change our old setTimeout example to just use a let declaration. However, when declaring types for objects, use interfaces instead of a type However, they alias for the object literal expression. These type system features allow succinctly specifying types and constructing Enum values can be accessed by their name or their index, while Type properties can only be accessed by their name. functions and methods) should not access this. It also helps in refactoring your code (the editor can figure out all places you used the enum value). Both only silence the TypeScript compiler, but do not insert any runtime terser names in the module that's imported. without introducing an extra interface. To help you decide whether to use a union, enum, or object, I've created a table that summarizes the key similarities and differences: Constant objects cannot be used directly as types, but it is simple to create a type of the key values: Unions can emulate named values when the values are strings, by making the value the same as the name. By the time the for loop has stopped executing, the value of i is 10. bugs. Alternatives to TypeScript enum. TypeScript Tutorial => const Enum Exception: Comparisons to the literal null value may use the == and the declaration of the symbol (this allows more precise type checking and error Understanding these distinctions can help developers write better code and create more efficient and maintainable projects. interesting technical reasons to prefer interface, Suppress the lint warning and document why, class / interface / type / enum / decorator / type description on constructor calls and property accesses. dwarves as 12). source code. In TypeScript, an enum is a way to define a set of named constants. It has confusing and contradictory usage: Instead, always use bracket notation to initialize arrays, or from to on-demand by volunteers. Use Enum for fixed sets of values, Type for defining data structures, and both judiciously based on the needs of the application. What is difference between enum vs const? | Javascript Job compiler, this pattern can be broken by rearranging the build rules, Variables declared in a catch clause also have similar scoping rules. automated check that is often a good sign. {});) discussed below. TypeScript: Documentation - Variable Declaration For example, suppose we have an object which resolves theme values for whether we should show a light or dark-mode color scheme. In this article we'll see why it's so verbose and what other solutions can improve our code. possible. the module keyword in the form module Foo { }. consider named parameters using object literals and destructuring. unions of enum types and other types) must not be implicitly coerced to The act of introducing a new name in a more nested scope is called shadowing. Also, see the Object spread also has a couple of other surprising limits. If an accessor is used to hide a class property, the hidden property may be In contrast, consider the following Type that represents a car: In this example, the properties of the Car Type can be modified or extended at runtime. So each time the given function gets called, it will print out 10! typescript - Enum vs As Const - Stack Overflow Others are dropped during the compile time. Only export symbols that are used outside of the module. In foo.ts: Results in error TS2614: Module '"./foo"' has no exported member 'fizz'. the function keyword. The syntax for defining an Enum is as follows: Here is an example of an Enum that represents days of the week: In this example, the numeric values for Monday through Sunday are 1 through 7, respectively. called API is conclusively known to throw non-Errors in violation of the above For example, when using RxJS the. Do not use private fields (also known as private identifiers): Instead, use TypeScript's visibility annotations: Private identifiers cause substantial emit size and Use descriptive names for Types that make it clear what they represent, such as "User" for a Type that represents a user object. const enum vs enum - this vs that - HTML DOM testing and private visibility time, they do not offer substantial benefits when static type checking is used const in an enum means the enum is fully erased during compilation. TS: Enum vs Union type in performance | by Suyeon Kang - Medium Entrepreneurship, Digital Marketing, Design & Ecommerce. If it really just doesn't matter that much -- if it's an obscure corner of the this pointer in general. are different in that they can be left out when constructing a value or calling 3. Different from other NB: TypeScript namespaces used to be called internal modules and used to use use setTimeout will run a function after some number of milliseconds, but only after the for loop has stopped executing; This often ends up being a source of bugs. The Difference Between TypeScript Unions, Enums, and Objects not to include.). in a function) then it must use lowerCamelCase. Exception: Symbols that are only exported to be consumed by tooling, such as Exception: There may be performance issues if try blocks are inside a loop. a local variable declared within a function, or a static field on a class nested expressions crossing file boundaries. very commonly used symbols, such as Jasmine's describe and it. How To Get String Value Of Enum In TypeScript - MarketSplash (T) or UpperCamelCase. (See, that for all examples here, union types will have "U" suffix, similarly to enums with "E", and const enums with "CE".) Explore how TypeScript extends JavaScript to add more safety and tooling. the non-Errors originate. Code should use relative imports (./foo) rather than absolute imports Code may rely on type inference as implemented by the TypeScript compiler for This can be used to name primitives, unions, Sometimes due to some local property of your code you can be sure that the That is, when creating an object, only objects may be used with // so we can use it just like any object: // => "margin-top" | "margin-right" | "margin-left", // | "margin-bottom" | "padding-top" | "padding-right", // => "color: red;" | "color: green;" | "color: blue;", everything you need to know about union types. just variable names though! let and const are two relatively new concepts for variable declarations in JavaScript. This also applies for readonly T[] vs ReadonlyArray. Identifiers should not generally use $, except when aligning with naming Hopefully this guide helped clear up the confusion between enums and unions, and made the advantages and disadvantages between enums, unions, and constant objects clear. require global rules. For more information, read our affiliate disclosure. For example, you can't set UP to '0'. often subject to change in TypeScript compiler versions. statements using a semicolon. Use Type for defining data structures, not for creating a limited set of constants. for more about what Names must be descriptive and clear to a new reader. and also violates optimization compatibility. Note that Typescript allows Enums to start with any numeric value, not just 0. that changes that we make to the language, don't break users. This optimization compatibility rule applies to all web apps. fields are always provided by the backend. hash, or dict). UserService and AjaxUserService. Avoid creating APIs that have return type only generics. This is great for improving the readability of code, essentially creating two sets: human-readable names, and machine-readable values. Restricting visibility of properties, methods, and entire types helps with fields of an interface change over time. (The label only exists For example, instead of using numeric values for days of the week, you can use an Enum to represent them: Types, on the other hand, are useful for defining the expected shape of data in the application, such as User objects, API responses, or database records. used: Further, never invoke the wrapper types as constructors (with new). Enum is a data type that consists of a set of named constants, which can be used to represent a fixed set of values. contains no code. Widening try blocks to cover a whole loop is ok. This is especially true with deeply nested destructuring, which gets really hard to understand even without piling on renaming, default values, and type annotations. Instead, just use regular imports and exports: Note: this does not apply to export as applied to a type definition, i.e. See. In short, union types are a way of modeling a discrete set of mutually exclusive types, or more simply put: unions model choices, where only one choice can be made. These Many or .., So, let's wrap things up by looking at a subjective opinion of what type I recommend you should use depending on the circumstances. For In the above example, all declarations of x actually refer to the same x, and this is perfectly valid. Use interfaces , an export type Foo = ;. Then, you need to remember to give a default for optional properties on the destructured property instead of the main initializer. declared lowerCamelCase. Most people expect the output to be. Using a constant object over an enum will result in a smaller bundle size, and more flexibility with being able to use dynamic values and interoperate with standard JavaScript syntax. Help us improve these pages by sending a Pull Request , How to provide types to functions in JavaScript, How to provide a type shape to JavaScript objects, How TypeScript infers types based on runtime behavior, 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. directory. Always use the simplest type construct that can possibly express your code. Only use an expression body if the return value of the function is actually When TypeScript originally introduced enums, they were nothing more than a set of numeric constants with the same type. implements, enum, private, override etc. allows to move the project around without introducing changes in these imports. callee's this is already bound, which increases confusion about what this keywords. An Enum in Typescript is a data type that allows developers to define a set of named constants. performance regressions when down-leveled by TypeScript, and are unsupported In the following sections, we will explore the differences between Enum and Type, and their respective use cases. object. TypeScript: TSConfig Option: isolatedModules One ); To this JavaScript: "use strict"; console.log(0 + 1); Shadowing should usually be avoided in the interest of writing clearer code. Another advantage of using these features over a more generic type like string is that the number of possible values is much lower and is enforced by the compiler. This can make programs However if the using these private properties, access the value through the accessor whenever and so situations where it does matter are undesirable because they will be and unquoted property access, for code hygiene. For example: Only keys can be easily derived, the values are not possible to subtype. This version of the loop will actually perform the summation correctly because the inner loops i shadows i from the outer loop. Enums vs Constant differences on Typescript - Stack Overflow Assignment By using a combination of Enums, Types, interfaces, classes, and generics, developers can create expressive and type-safe applications that are easy to maintain and extend. As with explicit conversions, values of enum types (including When passing a badFoo to a function that takes a Foo, the error will be at First of all, you need to remember to put the pattern before the default value. just defining a getter with no setter). TypeScript The variable x was declared within the if block, and yet we were able to access it from outside that block. It will counterintuitively a simple T[] nested in a more discussed here, and function expressions (doSomethingWith(function() expression. Instead, only throw (subclasses of) Error: When catching errors, code should assume that all thrown errors are instances constructed as a union type (string|null); similarly with undefined. program: Whether an annotation is required is decided by the code reviewer. Enums are useful for representing fixed sets of values, while Types are useful for defining the shape of data in the application. Instead, export individual constants and functions: There are four variants of import statements in ES6 and TypeScript: Both module and destructuring imports have advantages depending on the The badFoo object above relies on type inference. In general, engineers usually know best about what's needed in their code, so if It is also possible to define an enum as a const enum similar to the constant objects which we will talk about in this article. If the interpolated value is a union, multiple possibilities will be generated. The chapter on Interfaces has the details. Mapped & conditional types' evaluation model, in particular when combined It is a common feature to find in the majority of programming languages. We can create additional types that derive from a union, enum, or object. Typescript Enum vs Type: Understanding The Distinctions - MarketSplash Your Color is a perfect fit for enums. Recall that with our earlier setTimeout example, we ended up needing to use an IIFE to capture the state of a variable for every iteration of the for loop. projects should be consistent across So, unions and enums help make the code safer by automatically preventing simple typing mistakes. communicates intent. There's nothing wrong with TypeScript enums, it's people using the wrong tool for the job and then complaining that the tool is bad. Here, we have two local variables a and b. type aliases Their purpose is similar to union types, except they act on a much more basic principle. TypeScript code must not use the Array() constructor, with or without new. As we mentioned earlier, let is similar to var in some respects, but allows users to avoid some of the common gotchas that users run into in JavaScript. explicitly with comparison operators. Don't use that either. In contrast, here is a Type that represents a person: In this example, Person is a Type with three properties, name, age, and address. added to badFoo and the type is inferred based on the object itself. For the common pattern of conditionally exporting either of two values, first do I had previously used them in C# and felt a reassuring familiarity. are also allowed: Using the spread operator [foo]; {bar} is a convenient shorthand for out of two forms to prevent variation, we should choose one. Another great benefit of union types is that they can be easily transformed into other types. into its parent class because ES2015 provides a default class constructor if one For example, a check for unused code will We should usually match JavaScript style as well, because people often write If targeting ES2015, a modern runtime will throw an error; however, right now TypeScript is permissive and wont report this as an error. We've looked at a lot of examples and caveats of unions, enums, and objects. If declaring an interface is too heavyweight, you can use an inline object Now that we are a little bit familiar with unions, enums, and constant objects, let's compare them with each other and see how they stack up. Using strict typing can help catch errors at compile-time and make the code more maintainable and efficient. If you are tempted to create a Pair type, instead use a tuple type: However, often it's clearer to provide meaningful names for the properties. For example, here is an Enum that represents days of the week: In this example, you can access the Monday constant by its name or its index: In this example, you can only access the name, age, and address properties by their names: One final difference between Enum and Type is their flexibility at runtime. import an API through a different path. for users new to the language. We can use this construct as shown in the example below: the spread operator; when creating an array, only spread iterables. Yes, there are some ways to simulate enums there: By simply using key-valued objects However, those solutions are far from ideal. See Numeric enums We'll first start off with numeric enums, which are probably more familiar if you're coming from other languages. There are some differences between using an enum and a const object to define a set of related values in TypeScript:. Types are designed to define the shape of data in the application, such as User objects or API responses. Instead, module imports give a name to the entire relative, i.e. function move (direction:. Limit the amount of code inside a try block, if this can be done without hurting are also When there are two options that are equivalent in a superficial way, we containing class. Enums were added to TypeScript in anticipation of them becoming a standard JavaScript feature, however that currently seems unlikely based on the 4 year old, // Note: `Color` does not exist at run-time, so we, // ^^^^^ ERROR: 'Color' only refers, // to a type, but is being used as a value here. When a variable is declared using let, it uses what some call lexical-scoping or block-scoping. ). rule. 1 Answer Sorted by: 3 There are actually three different approaches: Type Allows you to specify a type that is restricted to the given values. For example, we can use union types to generate a large set of possible values: Or values can be narrowed down to just a subset: Neither enums nor unions have full support for dynamic values, but unions do have some partial support for dynamic value in the form of template literal types. while the others are truthy, which is likely to be unexpected. Consequently, engineers are accustomed to not thinking about this, Do not use bind in the expression that installs an event handler, because it TypeScript: Handbook - Enums modifiers or parameter decorators should not be omitted even if the body of Constructor calls must use parentheses, even when no arguments are passed: It is unnecessary to provide an empty constructor or one that simply delegates For example: copying arrays and objects. If we don't need the reverse mapping, this results in useless code being generated.