CODEWITHSUNDEEP
javascriptnode.jsdesign-patternses6-promise
alfredopacino
alfredopacino

Reputation: 3241

Method chaining with Promises

I want to implement the classic method chain pattern, the final usage should be 

DB
  .push(2)
  .push(3)

This is the current code, obviously doesn't work, I'm not clear how to return the reference to DB itself resolving the promise

let nodes = [];
let DB = {
    self:this,
    push: (i) => new Promise((resolve, reject) => {
        nodes.push(i)
        resolve(this)
    })
}

Upvotes: 2

Views: 148

Answers (1)

Roy Wang
Roy Wang

Reputation: 11260

Only a class or function instance has a this reference.

class DB {
  constructor() {
    this.nodes = [];
    this.promise = Promise.resolve();
  }
  push(i) {
    this.nodes.push(i);
    return this;
  }
  pushAsync(i) {
    return new Promise((resolve) => {
      this.nodes.push(i);
      resolve();
    });
  }
  pushAsyncChain(i) {
    this.promise.then(() => {
      this.promise = new Promise((resolve) => {
        this.nodes.push(i);
        resolve();
      });
    });
    return this;
  }
  then(callback) {
    this.promise.then(callback);
  }
}

const db = new DB();
db.push(2).push(3);
db.pushAsync(4).then(() => db.pushAsync(5));
db
  .pushAsyncChain(6)
  .pushAsyncChain(7)
  .then(() => console.log(db.nodes)); // or await db.promise; console.log(db.nodes);

Upvotes: 2

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