我有以下代码,它创建了一个承诺数组来保存一些数字,然后它产生承诺(使用 co 库)并打印出结果。然而,我不明白的是,当它打印输出时,它会打印相同的记录 10 次。
这是代码:
'use strict'
const Promise = require('bluebird');
const co = require('co');
const _ = require('lodash');
const mongoose = require('mongoose');
// plug in the bluebird promise library for mongoose
mongoose.Promise = Promise;
mongoose.connect('mongodb://localhost:27017/nodejs_testing');
const numSchema = new mongoose.Schema({
num: { type: Number, required: true }
});
const Num = mongoose.model('Num', numSchema);
let promises = [];
let x;
// create an array of promises to save some numbers
for (let i = 0; i < 10; ++i) {
let p = new Promise((resolve,reject) => {
x = Num();
x.num = i;
x.save((err) => {
if (err) {
reject(err);
} else {
resolve(x);
}
});
});
promises.push(p);
};
// yield all the promises, then print out the results
co(function * () {
let res = yield Promise.all(promises);
_.each(res, item => {
console.log(JSON.stringify(item));
});
mongoose.disconnect();
});
这是输出:
/tmp/test$ node m
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
{"__v":0,"num":9,"_id":"57d1931037a370055f51977c"}
请注意,如果我x
在 Promise 中声明变量,那么我会得到预期的结果(例如,输出中有 10 个不同的数字)。换句话说,如果我进行此更改(见下文),它将按预期工作:
let p = new Promise((resolve,reject) => {
let x = Num(); // <--- declare x inside the promise
.
.
});
我的问题是,为什么代码会这样?请注意,如果我重复完全相同类型的测试而不使用 mongodb/mongoose 并且只打印一些数字,即使x
在 Promise 之外声明它也会按预期工作。下面的示例代码:
'use strict'
const Promise = require('bluebird');
const co = require('co');
const _ = require('lodash');
class Number {
constructor(num) {
this.num = num;
}
};
let x;
let promises = [];
for (let i = 0; i < 10; ++i) {
let p = new Promise((resolve,reject) => {
setTimeout(() => {
x = new Number(i);
resolve(x);
}, 300);
});
promises.push(p);
};
co(function * () {
let res = yield Promise.all(promises);
_.each(res, item => {
console.log(JSON.stringify(item));
});
});
输出:
/tmp/test$ node t
{"num":0}
{"num":1}
{"num":2}
{"num":3}
{"num":4}
{"num":5}
{"num":6}
{"num":7}
{"num":8}
{"num":9}