Minimal Dataloader implementation

dataloader.js

/*
 * This is a minimal implementation of Dataloader. This implementation leaves out the caching functionality
 * and skips most of the error handling so it's not suitable to use in production. This is meant to help
 * you understand how Dataloader works. Once you understand this code check out the full Dataloader code:
 * https://github.com/graphql/dataloader
 */

class Dataloader {
  constructor(batchFunction) {
    this.batchFunction = batchFunction;
    this.batchKeys = [];
    this.batchPromises = [];
  }

  async load(key) {
    console.log(`load key: ${key}`);

    if (this.batchKeys.length === 0) {
      process.nextTick(() => this.dispatch());
    }

    this.batchKeys.push(key);

    const promise = new Promise((resolve, reject) => {
      this.batchPromises.push({ resolve, reject });
    });

    return promise;
  }

  async dispatch() {
    console.log(`batchKeys: ${this.batchKeys}`);

    const results = await this.batchFunction(this.batchKeys);

    for (let i = 0; i < this.batchPromises.length; i++) {
      const callback = this.batchPromises[i];

      if (results[i] instanceof Error) {
        callback.reject(results[i]);
      } else {
        callback.resolve(results[i]);
      }
    }

    this.batchKeys = [];
    this.batchPromises = [];
  }
}

module.exports = Dataloader;