TypeScript is a must know if you're a front developer or even a fullstack developer. Our curated list of TypeScript questions can be used for all levels of developers. Go through all questions and nail your next web developer tech interview in 2021.
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TypeScript is a free and open-source programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript, and adds optional static typing and class-based object-oriented programming to the language.
TypeScript has following benefits.
- It helps in code structuring.
- Use class based object oriented programming.
- Impose coding guidelines.
- Offers type checking.
- Compile time error checking.
- Intellisense.
TypeScript is a superset of JavaScript which primarily provides optional static typing, classes and interfaces. One of the big benefits is to enable IDEs to provide a richer environment for spotting common errors as you type the code. For a large JavaScript project, adopting TypeScript might result in more robust software, while still being deployable where a regular JavaScript application would run.
In details:
- TypeScript supports new ECMAScript standards and compiles them to (older) ECMAScript targets of your choosing. This means that you can use features of ES2015 and beyond, like modules, lambda functions, classes, the spread operator, destructuring, today.
- JavaScript code is valid TypeScript code; TypeScript is a superset of JavaScript.
- TypeScript adds type support to JavaScript. The type system of TypeScript is relatively rich and includes: interfaces, enums, hybrid types, generics, union and intersection types, access modifiers and much more. TypeScript makes typing a bit easier and a lot less explicit by the usage of type inference.
- The development experience with TypeScript is a great improvement over JavaScript. The IDE is informed in real-time by the TypeScript compiler on its rich type information.
- With strict null checks enabled (
--strictNullCheckscompiler flag) the TypeScript compiler will not allow undefined to be assigned to a variable unless you explicitly declare it to be of nullable type. - To use TypeScript you need a build process to compile to JavaScript code. The TypeScript compiler can inline source map information in the generated .js files or create separate .map files. This makes it possible for you to set breakpoints and inspect variables during runtime directly on your TypeScript code.
- TypeScript is open source (Apache 2 licensed, see github) and backed by Microsoft. Anders Hejlsberg, the lead architect of C# is spearheading the project.
Generics are able to create a component or function to work over a variety of types rather than a single one.
/** A class definition with a generic parameter */ class Queue<T> { private data = []; push = (item: T) => this.data.push(item); pop = (): T => this.data.shift(); }
const queue = new Queue<number>(); queue.push(0); queue.push("1"); // ERROR : cannot push a string. Only numbers allowed
Modules in Typescript helps in organizing the code. There are 2 types of Modules — Internal and External
Internal Modules are now replaceable by using Typescript’s namespace.
External Modules used to specify and load dependencies between multiple external js files. If there is only one js file used, then external modules are not relevant.
These are also called the primitive types in TypeScript:
- Number type: it is used to represent number type values and represents double precision floating point values.
var variable_name: number;- String type: it represents a sequence of characters stored as Unicode UTF-16 code. It is the same as JavaScript primitive type.
var variable_name: string;- Boolean type: in Typescript, it is used to represent a logical value. When we use the Boolean type, we get output only in true or false. It is also the same as JavaScript primitive type.
var variable_name: bool;- Null type: it represents a null literal and it is not possible to directly reference the null type value itself.
var variable_name:number = null;- Undefined type: it is the type of undefined literal. This type of built-in type is the sub-type of all the types.
var variable_name:number = undefined;Yes we do. Typescript is just a language Extension browsers can't interpret it. Converting from TypeScript to JavaScript is called compiling. Compiling doesn't mean binary code is created in this case. For this kind of translation, also the term transpilation is used instead of compilation.
JavaScript is the only client side language universally supported by all browsers. But JavaScript is not the best designed language. It’s not a class-based object-oriented language, doesn’t support class based inheritance, unreliable dynamic typing and lacks in compile time error checking. And TypeScript addresses all these problems. In other words, TypeScript is an attempt to “fix” JavaScript problems.
TypeScript is a free and open source programming language developed and maintained by Microsoft. It is a strict superset of JavaScript, and adds optional static typing and class-based object-oriented programming to the language. TypeScript is quite easy to learn and use for developers familiar with C#, Java and all strong typed languages. At the end of day “TypeScript is a language that generates plain JavaScript files.”
As stated on Typescript official website, “TypeScript lets you write JavaScript the way you really want to. TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. Any browser. Any host. Any OS. Open Source.” Where “typed” means that it considers the types of variables, parameters and functions.
A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators are functions that take their target as the argument. With decorators we can run arbitrary code around the target execution or even entirely replace the target with a new definition.
There are 4 things we can decorate in ECMAScript2016 (and Typescript): constructors, methods, properties and parameters.
One of TypeScript’s core principles is that type-checking focuses on the shape that values have.
An interface is a virtual structure that only exists within the context of TypeScript. The TypeScript compiler uses interfaces solely for type-checking purposes.
When you define your interface you’re saying that any object (not an instance of a class) given this contract must be an object containing interfaces properties.
We use classes as object factories. A class defines a blueprint of what an object should look like and act like and then implements that blueprint by initialising class properties and defining methods. Classes are present throughout all the phases of our code.
Unlike classes, an interface is a virtual structure that only exists within the context of TypeScript. The TypeScript compiler uses interfaces solely for type-checking purposes. Once code is transpiled to its target language, it will be stripped from interfaces.
A class may define a factory or a singleton by providing initialisation to its properties and implementation to its methods, an interface is simply a structural contract that defines what the properties of an object should have as a name and as a type.
In TypeScript, the const keyword cannot be used to declare class properties. Doing so causes the compiler to an error with "A class member cannot have the 'const' keyword." TypeScript 2.0 has the readonly modifier:
class MyClass { readonly myReadonlyProperty = 1;<span class="token cMod">myMethod</span><span class="token cBase">(</span><span class="token cBase">)</span> <span class="token cBase">{</span> console<span class="token cBase">.</span><span class="token cMod">log</span><span class="token cBase">(</span><span class="token cVar">this</span><span class="token cBase">.</span>myReadonlyProperty<span class="token cBase">)</span><span class="token cBase">;</span> <span class="token cBase">}</span>}
new MyClass().myReadonlyProperty = 5; // error, readonly
TypeScript supports getters/setters as a way of intercepting accesses to a member of an object. This gives you a way of having finer-grained control over how a member is accessed on each object.
class foo { private _bar:boolean = false;get bar():boolean { return this._bar; } set bar(theBar:boolean) { this._bar = theBar; } }
var myBar = myFoo.bar; // correct (get) myFoo.bar = true; // correct (set)
The answer is YES. There are 4 main principles to Object Oriented Programming:
- Encapsulation,
- Inheritance,
- Abstraction, and
- Polymorphism.
TypeScript can implement all four of them with its smaller and cleaner syntax.