Explore a curated collection of concise and insightful TypeScript interview questions in this repository. From fundamental concepts to advanced topics, these questions cover a range of aspects, helping you prepare for TypeScript-related interviews.
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.
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 the 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 the 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;
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 allowedModules in TypeScript help in organizing the code. There are 2 types of Modules — Internal and External.
-
Internal Modules: Internal Modules are now replaceable by using TypeScript’s namespace.
-
External Modules: External Modules are 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.
TypeScript is a superset of JavaScript, providing optional static typing, classes, and interfaces. One of the significant benefits is enabling IDEs to offer a richer environment for spotting common errors as you type. For a large JavaScript project, adopting TypeScript might result in more robust software while remaining deployable where a regular JavaScript application would run.
- TypeScript supports new ECMAScript standards, compiling them to chosen ECMAScript targets.
- JavaScript code is valid TypeScript code; TypeScript is a superset of JavaScript.
- TypeScript adds type support, including interfaces, enums, hybrid types, generics, union and intersection types, access modifiers, and more.
- Development experience with TypeScript is an improvement over JavaScript, providing real-time feedback from the TypeScript compiler.
- With strict null checks enabled, TypeScript ensures better handling of null and undefined values.
- A build process is required to compile TypeScript to JavaScript, with support for source maps for debugging.
- TypeScript is open source (Apache 2 licensed) and backed by Microsoft, led by Anders Hejlsberg, the lead architect of C#.
Yes, TypeScript files need to be compiled because browsers cannot directly interpret TypeScript, as it is a language extension. The process of converting TypeScript to JavaScript is referred to as compiling. It's essential to understand that in this context, compilation doesn't involve creating binary code; instead, the term "transpilation" is more accurate. Transpilation ensures that TypeScript code is translated into JavaScript, which can be executed by browsers.
TypeScript offers the following benefits:
- Code structuring
- Class-based object-oriented programming
- Imposition of coding guidelines
- Type checking
- Compile-time error checking
- Intellisense
We can call the base class constructor using super().
JavaScript, though universally supported, has design flaws such as lack of class-based object-oriented features, unreliable dynamic typing, and a lack of compile-time error checking. TypeScript, developed by Microsoft, addresses these issues. It is a free and open-source programming language, a strict superset of JavaScript, adding optional static typing and class-based object-oriented programming. TypeScript is suitable for developers familiar with C#, Java, and other strongly typed languages.
String interpolation in TypeScript can be performed using template literals, which are literals delimited with backticks (`). For example:
let value = 100;
console.log(`The size is ${value}`);- Use
.tsfor pure TypeScript files. - Use
.tsxfor files that contain JSX.
- Classes are used as object factories, defining blueprints with initializations and methods.
- Interfaces are virtual structures for type-checking, existing only in TypeScript context.
- An interface is a virtual structure for type-checking purposes.
- It defines the properties an object should have in terms of name and type.
- Decorators are special declarations attached to classes, methods, properties, etc.
- They are functions that take their target as an argument, allowing code execution around the target.
- TypeScript is an object-oriented programming language.
- TypeScript has static typing with various ways to declare variables.
- TypeScript supports interfaces.
- TypeScript features optional parameters and Rest Parameter.
- TypeScript supports generics.
- TypeScript supports modules.
- Number, string, etc., are interfaces in TypeScript.
- Use classes when creating instances with type-checking for arguments, return types, or generics.
- Use interfaces when not creating instances, benefiting from virtual type-checking without generating source code.
In TypeScript, use the readonly modifier as the const keyword cannot be used for class properties.
class MyClass {
readonly myReadonlyProperty = 1;
myMethod() {
console.log(this.myReadonlyProperty);
}
}
new MyClass().myReadonlyProperty = 5; // error, readonly-Just use:
if (value) {
}
It will evaluate to true if value is not:
- null
- undefined
- NaN
- empty string ''
- 0
- false TypesScript includes JavaScript rules.
Yes, TypeScript supports all four main principles of Object-Oriented Programming:
- Encapsulation
- Inheritance
- Abstraction
- Polymorphism
TypeScript can implement all these principles with its smaller and cleaner syntax.
TypeScript supports the following object-oriented terms:
- Modules
- Classes
- Interfaces
- Data Types
- Member functions
TypeScript supports getters/setters as a way of intercepting accesses to a member of an object. This provides 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;
}
}
const myFoo = new Foo();
const myBar = myFoo.bar; // Correct (get)
myFoo.bar = true; // Correct (set).map files are source map files that allow tools to map between the emitted JavaScript code and the TypeScript source files that created it. Debuggers, such as Visual Studio or Chrome's dev tools, can consume these files, enabling debugging of TypeScript code instead of the generated JavaScript.
Stringis the JavaScript String type used to create new strings.stringis the TypeScript string type used to type variables, parameters, and return values.
The pipe (|) represents a union type in TypeScript. A union type describes a value that can be one of several types. For example, number | string | boolean is the type of a value that can be a number, a string, or a boolean.
Example:
let something: number | string | boolean;
something = 1; // ok
something = '1'; // ok
something = true; // ok
something = {}; // Error: Type '{}' is not assignable to type 'string | number | boolean'Using the ReadonlyArray type describes arrays that can only be read from. Any variable with a reference to a ReadonlyArray can't add, remove, or replace any elements of the array.
Example:
function foo(arr: ReadonlyArray<string>) {
arr.slice(); // okay
arr.push("hello!"); // error!
}The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand. Its main purpose is to simplify assigning default values. The nullish coalescing operator tests for null and undefined only.
// Nullish coalescing
console.log(0 ?? 'error'); // 0
console.log('' ?? 'error'); // ''
console.log(null ?? 'error'); // 'error'- Everything in a class is public if not specified.
- Everything in a module is private unless the
exportkeyword is used.
- The
readonlykeyword is used to make a property or variable immutable. Once a value is assigned, it cannot be changed. This is particularly useful for creating objects with constant values.
- Type inference is the ability of the TypeScript compiler to automatically deduce and assign types to variables based on their values. This allows developers to write code without explicitly specifying types in every case, making the code cleaner and more concise.
- TypeScript supports mixins, a concept where a class can extend multiple classes to inherit their properties and methods. This is achieved by combining classes and using intersection types to merge their functionality.
- The
nevertype in TypeScript represents values that will never occur. It is often used as the return type for functions that always throw an exception or never return. It is also inferred for functions that have an infinite loop or always throw errors.