Javascript Promise within Class

February 16, 2022

Fun with promises today … I’m trying to figure out why I can’t refer to this.promise to resolve it with another method, it’s always null.

Any enlightenment is appreciated 🙂

export default class Example {
  constructor() {
    this.promise = null
  }

  test() {
    this.promise = new Promise((resolve, reject) => {
      this.something()
    }).then(resolve => {
      // resolved, do something else
    }).catch(reject => {
      console.log('reject', reject)
    })
  }

  something() {
    this.promise.resolve()
  }
}

>Solution :

You’re trying to use this.promise in something before it’s value changes from null. Another way to do this is to pass the resolve as an argument:

class Example {
  constructor() {
    this.promise = null;
  }
  test() {
    this.promise = new Promise((resolve, reject) => {
      this.something(resolve);
    }).then(result => {
      console.log('resolve', result);
    }).catch(error => {
      console.log('reject', error);
    });
  }
  something(resolve) {
    resolve(1);
  }
}
const example = new Example();
example.test();