Typescript: meaning of ? and ! in class properties

Question

@Column({ name: 'device_kind', type: 'int2', nullable: false })
deviceKind?: number;

Can anyone explain this code? I didn't understand why they added the '?' mark. and some of them has '!' instead of question mark. What do they mean?

Answer 1

This is a really a Typescript question, not TypeORM.

When you define a property like this:

type Foo = {
  prop1?: number
}

You are saying prop1 is optional.

When a property is preceded with ! it means you are telling Typescript to not warn you that you didn't initialize it in the constructor (which it normally will complain about in strict mode).

Example:

class Foo {
  // Typescript does not complain about `a` because we set it in the constructor
  public a: number;

  // Typescript will complain about `b` because we forgot it.
  public b: number;

  // Typescript will not complain about `c` because we told it not to.
  public c!: number;

  // Typescript will not complain about `d` because it's optional and is
  // allowed to be undefined.
  public d?: number;

  constructor() {
    this.a = 5;
  }

}

It should be noted that the c! case in the above class is really a way to tell Typescript: "I know what I'm doing, I know I'm setting this somewhere, just not in the constructor. Please don't complain".

This is the not the same as the d? case, because this just means that d is allowed to be a number or undefined.