Promise.all

promise.all.js

Promise.all = function (iterators) {
  return new Promise((resolve, reject) => {
    if (!iterators || iterators.length === 0) {
      resolve([]);
    } else {
      // used to determine whether all tasks are completed
      let count = 0; 
      let result = []; // result array
      for (let i = 0; i < iterators.length; i++) {
        // Considering that iterators[i] may be ordinary object, 
        // they are uniformly packaged as Promise object
        Promise.resolve(iterators[i]).then(
          (data) => {
            // Save the corresponding results in order
            result[i] = data; 
            // When all tasks are executed, 
            // return the result uniformly
            if (++count === iterators.length) {
              resolve(result);
            }
          },
          (err) => { 
           // If any Promise object fails to execute, 
           // the reject() method is called
            reject(err); 
            return;
          }
        );
      }
    }
  });
};