node.js - ES5，如何在循环中使用 Promise

Question

说 command_arr.length === 2

当我运行以下函数device_execute_command时。执行消息顺序为

finish one command
finish one command
has latest state?
has latest state?

我期望的是:

finish one command
has latest state?
finish one command
has latest state?

代码

var device_execute_command = function(command_arr) {
  // This is the main loop
  var i = 0;
  for(i=0; i < command_arr.length; i++) {
    var command = command_arr[i];
    var command_id = command.command_id;

    device_run_single_command(command).then(function(command){
      console.log();
      console.log("finish one command");
      console.log(command);

      return is_device_has_latest_state(command);
    }).then(function(command_with_condi){
      console.log();
      console.log("has latest state?");
      console.log(command_with_condi);

    });


  }

}

Answer 1

这是由于 javascript 的异步特性造成的。你要的就是一一兑现 promise 。这不能通过简单地在循环迭代中调用 Promise 来实现。实现这一点的最简单方法可能是使用 bluebird Promise 实现，它附带了许多用于 Promise 执行流程控制的方法。

例如，您的案例中的顺序执行可以通过以下方式实现:

const Promise = require('bluebird');

Promise.each(command_arr, function(command) {
  return device_run_single_command(command).then(function(command) {
    console.log();
    console.log("finish one command");
    console.log(command);    
    return is_device_has_latest_state(command);
  }).then(function(command_with_condi) {
    console.log();
    console.log("has latest state?");
    console.log(command_with_condi);    
  });
});