如何在 typescript 中的 object 文字中键入带有“this”的方法？

Question

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' 'a' | 'b' | 'c' '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.

Answer 1

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

Answer 2

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'

Answer 3

You should create a type or interface for log :

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