代码前提：只需要替换两个与自己相关的参数key和mch_id

1、小程序开通微信支付成功，去公众平台(https://mp.weixin.qq.com/)；
成功后可以知道自己的mch_id，即商户号。
2、去这里：商户平台(https://pay.weixin.qq.com/)，获取key = API密钥
图片地址：[https://mmbiz.qlogo.cn/mmbiz_png/UDa9R1yl9UZoyY753HG2kIgkLluHnCVBJLjUAyjbric01Wr142fcGRzUu5yUibyhEgTIZt0wXBtBDVhUQSZYcETw/0?]

小程序端：

 testWxCloudPay: function () {
    wx.cloud.callFunction({
      name: 'getPay',
      // data: {body:"body",attach:"attach",total_fee:1}, // 可传入相关参数。
      success: res => {
        console.log(res.result)
        if (!res.result.appId) return
        wx.requestPayment({
          ...res.result,
          success: res => {
            console.log(res)
          }
        })
      }
    })
  }

云函数getPay：

const key = "ABC...XYZ"//换成你的商户key，32位
const mch_id = "1413092000"//换成你的商户号
//以下全部照抄即可
const cloud = require('wx-server-sdk')
const rp = require('request-promise')
const crypto = require('crypto')
cloud.init()
function getSign(args) {
  let sa = []
  for (let k in args) sa.push(k + '=' + args[k])
  sa.push('key=' + key)
  return crypto.createHash('md5').update(sa.join('&'), 'utf8').digest('hex').toUpperCase()
}
function getXml(args) {
  let sa = []
  for (let k in args) sa.push('<' + k + '>' + args[k] + '')
  sa.push('' + getSign(args) + '')
  return '' + sa.join('') + ''
}
exports.main = async (event, context) => {
  const wxContext = cloud.getWXContext()
  const appId = appid = wxContext.APPID
  const openid = wxContext.OPENID
  const attach = 'attach' //可从event.attach传入
  const body = 'body' //可从event.body传入
  const total_fee = 1 //可从event.total_fee传入或改成自己的金额。
  const notify_url = "https://mysite.com/notify" //支付成功后的回调地址，没有的话可任意写。
  const spbill_create_ip = "118.89.40.200" //任意写
  const nonceStr = nonce_str = Math.random().toString(36).substr(2, 15)
  const timeStamp = parseInt(Date.now() / 1000) + ''
  const out_trade_no = "otn" + nonce_str + timeStamp //订单号，可用于订单查询，可自己写命名算法。
  const trade_type = "JSAPI"
  const xmlArgs = { appid, attach, body, mch_id, nonce_str, notify_url, openid, out_trade_no, spbill_create_ip, total_fee, trade_type }
  let xml = (await rp({ url: "https://api.mch.weixin.qq.com/pay/unifiedorder", method: 'POST', body: getXml(xmlArgs) })).toString("utf-8")
  if (xml.indexOf('prepay_id') < 0) return xml //如果发生错误，不含prepay_id，传回xml
  let prepay_id = xml.split("")[0]
  let payArgs = { appId, nonceStr, package: ('prepay_id=' + prepay_id), signType: 'MD5', timeStamp }
  return { ...payArgs, paySign: getSign(payArgs) }
}

packge.json:

{
  "name": "getPay",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "zfe",
  "license": "ISC",
  "dependencies": {
    "wx-server-sdk": "latest",
    "crypto": "^1.0.1",
    "request-promise": "^4.2.2"
  }
}