小程序云开发接入微信支付

1.登录到微信支付(https://pay.weixin.qq.com/)，提交各种企业资料开通微信支付功能
2.获取商户号和API密钥(初次设置需要先设置操作密码)

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] + '</' + k + '>')
  sa.push('<sign>' + getSign(args) + '</sign>')
  return '<xml>' + sa.join('') + '</xml>'
}

exports.main = async(event, context) => {
  const wxContext = cloud.getWXContext()
  const appId = appid = wxContext.APPID
  const openid = wxContext.OPENID
  const attach = 'attach'
  const body = 'body'
  const total_fee = 1
  const notify_url = "https://mysite.com/notify"
  const spbill_create_ip = "118.89.40.200"
  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
  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
  let prepay_id = xml.split("<prepay_id><![CDATA[")[1].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"
  }
}

相关文档：
Cloud.getWXContext()返回结果详情

参考链接：https://developers.weixin.qq.com/community/develop/article/doc/0004c4a50a03107eaa79f03cc56c13

更新：
云开发后期支持了云开发的支付接口，因此使用如下更简单的方式
https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/open/pay/CloudPay.unifiedOrder.html

参考文档：
小程序云开发：现已原生支持微信支付