Typescript Union Types inside Class

Question

I have a simple example on setting Union types inside a class.

class Foo {
  constructor() {}
  progress: number | string = 0;

  bar() {
        this.progress = this.progress + 5;
    }
}

I expected to increment by 5 but instead I recieve a ts(2365) error.

Operator '+' cannot be applied to types 'string | number' and '5'.ts(2365)

I thought this would work because if I set this up outside of a class like this code below.

let progress: number | string = 0;
progress = progress + 5;

There are no errors.

Ultimately what I'm trying to do to is to set a variable inside a class declaration that accepts a number from setInterval, so I'm trying to set the variable to the union types of number and NodeJS.Timer since setInterval returns NodeJS.Timer.

export class CurrentTrainingComponent implements OnInit {
  interval: number | NodeJS.Timer;
  progress: number = 0;

  constructor() {}

  ngOnInit() {
    this.interval = setInterval(() => {
      this.progress = this.progress + 5;
      if (this.progress >= 100) {
        clearInterval(this.interval);
      }
    }, 100);
  }

  onStop() {
    clearInterval(this.interval);
  }
}

I know I can simply assign <any> to solve this issue this.interval = <any>setInterval( ()=>{} ), but I wanted to see if I could try Union types.

Last thing that's not related to this issue, setting <number> to setInterval shows a warning Conversion of type 'Timer' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.ts(2352)

Valerii · Accepted Answer · 2019-05-07 16:36:31Z

1

In bar function you can't be sure that your value now is a number. You can add additional check to make it work

class Foo {
    constructor() {}
    progress: number | string = 0;

    bar() {
        if (typeof this.progress === 'number')
            this.progress = this.progress + 5;
    }
}

In case of

let progress: number | string = 0;
progress = progress + 5;

value is definitely a number (it was assigned 1 line above)

answered May 7, 2019 at 16:36

Valerii

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