Question

在_.mapValues里面我希望得到一些具有一些延迟的修改后的值（例如来自DB），但是我遇到了问题：当我在同步模式下修改值时，一切都很好，当我尝试使用promises或回调它的工作时不正确（在第一种情况下，我得到Promise对象，在第二种情况下：未定义的值）。这里有一些简化的例子，我如何在mapValues中重写代码来解决这个问题？

'use strict';
const _ = require('lodash');
const Promise = require('bluebird');

let obj = {
  one: 1,
  two: 2,
};

let increment = (value) => {
  return value + 1;
};

let incrementProm = (value) => {
  return Promise.resolve(value + 1);
};

let incrementCb = (value, cb) => {
  let res = value + 1;
  let err = null;
  setTimeout(cb.bind(undefined, err, res), 100);
};

let t1 = _.mapValues(obj, (value) => {
  return increment(value);
});

let t2 = _.mapValues(obj, (value) => {
  return incrementProm(value);
});

let t3 = _.mapValues(obj, (value) => {
  let temp;
  incrementCb(value, (err, res) => {
    temp = res;
  });
  return temp;
});

console.log('Sync res:');
console.log(t1);
console.log('Promise res:');
console.log(t2);
console.log('Callback res:');
console.log(t3);

Answer 1

您可以使用bluebird的props()函数来解析所有带承诺的属性。

Promise.props(_.mapValues(obj, incrementProm))
.then(result => console.log(result));

var obj = {
  one: 1,
  two: 2
};

var incrementProm = value => Promise.resolve(value + 1);

Promise.props(_.mapValues(obj, incrementProm))
.then(result => console.log(result));

<script src="https://cdn.jsdelivr.net/lodash/4.13.1/lodash.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bluebird/3.4.1/bluebird.js"></script>

Answer 2

你正在映射到承诺，所以我做的事情可能是：

var _ = require('lodash');
let incrementProm = (value) => {
  return Promise.resolve(value + 1);
};

let obj = {
    foo: 1,
    bar: 2
}

let keys = _.keys(obj);
let promises = _.map(keys, k => {
    return incrementProm(obj[k])
    .then(newValue => { return { key: k, value: newValue } });
})
Promise.all(promises).then(values => {
    values.forEach(v => obj[v.key] = v.value)
})
.then(() => {
    // now your object is updated: foo = 2 and bar = 3
    console.log(obj);
});

Answer 3

您可以实现mapValues的异步版本。像...

async function mapValuesAsync(object, asyncFn) {
  return Object.fromEntries(
    await Promise.all(
      Object.entries(object).map(async ([key, value]) => [
        key,
        await asyncFn(value, key, object)
      ])
    )
  );
}

然后像调用_.mapValues一样调用它：

let t2 = await mapValuesAsync(obj, (value) => {
    return incrementProm(value);
});

或者简单地

let t2 = await mapValuesAsync(obj, incrementProm);

lodash mapValues async

3 个答案: