Senior Software Engineer

‘Null’ is a keyword in JavaScript that signifies ‘no value’ or nonexistence of any value. If you wish to shred a variable off its assigned value, you can simply assign ‘null’ to it. Besides this, like any other object, it is never implicitly assigned to a variable by JavaScript.

Yes. When an array of object references is declared and initialized , all the values inside it will be null by default. Remember that the length of an array will never vary, despite the contents (null elements) in the array

Middleware is software that provides common services and capabilities to applications outside of what’s offered by the operating system. Data management, application services, messaging, authentication, and API management are all commonly handled by middleware. Middleware helps developers build applications more efficiently. It acts like the connective tissue between applications, data, and users.

The general way to define a custom pipe is:

import { Pipe, PipeTransform } from '@angular/core'; 
@Pipe({name: 'Pipename'}) 

export class Pipeclass implements PipeTransform { 
transform(parameters): returntype { } 
} 
Where,
'Pipename' − This is the name of the pipe.
Pipeclass − This is name of the class assigned to the custom pipe.
Transform − This is the function to work with the pipe.
Parameters − This are the parameters which are passed to the pipe.
Returntype − This is the return type of the pipe.

Dependency Injection is passing dependency to other objects or framework( dependency injector). Dependency injection makes testing easier. The injection can be done through constructor. 
Dependency injection is basically providing the objects that an object needs (its dependencies) instead of having it construct them itself. It's a very useful technique for testing, since it allows dependencies to be mocked or stubbed out. Dependencies can be injected into objects by many means (such as constructor injection or setter injection). One can even use specialized dependency injection frameworks (e.g. Spring) to do that, but they certainly aren't required. You don't need those frameworks to have dependency injection. Instantiating and passing objects (dependencies) explicitly is just as good an injection as injection by framework.