CODEWITHSUNDEEP
typescript
Quuxuu
Quuxuu

Reputation: 669

How can I type a method with 'this' inside object literal in typescript?

const log = {
  counter: {
    a: 1,
    b: 2,
    c: 3,
  },
  increment(entry: keyof typeof this.counter){
    this.counter[entry]++;
  }
};

function incrementLog(entry:keyof typeof log.counter){
    log.counter[entry]++;
}

incrementLog('a'); // ok
incrementLog('d'); // error, must be 'a' | 'b' | 'c'
log.increment('a'); // ok
log.increment('d'); // no error

Playground Link

I want to enforce the argument type of increment method to be keyof typeof log.counter, which is 'a' | 'b' | 'c'. I can achieve it in the standalone function, but it doesn't work in the increment method: 'this' is not defined.

I've also tried log.counter instead of this.counter on the method definition, but that creates a 'circular initializer' which also doesn't work as intended.

I hope not to manually type the log or manually type the counter, because when I make changes to the object, I hope to only make changes in one place.

Upvotes: 2

Views: 464

Answers (4)

Alex
Alex

Reputation: 36

They fixed it.

P.S.: I have to write this to go over the minimum thirty characters.

Upvotes: 0

Nenad
Nenad

Reputation: 26627

Define counter before log. You cannot reference type in the middle of expression which defines the type. You can easily avoid duplicating definition/initialization.

const counter = {
    a: 1,
    b: 2,
    c: 3,
};
const log = {
  counter,
  increment(entry: keyof typeof counter){
    this.counter[entry]++;
  }
};

function incrementLog(entry:keyof typeof log.counter){
    log.counter[entry]++;
}

incrementLog('a'); // ok
incrementLog('d'); // error, must be 'a' | 'b' | 'c'
log.increment('a'); // ok
log.increment('d'); // error, must be 'a' | 'b' | 'c'

Upvotes: 2

Patrick Roberts
Patrick Roberts

Reputation: 51816

When writing object-oriented code in TypeScript, it's a lot easier to use class syntax than it is to force things to work with plain object literals:

class Log {
  counter = {
    a: 1,
    b: 2,
    c: 3
  };

  increment(entry: keyof Log['counter']) {
    this.counter[entry]++;
  }
}

const log = new Log();

function incrementLog(entry:keyof Log['counter']) {
  log.counter[entry]++;
}

incrementLog('a'); // ok
incrementLog('d'); // error
log.increment('a'); // ok
log.increment('d'); // error

Upvotes: 2

Marvin Scharle
Marvin Scharle

Reputation: 97

You should create a type or interface for log:

interface Log<T extends {[key: string]: number}> {
    counter: T;
    increment(element: keyof T): void;
}

Upvotes: 0

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