Resolve async function inside sync function

Question

I have this function that returns a Promise which resolves to an object that is later inserted into db.

lastPrice (crypto) {
    return axios.get('https://www.bitmex.com/api/v1/instrument/active').then(res => {
      return _.map(_.filter(res.data, instrument => isMatch(instrument, crypto)), y => {
        return { exchange: getExchangeName(y.symbol), order: getOrder(y.symbol), crypto, lastPrice: getLastPrice(y) }
      })
    }).catch((e) => { Promise.resolve() })
  }

isMatch, getExchangeName, getOrder and getLastPrice used to be sync functions. Now I need getLastPrice to be async.

function getLastPrice (instrument) {
  const regex = /^(ETH)[FNGQHUJVKXZM]\d{2}$/
  let regexResult = regex.exec(instrument.symbol) || []
  if (regexResult.length > 0) {
    const app = require('../../app')
    return app.service('exchanges').find({
      query: {
        exchange: 'BMEXperp',
        crypto: 'ETH'
      }
    }).then(res => {
      if (res.total === 0) {
        return 0
      }
      return res.data[0].lastPrice * instrument.lastPrice
    })
  } else {
    return instrument.lastPrice
  }

I need to keep the same functionality but with an async lastPrice(crypto). Basically getLastPrice should still return a Promise that resolves to an object with no promises in it.

My idea is not to modify the function that calls lastPrice, which is this one:

exchange.lastPrice(crypto).then(res => {
      res = res || []
      res.forEach(element => {
        exchangesService.create(element)
      })
    })

in a .then you don't need to specifically return Promise.resolve(x) ... since .then always returns a Promise anyway - this isn't your issue but makes your code harder to read — Jaromanda X
– Jaromanda X, Commented Aug 5, 2019 at 0:56
it returns a Promise that resolves into { exchange: 'BMEXSep', order: 6, crypto: 'ETH', lastPrice: Promise { <pending> } } and I need lastPrice to be a number — Vallo
– Vallo, Commented Aug 5, 2019 at 1:50

alanning · Accepted Answer · 2019-08-05 02:03:12Z

3

Missing part is Promise.all.

If you can use async/await then I think something like this is what you want:

lastPrice (crypto) {
  return axios.get('https://www.bitmex.com/api/v1/instrument/active')
    .then(res => {
      const filteredList = _.filter(res.data, instrument => isMatch(instrument, crypto))
      const promises = _.map(filteredList, async (y) => {
        return {
          exchange: getExchangeName(y.symbol),
          order: getOrder(y.symbol),
          crypto,
          lastPrice: await getLastPrice(y),
        }
      })
      return Promise.all(promises)
    }).catch((e) => { Promise.resolve() })
}

answered Aug 5, 2019 at 2:03

alanning

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