TypeScript starts from the same syntax and semantics that millions of JavaScript developers know today. Don't miss that advanced list of TypeScript interview questions and answers and nail your next web developer tech interview in style.
You could also find all the answers here 👉 https://www.fullstack.cafe/TypeScript.
Answer:
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;🔗 Source: FullStack.Cafe
Answer:
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.
🔗 Source: FullStack.Cafe
Answer:
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.
🔗 Source: FullStack.Cafe
Answer:
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🔗 Source: basarat.gitbooks.io
Questions Details:
Answer:
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.
🔗 Source: stackoverflow.com
Answer:
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.
🔗 Source: talkingdotnet.com
Answer:
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.
🔗 Source: talkingdotnet.com
Answer:
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.
🔗 Source: stackoverflow.com
Answer:
We can call base class constructor using super().
🔗 Source: http://www.talkingdotnet.com
Answer:
- It is an object oriented programming language (not pure).
- Here it is static typing (We can declare a variable in multiple ways). ex: var num : number.
- It has interfaces.
- It has optional parameter feature.
- It has Rest Parameter feature.
- Supports generics.
- Supports Modules
- Number, string etc. are the interfaces.
🔗 Source: FullStack.Cafe
Answer:
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.
🔗 Source: medium.com
Answer:
If you need/wish to create an instance of perhaps a custom object, whilst getting the benefits of type-checking things such as arguments, return types or generics - a class makes sense.
If you’re not creating instances - we have interfaces at our disposal, and their benefit comes from not generating any source code, yet allowing us to somewhat “virtually” type-check our code.
🔗 Source: toddmotto.com
Answer:
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.
🔗 Source: toddmotto.com
Answer:
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.
🔗 Source: www.sparkbit.pl
Answer:
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)🔗 Source: typescriptlang.org
Answer:
Just use:
if (value) {
}It will evaluate to true if value is not:
nullundefinedNaN- empty string
'' 0false
TypesScript includes JavaScript rules.
🔗 Source: stackoverflow.com
Answer:
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;
myMethod() {
console.log(this.myReadonlyProperty);
}
}
new MyClass().myReadonlyProperty = 5; // error, readonly🔗 Source: stackoverflow.com
Answer:
Typescript doesn’t only work for browser or frontend code, you can also choose to write your backend applications. For example you could choose Node.js and have some additional type safety and the other abstraction that the language brings.
- Install the default Typescript compiler
npm i -g typescript- The TypeScript compiler takes options in the shape of a tsconfig.json file that determines where to put built files and in general is pretty similar to a babel or webpack config.
{
"compilerOptions": {
"target": "es5",
"module": "commonjs",
"declaration": true,
"outDir": "build"
}
}- Compile ts files
tsc- Run
node build/index.js🔗 Source: jonathanmh.com
Answer:
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.
🔗 Source: jonathanmh.com
Answer:
TypeScript supports following object oriented terms:
- Modules
- Classes
- Interfaces
- Data Types
- Member functions
🔗 Source: http://www.talkingdotnet.com
Answer:
.map files are source map files that let tools map between the emitted JavaScript code and the TypeScript source files that created it. Many debuggers (e.g. Visual Studio or Chrome's dev tools) can consume these files so you can debug the TypeScript file instead of the JavaScript file.
🔗 Source: stackoverflow.com
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Questions Details:
Consider the code:
class Foo {
save(callback: Function) : void {
//Do the save
var result : number = 42; //We get a number from the save operation
//Can I at compile-time ensure the callback accepts a single parameter of type number somehow?
callback(result);
}
}
var foo = new Foo();
var callback = (result: string) : void => {
alert(result);
}
foo.save(callback);Can you make the result parameter in save a type-safe function? Rewrite the code to demonstrate.
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Consider:
class Point {
x: number;
y: number;
}
interface Point3d extends Point {
z: number;
}
let point3d: Point3d = {x: 1, y: 2, z: 3};See 👉 Answer
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// something is wrong
function reverse(s: String): String;See 👉 Answer
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interface X {
a: number
b: string
}
type X = {
a: number
b: string
};See 👉 Answer
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/* WRONG */
interface Fetcher {
getObject(done: (data: any, elapsedTime?: number) => void): void;
}See 👉 Answer
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