[Typescript] Make TypeScript Class Usage Safer with Strict Property Initialization

By setting the strictPropertyInitialization flag in the .tsconfig file, TypeScript will start throwing errors unless we initialize all properties of classes on construction. We’ll explore how you can fix the errors by assigning to them directly or in the constructor body. And if you can’t initialize directly but you’re sure it will be assigned to at runtime by a dependency injection library, you can use the definite assignment assertion operator to ask TypeScript to ignore that property.

For example, code belkow, 'title' is undefined. WIll cause the problem when we call '.filter'.

class Library {
  titles: string[];

  constructor() {}
}
const library = new Library();

// sometime later & elsewhere in our codebase..

const shortTitles = library.titles.filter(
  title => title.length < 5
);

First we want our IDE to help us to detect the problem even before compiling...

tsconfig.json:

{
  "compilerOptions": {
    "strictPropertyInitialization": true,
    "strictNullChecks": true
  }
}

After setting up 'strictPropertyInitialization' & 'strictNullChecks', IDE will tell us that 'title' is undefined.

In the code, we can also utitlize: 

'use strict'

or add some if checking.

Using definite assignment operator from Typescript:

class Library {
  titles!: string[]

  constructor() {}
}

'!' tell typescript, this object is not undefine or null, we will assign the value later, so in this case, IDE won't complain:

class Library {
  titles!: string[]

  constructor() {

     this.titles = []
  }
}