TypeScript Interview Questions and Answers

To enable decorators in TypeScript, developers first have to enable the option of experimentalDecorators via command line or tsconfig.json.

Here is the command line to use.

• $tsc --target ES5 --experimentalDecorators
• For tsconfig.json, use the following syntax.
  {
   "compilerOptions": {
      "target": "ES5",
      "experimentalDecorators": true
   }
}

In TypeScript, static typing means parameters, variables, and object members have types that compiler recognizes at the time of compile. This helps in detecting early errors faster than unit test runs. It also greatly helps IDEs with auto-completion and API exploration with statically typed DOM. Static typing is an important chapter which supports a number of TypeScript Interview Questions, so it’s crucial to practice this one.

The prototype property in TypeScript allows users to include methods and properties to an object. It allows cloning objects (complex ones also) without coupling to their specific classes. Prototype objects can create full copies due to the accessibility of objects to each other’s private field.

In TypeScript, the callback function is used to schedule a call after some asynchronous processing is completed. It is passed to another function as a parameter that allows callback function to be called when the process of asynchronous processing is completed.

In Typescript, a function in TypeScript can be written in two ways:

Named Function

This type of function is written and called using its own name.

Example:

function display() {
console.log("Hello Best Interview Question Team!");
}
display(); //Output: Hello Best Interview Question Team

Anonymous Function

In this function, the code is written and stored through expressions or variables. The variable/expression name is used to call the stored data types.

Example:

let greeting = function() {
console.log("Hello Best Interview Question Team");
};

greeting(); //Output: Hello Best Interview Question Team

Here’s how to declare modules in Typescript:

Declaring any of the external modules having no exposed types or values:

declare module 'Foo' {
   var x: any;
   export = x;
}

For declaring individual classes

declare module 'Foo' {
   export type cls = any;
   export var cls: any;
}

Modules can be imported using the import keyword. Here’s a detailed info into it:

Importing individual exports from within a module:

import { Employee } from "./Employee";
let empObj = new Employee("Best Interview Question", 1);
empObj.displayEmployee(); //Output: Employee Code: 1, Employee Name: Best Interview Question

Now, to import the exports within a module as a variable into another module, use this code:

import * as Emp from "./Employee"
console.log(Emp.age); // 10
let empObj = new Emp.Employee("Best Interview Question" , 2);
empObj.displayEmployee(); //Output: Employee Code: 2, Employee Name: Best Interview Question

Compiling Typescript into Javascript is done using a JS function. During this process of compiling, the code structure could be changed a bit. If you have multiple modules or classes in the typescript, then they would become a part of the path to call the specific function.

The typeof command is used to check the data type of any variable in Typescript.

Example:
Variable: abc:number|string;
if (typeof abc === "number") {
   // do something
}

• Install the Typescript compiler, tsc
• Go to the Solutions Explorer and Click on properties
• Now, go to the Typescript Build Tab and change the Typescript version as per requirement.