Disclaimer: This was originally posted on my own blog
This post might get updated from time to time.
TL;DR: GitHub-Gist
Skip all the explanations and just see types
I've grown pretty fond of Typescript in recent years.
In particular I very much like its Turing-complete type system.
Typescript provides a fair share of built-in Utility-Types, however, I always come back copy-pasta-ing from my old code when requiring more complex util-types.
So here's kind of a cheat-sheet, with examples and explanations, for some of the advanced types I've built and used within the last years.
Also, check out SimplyTyped or TypeFest. They contain some truly mind-boggling, but extremely useful stuff.
I try not to include it in every one of my projects, so there might be some duplicates.
As I find it pretty hard to explain more in-depth types, let me know where you would like to see clarification.
Also let me know if you know any other cool, usefull or mindbending stuff!
So, without further ado, Let's dive in
Await Type
This one is pretty straightforward.
We use the built-in PromiseLike type to check if the input really is a promise.
If it is, use the infer keyword to deduce its type-argument and recursively 'call' Await again.
If not, just return the input.
type Await<T> = T extends PromiseLike<infer U> ? Await<U> : T;const asyncFn = async (): Promise<number> => new Promise((res) => res(1337));type ResolvedReturnType = Await<ReturnType<typeof asyncFn>>; // = number
Length Type
Another simple one is the Length type.
You can use this for example to infer the number of parameters to a given function or (readonly) array-lengths
There is, however, another clever use, to which we will come back later.
export type Length<T> = T extends { length: infer L } ? L : never;type MyFnType = (a: number, b: string, c: number) => unknown;type NumParams = Length<Parameters<MyFnType>>; // = 3
KeysOfType
I find this one particularly useful from time to time.
It returns the keys of a given interface O, but only if they are of type T
export type KeysOfType<O, T> = { [K in keyof O]: O[K] extends T ? K : never;}[keyof O];// example interface MyInterface { keyA: number, keyB: string, keyC: Record<string, MyInterface>, keyD: 1337};type KeysOfTypeNumber = KeysOfType<MyInterface, number>; // 'keyA' | 'keyD'This one is not as easy to unserstand, so how does it work?
First, we create a new type that contains all of the keys of O,
then we get the valueType of O by its key O[K] and then check if it is of type T
If so, we just use the key K as our value, otherwise, we use never.
This would result in something like { keyA: 'keyA'; keyB: never }
The last step now is to look up this intermediate type, by all the keys of our interface O.
The never type gets discarded when looking up a type like this.
But because of this, if we wanted to filter our interface, we cannot just leave out the lookup.
Bonus: PickByType
// ConvertLiterals would convert literal types like `1337` to their base type like `number` if set to true export type PickByType<O, T, ConvertLiterals extends boolean = false> = { [K in KeysOfType<O, T>]: ConvertLiterals extends true ? T : O[K]};type PickedByNumber = PickByType<MyInterface, number>; // { keyA: number; keyD: 1337 }
OneOf
Suppose you have an Interface (or intersecting Interfaces) and you want to create a (strict) Union type from it.
In the aforementioned SimplyTyped library, there exist types with which you can do the same thing.
I personally like this version better, though, as it's a single 'one-liner' (cough) type that one can even parameterize.
Let's start with just the type
export type OneOf<T, Strict extends boolean = true> = { [OuterKey in keyof T]: Strict extends false ? { [K in OuterKey]: T[K] } : { [InnerKey in OuterKey|keyof T]?: InnerKey extends OuterKey ? T[OuterKey] : never } & { [TheKey in OuterKey]: T[OuterKey] } }[keyof T];
Now break this down somewhat.
First, we omit the second type parameter Strict and only use the then false-case and just look at a simpler Unionize type
export type Unionize<T> = { [OuterKey in keyof T]: { [K in OuterKey]: T[K] }}[keyof T];
This creates a type with the keys of the input type T as key and a new dictionary type containing a single key-of T and the corresponding value type of T as value.
Afterward, we use the look-up trick we already used above to get a union type of the inner dictionary types (hence the unionize name)
interface MyIface { keyA: number; keyB: string;}type Demo = Unionize<MyIface>; // = { keyA: number } | { keyB: string }
However...
const a: Unionize<MyIface> = { keyA: 1337 }; // okconst b: Unionize<MyIface> = { keyA: 1337, keyB: 'foo' }; // also okay
This is because union dictionary types are not exclusive.
In some cases, it can be okay, other times you'd want it to be strict.
So let's look at that
export type OneOf<T> = { [OuterKey in keyof T]: { [InnerKey in OuterKey|keyof T]?: InnerKey extends OuterKey ? T[OuterKey] : never } & { [TheKey in OuterKey]: T[TheKey] } }[keyof T];
Now, this is a bit more complex.
If we look closely, however, it is the same as the Unionize type shown above except for an intersecting type: { [InnerKey in OuterKey|keyof T]?: InnerKey extends OuterKey ? T[OuterKey] : never }.
Let's break this down some more.
OuterKey is a single key of our input-type T. Create a union with that and with all keys of T as keys for the inner dictionary ([InnerKey in OuterKey|keyof T]: ...).
For the value of the inner dictionary, we check if the current InnerKey is the OuterKey (InnerKey extends OuterKey ...).
If it is, we use the value-type of our original type T, otherwise never.
The last part is to make every key for this dictionary optional (the ? after the key declaration).
Now this gives us a type which, has all keys of the input-type, but only a single key is allowed to be assigned with a value-type other than undefined.
That means we can still use undefined, though.
We can still use undefined, though.
To mitigate this, we add an intersection to an inner dictionary, that being the one we've seen in the Unionize type, as it doesn't allow undefined.
The result is the something like this
export type OneOf<T, Strict extends boolean = true> = { [OuterKey in keyof T]: Strict extends false ? { [K in OuterKey]: T[K] } : { [InnerKey in OuterKey|keyof T]?: InnerKey extends OuterKey ? T[OuterKey] : never } & { [TheKey in OuterKey]: T[TheKey] } }[keyof T];// exampleinterface MyInterface { keyA: number, keyB: string, keyC: Record<string, unknown>, keyD: 1337};type OnlyOne = OneOf<MyInterface>; // ^= more or less strict version of { keyA: number } | { keyB: string } | { keyC: Record<string, MyInterface> } | { keyD: 1337 }type NonStrictVersion = OneOf<MyInterface, false>; // { keyA: number } | { keyB: string } | { keyC: Record<string, MyInterface> } | { keyD: 1337 }
Tuple Manipulation
Tuples, combined with the ability to spread them and infer single or even rest-types, are an incredibly useful tool when dealing with more complex types.
So here are a few basic manipulation types starting with:
Push, PushFront, Pop, PopFront, Shift, ShiftRight
type Push<T extends unknown[], U> = T extends [...infer R] ? [...T, U] : never;type PushFront<T extends unknown[], U> = T extends [...infer R] ? [U, ...T] : never;type Pop<T extends unknown[]> = T extends [...infer R, infer U] ? U : never;type PopFront<T extends unknown[]> = T extends [infer U, ...infer R] ? U : never;type Shift<T extends unknown[]> = T extends [infer U, ...infer R] ? R : never;type ShiftRight<T extends unknown[]> = T extends [...infer R, infer U] ? R : never;
These should all be pretty straightforward.
Note: You could just as well use any instead of infer X where X isn't used.
Reverse
Since there are no loop-functions for TypeScripts type-system, and we somehow need to iterate over the input tuple, we have to use recursion.
To check when the recursion needs to end, we can use the aforementioned Length-type. Afterward, just Pop from one tuple, and Push to another.
type Reverse<T extends unknown[], U extends unknown[] = []> = Length<T> extends 1 ? Push<U, Pop<T>> : Reverse<ShiftRight<T>, Push<U, Pop<T>>>;
A little bit special here is the second type-parameter, which is default initialized to an empty tuple.
We basically use that as our return value, after recursively filling it from the input T.
If you wanted to restrict any other devs in your codebase to do nasty things with the second type parameter, you can just wrap the above implementation.
Filter, TupleIncludes
Next, let's look at a filter-type. This one's also pretty simple.
First, check if our tuple is empty. If it's not, infer the first Element F and the rest of the tuple as another type R.
Then we check if F is equal to, or extends, the type we want to filter out.
If it is, return Filter again, but this time on the rest R without F.
Otherwise, we return a new tuple, with F at the front and a spread (...) of the filtered-rest.
Combining the Filter type with our handy-dandy Length-type, we can also build an Includes-type by simply comparing the lengths of the filtered and unfiltered tuple!
type Filter<T extends unknown[], U> = T extends [] ? [] : T extends [infer F, ...infer R] ? F extends U ? Filter<R, U> : [F, ...Filter<R, U>] : nevertype TupleIncludes<T extends unknown[], U> = Length<Filter<T, U>> extends Length<T> ? false : true
You may have noticed that I didn't actually call the Includes-type like that, but rather TupleIncludes.
We'll come back to that just below...
Template Literal Types
My favorite "subtype" of Types.
Introduced in TypeScript 4.1, they really filled a gap for me.
With them, you can do so much fancy and mindblowing stuff
Someone even wrote a full-blown CSS-Parser 🤯!
Fancyness aside, I also want to share a few types I've found useful to me.
StringIncludes
The nice part about template literals is that you can infer literal types from them.
Even nicer: You can infer empty strings!
Which makes a type that can check if any string includes another pretty trivial.
type StringIncludes<S extends string, D extends string> = S extends `${infer T}${D}${infer U}` ? true : false;type Demo1 = StringIncludes<'a.b', '.'>; // = truetype Demo2 = StringIncludes<'ab', '.'>; // = falsetype Demo3 = StringIncludes<'a.', '.'>; // = truetype Demo4 = StringIncludes<'.', '.'>; // = true
As we now also have a StringIncludes type, we can now combine that with the TupleIncludes from above, to get a completely generic Includes type
type Includes<T extends unknown[]|string, U> = T extends unknown[] ? TupleIncludes<T, U> : T extends string ? U extends string ? StringIncludes<T, U> : never : never;
Template literal types are (or were, at the time of initially writing most of this), however, for some reason, more restricted than tuples, so we need to build a few types to help with that.
Split
type Split<S extends string, D extends string> = string extends S ? string[] : S extends '' ? [] : S extends `${infer T}${D}${infer U}` ? [T, ...Split<U, D>] : [S];
You may wonder why the first conditional is string extends S.
If you haven't guessed, it's the check if string itself is passed as a type argument, instead of any literal.
Any literal string extends string - but string doesn't extend any literal string
Here's some code to better explain:
type Demo<S extends string> = string extends S ? 'used string' : `used literal: ${S}`;type D1 = Demo<'str'>; // = 'used literal: str'type D2 = Demo<string>; // = 'used string'
Join
type Join<T extends unknown[], D extends string> = string[] extends T ? string : T extends string[] ? PopFront<T> extends string ? Length<T> extends 1 ? `${PopFront<T>}` : `${PopFront<T>}${D}${Join<Shift<T>, D>}` : never : never;
Using what we've learned so far and some of the tuple manipulation types above, we can now join any string tuple back to a string.
Example using all of this
A cool and really useful example, using most of the above util-types, is dealing with "Paths" and nested-types of dictionaries.
(Explanations for those are left as an excersice for the reader)
Let's start with valid Paths
ValidPaths, ValidPathTuples
At the time of initially writing this, the Tuple version was the only one working.In most recent TS versions both work, however, the string literal version has got some gotchas and so the tuple version is somewhat preferred.
const dictionary = { someProp: 123, nested: { moreProps: 333, deeper: { evenDeeper: { deepest: 'string' } }, alsoDeeper: { randomProp: { anotherProp: 'wtf' } } }} as const;type MyDict = typeof dictionary;// If you look at type "Debug" below, this surprisingly works.// You may get a `Type instantiation is excessively deep and possibly infinite.`-error if you're using an older TS version, thoughtype ValidPaths<T> = keyof T extends never ? never : ({ [K in keyof T]: T[K] extends never ? never : T[K] extends Record<string|number|symbol, unknown> ? K extends string ? `${K}.${ValidPaths<T[K]>}` | K : never : K })[keyof T] & string;type Debug = ValidPaths<MyDict>;// With Tuples.type ValidPathTuples<T> = keyof T extends never ? never : ({ [K in keyof T]: T[K] extends never ? never : T[K] extends Record<string|number|symbol, unknown> ? [K, ...ValidPathTuples<T[K]>] | [K] : [K] })[keyof T];type DebugTuples = ValidPathTuples<MyDict>;// on recent TS versions you can even join the tuples back to "PathStrings"type DebugTuples1 = Join<ValidPathTuples<MyDict>, '.'>;
typescript
Playground
NestedType, NestedTypeByTuple
We can also craft a type that gets us the nested-type of a dictionary for any valid path, or throw an error if an invalid path is passed.
And of course we can put it all together in a beautiful function where only valid Paths are allowed as parameter and the return type is automagically deduced for us.
// string versiontype NestedType<T, P extends string> = ( Includes<P, '.'> extends true ? PopFront<Split<P, '.'>> extends keyof T ? NestedType<T[PopFront<Split<P, '.'>>], Join<Shift<Split<P, '.'>>, '.'>> : never : P extends keyof T ? T[P] : never);type DemoNested1 = NestedType<MyDict, "nested.moreProps">; // 333type DemoNested2 = NestedType<MyDict, "nested.deeper.evenDeeper">; // { readonly deepest: "string" }type DemoNestedNever = NestedType<MyDict, "nested.randomProp">; // never// tuple versiontype NestedTypeByTuple<T, P extends string[]> = ( Length<P> extends 1 ? Pop<P> extends keyof T ? T[Pop<P>] : never : PopFront<P> extends keyof T ? Shift<P> extends string[] ? NestedTypeByTuple<T[PopFront<P>], Shift<P>> : never : never);type DemoNestedTuple1 = NestedTypeByTuple<MyDict, ["nested" ,"moreProps"]>; // 333type DemoNestedTuple2 = NestedTypeByTuple<MyDict, ["nested" , "deeper", "evenDeeper"]>; // { readonly deepest: "string" }type DemoNestedTupleNever = NestedTypeByTuple<MyDict, ["nested", "sillyProp"]>; // never// String version internally using tuples// Bonus: Also errors nowtype NestedTypeUsingTuplesAgain<T, P extends ValidPaths<T>> = NestedTypeByTuple<T, Split<P, '.'>>;type Convoluted = NestedTypeUsingTuplesAgain<MyDict, 'nested.alsoDeeper'>;type ConvolutedError = NestedTypeUsingTuplesAgain<MyDict, 'nested.nonExistant'>;// And now we can finally give lodash's `get` function a run for it's moneyfunction GetByPath<T, P extends ValidPaths<T>>(dict: T, path: P): NestedType<T, P> { // internal impl. must be different. only demoing types here! return (dict as any)[path];}
FAQs
What advanced TypeScript type allows you to define a type from another type? ›
Extract. Extract allows you to construct a type by picking properties that are present in two different types. The utility will extract from T all properties that are assignable to U .
What are the data types available in TypeScript and explain in detail? ›| Data type | Keyword | Description |
|---|---|---|
| String | string | Represents a sequence of Unicode characters |
| Boolean | boolean | Represents logical values, true and false |
| Void | void | Used on function return types to represent non-returning functions |
| Null | null | Represents an intentional absence of an object value. |
TypeScript has two special types, null and undefined , that have the values null and undefined respectively. We mentioned these briefly in the Basic Types section. By default, the type checker considers null and undefined assignable to anything. Effectively, null and undefined are valid values of every type.
Should I avoid any in TypeScript? ›❌ Don't use any as a type unless you are in the process of migrating a JavaScript project to TypeScript. The compiler effectively treats any as “please turn off type checking for this thing”. It is similar to putting an @ts-ignore comment around every usage of the variable.
What are the three main simple types in TypeScript? ›- boolean - true or false values.
- number - whole numbers and floating point values.
- string - text values like "TypeScript Rocks"
KEY DIFFERENCES:
= is called as assignment operator, == is called as comparison operator whereas It is also called as comparison operator. = does not return true or false, == Return true only if the two operands are equal while === returns true only if both values and data types are the same for the two variables.
These three dots are called the spread syntax or spread operator. The spread syntax is a feature of ES6, and it's also used in React. Spread syntax allows you to deconstruct an array or object into separate variables.
What data type is {} in TypeScript? ›const {variable name}: {variable type} = {variable value}
This is the convention that the majority of TypeScript data types are declared with the exception of functions and objects.
| Data Type | Used for | Example |
|---|---|---|
| Integer | Whole numbers | 7, 12, 999 |
| Float (floating point) | Number with a decimal point | 3.15, 9.06, 00.13 |
| Character | Encoding text numerically | 97 (in ASCII, 97 is a lower case 'a') |
| Boolean | Representing logical values | TRUE, FALSE |
Introduction to TypeScript JSON type. The TypeScript comes up with the functionality of working with JSON Type data. JSON being the JavaScript Object Notation, is used to make a data model that is easy to write and read. We can easily analyze large and complex data set with this TypeScript JSON type.
What language is TypeScript most similar to? ›
Syntactically, TypeScript is very similar to JScript .NET, another Microsoft implementation of the ECMA-262 language standard that added support for static typing and classical object-oriented language features such as classes, inheritance, interfaces, and namespaces.
What is disadvantage of TypeScript? ›Another disadvantage of TypeScript is the required extra compilation step. This step can slow down the build time and complicate the bundler setup. Since TypeScript adds many new features that can be unknown to a frontend developer, it increases the learning curve of a codebase.
Is TypeScript Overhyped? ›TypeScript is a good language but a bit too much overhyped. You could still try to advocate for it in large codebases. It might have some advantages if your team has the knowledge. However, for small projects or MVP applications, I would advise against it.
Why is TypeScript so loved? ›TypeScript extends JavaScript and improves the developer experience. It enables developers to add type safety to their projects. Moreover, TypeScript provides various other features, like interfaces, type aliases, abstract classes, function overloading, tuple, generics, etc.
Is TypeScript functional or OOP? ›Although TypeScript is a superset of JavaScript, object-oriented programming in TypeScript differs from Object-oriented programming in JavaScript because, unlike JavaScript, TypeScript has full class support, has access modifiers, and type annotations like most object-oriented programming languages.
What does {} do in JavaScript? ›It's used to reference a variable within string: let someVar = "World!" console. log(`Hello ${someVar}`); // Output is Hello World!
Which is easier TypeScript or JavaScript? ›All of these languages are great, but TypeScript has one key advantage over them that makes it more suitable for frontend development: TypeScript is far easier to learn for current JavaScript developers, mainly because it's just augmented JavaScript.
Is TypeScript an OOP language? ›TypeScript on the other hand can be treated as an object-oriented language because of the language constructs it introduces on top of JavaScript closures. In this chapter, we will discuss each of the core concepts behind object-oriented ...
How long does it take to learn TypeScript? ›If TypeScript is your first language, it will likely take you anywhere from 3 months to a year to learn. Mainly because you need to learn JavaScript and type systems as well! However, if you understand JavaScript and how type systems work, you can easily get the basics of TypeScript down within a day to a week.
Is TypeScript going to replace JavaScript? ›Since this typed programming language was meant to work with JS, it will never replace JavaScript. It will always build on JS as a solid foundation. It is expected that TypeScript will keep growing in popularity for the foreseeable future as it offers many benefits to developers.
What is '!' In JavaScript? ›
Logical NOT (!) The logical NOT ( ! ) (logical complement, negation) operator takes truth to falsity and vice versa. It is typically used with boolean (logical) values. When used with non-Boolean values, it returns false if its single operand can be converted to true ; otherwise, returns true .
What does || mean in TypeScript? ›The logical OR ( || ) (logical disjunction) operator for a set of operands is true if and only if one or more of its operands is true. It is typically used with boolean (logical) values.
What does _ do in TypeScript? ›If you use _ , it explicitly states that the function will be passed one argument, but that you don't care about it. The function's . length will be 1, which might matter in some frameworks.
Is TypeScript static or dynamic? ›It's inferred as any. And the reasons why it happens are very important. No, it's not because TypeScript's type inference is incapable. Thing is, that despite the fact that there are both type annotations and type inference, it is still dynamic language.
What type is array in TypeScript? ›An array is a special type of data type which can store multiple values of different data types sequentially using a special syntax. TypeScript supports arrays, similar to JavaScript.
How do you declare a variable in TypeScript? ›The type syntax for declaring a variable in TypeScript is to include a colon (:) after the variable name, followed by its type. Just as in JavaScript, we use the var keyword to declare a variable.
What are the 5 main data types? ›Most modern computer languages recognize five basic categories of data types: Integral, Floating Point, Character, Character String, and composite types, with various specific subtypes defined within each broad category.
What are the 4 main data types? ›4 Types of Data: Nominal, Ordinal, Discrete, Continuous | upGrad blog.
Is JSON is a serialization type? ›Json namespace provides functionality for serializing to and deserializing from JavaScript Object Notation (JSON). Serialization is the process of converting the state of an object, that is, the values of its properties, into a form that can be stored or transmitted.
What is the difference between interface and type in TypeScript? ›Type aliases and interfaces are very similar, and in many cases you can choose between them freely. Almost all features of an interface are available in type , the key distinction is that a type cannot be re-opened to add new properties vs an interface which is always extendable.
Can JSON have boolean values? ›
JSON Booleans
Values in JSON can be true/false.
Learn TypeScript by Codecademy is an excellent way to learn TypeScript through guided tutorials. In this free course, you'll apply the JavaScript syntax you already know to TypeScript's type system. By the end of the course, you'll be familiar with how TypeScript works.
What should I learn before TypeScript? ›The answer is that you can't learn TypeScript without learning JavaScript! TypeScript shares syntax and runtime behavior with JavaScript, so anything you learn about JavaScript is helping you learn TypeScript at the same time.
What is TypeScript for dummies? ›What is TypeScript? TypeScript is a superset of JavaScript, meaning that it does everything that JavaScript does, but with some added features. The main reason for using TypeScript is to add static typing to JavaScript. Static typing means that the type of a variable cannot be changed at any point in a program.
Is TypeScript more oop than JavaScript? ›We know that TypeScript offers more features than JavaScript. In a nutshell, TypeScript is an object-oriented programming language, whereas JavaScript is a scripting language. Thus, TypeScript offers interfaces and modules through ES6 features; on the other hand, JavaScript doesn't offer such features.
Why do people prefer TypeScript over JavaScript? ›TypeScript is better than JavaScript in terms of language features, reference validation, project scalability, collaboration within and between teams, developer experience, and code maintainability.
Is TypeScript easier than Python? ›Features like generics and static typing make it easier to do functional programming in TypeScript than in Python. This could be an advantage because demand for functional code is growing due to developments in data science, parallel programming, asynchronous programming, and more.
How do you change from one type to another in TypeScript? ›Converting a variable from one type to another is possible with type casting in Typescript. Type casting in TypeScript can be done with the 'as' keyword or the '<>' operator.
How do I create a new type from existing type in TypeScript? ›- Generics - Types which take parameters.
- Keyof Type Operator - Using the keyof operator to create new types.
- Typeof Type Operator - Using the typeof operator to create new types.
- Indexed Access Types - Using Type['a'] syntax to access a subset of a type.
To inherit the interface from one to another interface, in TypeScript we can use the keyword extends that allow us to extend the set of properties and attributes from one interface to another and access it into the child interfaces accordingly.
How TypeScript define nested types? ›
TypeScript provides you with multiple ways to define type definitions for objects. Object properties can be assigned type definitions using the `type` keyword in TypeScript. Nested objects can be defined using the type keyword itself.
How do I compare two objects values in TypeScript? ›- JSON.stringify(obj1) === JSON.stringify(obj2)
- Object.entries(obj1).toString() === Object.entries(obj2).toString()
You can't. You have to return an object. (Arrays are objects, often used for exactly this use case.)
How do you assign a variable to a type in TypeScript? ›Variable Declaration in TypeScript
The type syntax for declaring a variable in TypeScript is to include a colon (:) after the variable name, followed by its type. Just as in JavaScript, we use the var keyword to declare a variable.
// One major difference between type aliases vs interfaces are that interfaces are open and type aliases are closed. This means you can extend an interface by declaring it a second time. // In the other case a type cannot be changed outside of its declaration.
What are interfaces in TypeScript? ›An Interface is a structure which acts as a contract in our application. It defines the syntax for classes to follow, means a class which implements an interface is bound to implement all its members.
Can we implement multiple interfaces in TypeScript? ›An interface can be extended by other interfaces. In other words, an interface can inherit from other interface. Typescript allows an interface to inherit from multiple interfaces. Use the extends keyword to implement inheritance among interfaces.
Can you extend interfaces in TypeScript? ›TypeScript lets you augment an interface by simply declaring an interface with an identical name and new members. This lets you extend existing JavaScript code without creating a new named type.
What is the difference between implements and extends in TypeScript? ›Today, a friend ask about the difference between extends and implements . The short answer for him was: extends: The class get all these methods and properties from the parent, so you don't have to implement. implements: The class has to implement methods and properties.
How TypeScript define Singleton? ›- Restrict the usage of the new keyword on the class (using the new keyword on the class creates a new class instance.
- Provide a new way to instantiate the class.
What is the difference between array and [] in TypeScript? ›
There is no difference at all. Type[] is the shorthand syntax for an array of Type . Array<Type> is the generic syntax. They are completely equivalent.
Why we use typeof in TypeScript? ›typeof is used to differentiate between the different types in TypeScript. By the use of typeof we can differentiate between number, string, symbol, Boolean, etc. typeof can be used with any type in TypeScript, by the use of it we can re-use the code by passing any parameter type.