Promise

[phymo]

• promise.js

  
  class PromiseSimple {
  constructor(executionFunction) {
    this.promiseChain = [];
    this.handleError = () => {};
  
    this.onResolve = this.onResolve.bind(this);
    this.onReject = this.onReject.bind(this);
  
    // this.onResolve;
    // this.onReject;
  
    executionFunction(this.onResolve, this.onReject);
  }
  then(handleSuccess) {
    this.promiseChain.push(handleSuccess);
    return this;
  }
  
  catch(handleError) {
    this.handleError = handleError;
    return this;
  }
  
  onResolve(value) {
    let storedValue = value;
  
    try {
      this.promiseChain.forEach((nextFunction) => {
        storedValue = nextFunction(storedValue);
      });
    } catch (error) {
      this.promiseChain = [];
  
      this.onReject(error);
    }
  }
  
  onReject(error) {
    console.log(error);
    this.handleError(error);
  }
  
  }
  
  const aa = new PromiseSimple((resolve, reject) => {
  setTimeout(() => {
    reject('ss!');
  }, 1000);
  });
  
  aa.then((value) => {
  console.log('aa', value);
  }).catch((error) => {
  console.log('bb', error);
  });

module.exports = PromiseSimple

[phymo]

• promiseTest.js

  
  const PromiseSimple = require('./promise');

console.log(PromiseSimple);

fakeApiBackend = () => { const user = { userName: 'user', password: 'password', profile: 'ssssssssssssss', };

if (Math.random() > 0.1) { return { data: user, statusCode: 200, }; } else { return { error: 'error', message: 'error, please try later', statusCode: 500, }; } }

const makeApiCall = () => { return new PromiseSimple((resolve, reject) => { setTimeout(() => { const response = fakeApiBackend();

  if (response.statusCode === 200) {
    resolve(response.data);
  } else {
    reject(response);
  }
}, 3000);

}) }

makeApiCall() .then((data) => { console.log(data); return data; }).then((data) => { console.log(data); }).catch((error) => { console.log(error); });

[phymo]

https://www.lydiahallie.com/blog/promise-execution Long story short, Promises are just objects with some additional functionality to change their internal state.

[phymo]

Promises From The Ground Up https://www.joshwcomeau.com/javascript/promises/