微信支付流程##

步骤
（一）获取用户的信息
（二）统一下单（返回的prepay_id用于第（三）步的请求参数）
（三）发起支付请求

操作（这边假设你已经获得了用户的openId）##

（一）.获取用户openId（略）

（二）.统一下单

1.接口链接
URL地址：https://api.mch.weixin.qq.com/pay/unifiedorder

2.统一下单需要的参数（有11个参数是必须的）

    appid: appid,//小程序的appid
    body: body,//说明
    mch_id: mch_id,//商户id
    nonce_str: nonce_str,//随机字符串
    notify_url: notify_url,//通知的url(用于后台操作)
    openid: openid,//用户的openId
    out_trade_no: out_trade_no,//商户订单号，用户自己定义
    spbill_create_ip: spbill_create_ip,//终端ip，这个其实可以比较随意
    total_fee: total_fee,//支付的金额，单位分（total_fee=1表示1分）
    trade_type: 'JSAPI'//固定值
    sign：sign//签名（下面详细说明sign的获取）

3.sign的获取
①整体步骤

②先把上述参数按字典序排序，假设参数appid和body，一定是appid在前，body在后，把所有的要发送的参数按照参数名的字典序排序形成如下字符串

③拼接key
把第②步获得的结果+’&key=‘+key。示例如下

②③步可以使用一下js代码

/*
  生成统一下单的签名
*/
function paysignjsapi(appid, body, mch_id, nonce_str, notify_url, openid, out_trade_no, spbill_create_ip, total_fee,key) {
  var ret = {
    appid: appid,
    body: body,
    mch_id: mch_id,
    nonce_str: nonce_str,
    notify_url: notify_url,
    openid: openid,
    out_trade_no: out_trade_no,
    spbill_create_ip: spbill_create_ip,
    total_fee: total_fee,
    trade_type: 'JSAPI'
  };
  var str = raw(ret);
  str = str + '&key=' + key;
  var md5Str = MD5Util.hex_md5(str).toUpperCase(); //MD5加密后转化为大写字符，代码在下述步骤。var MD5Util = require('../utils/md5.js');
  return md5Str;
};
function raw(args) {//排序拼接
  var keys = Object.keys(args);
  keys = keys.sort();
  var newArgs = {};
  keys.forEach(function (key) {
    newArgs[key] = args[key];
  });

  var str = '';
  for (var k in newArgs) {
    str += '&' + k + '=' + newArgs[k];
  }
  str = str.substr(1);
  return str;
};


/*
生成32为随机数算法    
*/
function getNonce_str(){
  var chars = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
  var res = "";
  for (var i = 0; i < 32; i++) {
    var id = Math.ceil(Math.random() * 35);
    res += chars[id];
  }
  return res;
}

④得到上述第③步得到的字符串后进行MD5加密后得到sign，我试了网上很多的MD5加密得到的结果和文档中提供的验证工具得到的结果不一致，最后使用下面的MD5算法成功。

//md5.js
/*
 * A JavaScript implementation of the RSA Data Security, Inc. MD5 Message
 * Digest Algorithm, as defined in RFC 1321.
 * Version 2.2 Copyright (C) Paul Johnston 1999 - 2009
 * Other contributors: Greg Holt, Andrew Kepert, Ydnar, Lostinet
 * Distributed under the BSD License
 * See http://pajhome.org.uk/crypt/md5 for more info.
 */

/*
 * Configurable variables. You may need to tweak these to be compatible with
 * the server-side, but the defaults work in most cases.
 */
var hexcase = 0;   /* hex output format. 0 - lowercase; 1 - uppercase        */
var b64pad = "";  /* base-64 pad character. "=" for strict RFC compliance   */

/*
 * These are the functions you'll usually want to call
 * They take string arguments and return either hex or base-64 encoded strings
 */
function hex_md5(s) { return rstr2hex(rstr_md5(str2rstr_utf8(s))); }
function b64_md5(s) { return rstr2b64(rstr_md5(str2rstr_utf8(s))); }
function any_md5(s, e) { return rstr2any(rstr_md5(str2rstr_utf8(s)), e); }
function hex_hmac_md5(k, d)
{ return rstr2hex(rstr_hmac_md5(str2rstr_utf8(k), str2rstr_utf8(d))); }
function b64_hmac_md5(k, d)
{ return rstr2b64(rstr_hmac_md5(str2rstr_utf8(k), str2rstr_utf8(d))); }
function any_hmac_md5(k, d, e)
{ return rstr2any(rstr_hmac_md5(str2rstr_utf8(k), str2rstr_utf8(d)), e); }

/*
 * Perform a simple self-test to see if the VM is working
 */
function md5_vm_test() {
  return hex_md5("abc").toLowerCase() == "900150983cd24fb0d6963f7d28e17f72";
}

/*
 * Calculate the MD5 of a raw string
 */
function rstr_md5(s) {
  return binl2rstr(binl_md5(rstr2binl(s), s.length * 8));
}

/*
 * Calculate the HMAC-MD5, of a key and some data (raw strings)
 */
function rstr_hmac_md5(key, data) {
  var bkey = rstr2binl(key);
  if (bkey.length > 16) bkey = binl_md5(bkey, key.length * 8);

  var ipad = Array(16), opad = Array(16);
  for (var i = 0; i < 16; i++) {
    ipad[i] = bkey[i] ^ 0x36363636;
    opad[i] = bkey[i] ^ 0x5C5C5C5C;
  }

  var hash = binl_md5(ipad.concat(rstr2binl(data)), 512 + data.length * 8);
  return binl2rstr(binl_md5(opad.concat(hash), 512 + 128));
}

/*
 * Convert a raw string to a hex string
 */
function rstr2hex(input) {
  try { hexcase } catch (e) { hexcase = 0; }
  var hex_tab = hexcase ? "0123456789ABCDEF" : "0123456789abcdef";
  var output = "";
  var x;
  for (var i = 0; i < input.length; i++) {
    x = input.charCodeAt(i);
    output += hex_tab.charAt((x >>> 4) & 0x0F)
      + hex_tab.charAt(x & 0x0F);
  }
  return output;
}

/*
 * Convert a raw string to a base-64 string
 */
function rstr2b64(input) {
  try { b64pad } catch (e) { b64pad = ''; }
  var tab = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  var output = "";
  var len = input.length;
  for (var i = 0; i < len; i += 3) {
    var triplet = (input.charCodeAt(i) << 16)
      | (i + 1 < len ? input.charCodeAt(i + 1) << 8 : 0)
      | (i + 2 < len ? input.charCodeAt(i + 2) : 0);
    for (var j = 0; j < 4; j++) {
      if (i * 8 + j * 6 > input.length * 8) output += b64pad;
      else output += tab.charAt((triplet >>> 6 * (3 - j)) & 0x3F);
    }
  }
  return output;
}

/*
 * Convert a raw string to an arbitrary string encoding
 */
function rstr2any(input, encoding) {
  var divisor = encoding.length;
  var i, j, q, x, quotient;

  /* Convert to an array of 16-bit big-endian values, forming the dividend */
  var dividend = Array(Math.ceil(input.length / 2));
  for (i = 0; i < dividend.length; i++) {
    dividend[i] = (input.charCodeAt(i * 2) << 8) | input.charCodeAt(i * 2 + 1);
  }

  /*
   * Repeatedly perform a long division. The binary array forms the dividend,
   * the length of the encoding is the divisor. Once computed, the quotient
   * forms the dividend for the next step. All remainders are stored for later
   * use.
   */
  var full_length = Math.ceil(input.length * 8 /
    (Math.log(encoding.length) / Math.log(2)));
  var remainders = Array(full_length);
  for (j = 0; j < full_length; j++) {
    quotient = Array();
    x = 0;
    for (i = 0; i < dividend.length; i++) {
      x = (x << 16) + dividend[i];
      q = Math.floor(x / divisor);
      x -= q * divisor;
      if (quotient.length > 0 || q > 0)
        quotient[quotient.length] = q;
    }
    remainders[j] = x;
    dividend = quotient;
  }

  /* Convert the remainders to the output string */
  var output = "";
  for (i = remainders.length - 1; i >= 0; i--)
    output += encoding.charAt(remainders[i]);

  return output;
}

/*
 * Encode a string as utf-8.
 * For efficiency, this assumes the input is valid utf-16.
 */
function str2rstr_utf8(input) {
  var output = "";
  var i = -1;
  var x, y;

  while (++i < input.length) {
    /* Decode utf-16 surrogate pairs */
    x = input.charCodeAt(i);
    y = i + 1 < input.length ? input.charCodeAt(i + 1) : 0;
    if (0xD800 <= x && x <= 0xDBFF && 0xDC00 <= y && y <= 0xDFFF) {
      x = 0x10000 + ((x & 0x03FF) << 10) + (y & 0x03FF);
      i++;
    }

    /* Encode output as utf-8 */
    if (x <= 0x7F)
      output += String.fromCharCode(x);
    else if (x <= 0x7FF)
      output += String.fromCharCode(0xC0 | ((x >>> 6) & 0x1F),
        0x80 | (x & 0x3F));
    else if (x <= 0xFFFF)
      output += String.fromCharCode(0xE0 | ((x >>> 12) & 0x0F),
        0x80 | ((x >>> 6) & 0x3F),
        0x80 | (x & 0x3F));
    else if (x <= 0x1FFFFF)
      output += String.fromCharCode(0xF0 | ((x >>> 18) & 0x07),
        0x80 | ((x >>> 12) & 0x3F),
        0x80 | ((x >>> 6) & 0x3F),
        0x80 | (x & 0x3F));
  }
  return output;
}

/*
 * Encode a string as utf-16
 */
function str2rstr_utf16le(input) {
  var output = "";
  for (var i = 0; i < input.length; i++)
    output += String.fromCharCode(input.charCodeAt(i) & 0xFF,
      (input.charCodeAt(i) >>> 8) & 0xFF);
  return output;
}

function str2rstr_utf16be(input) {
  var output = "";
  for (var i = 0; i < input.length; i++)
    output += String.fromCharCode((input.charCodeAt(i) >>> 8) & 0xFF,
      input.charCodeAt(i) & 0xFF);
  return output;
}

/*
 * Convert a raw string to an array of little-endian words
 * Characters >255 have their high-byte silently ignored.
 */
function rstr2binl(input) {
  var output = Array(input.length >> 2);
  for (var i = 0; i < output.length; i++)
    output[i] = 0;
  for (var i = 0; i < input.length * 8; i += 8)
    output[i >> 5] |= (input.charCodeAt(i / 8) & 0xFF) << (i % 32);
  return output;
}

/*
 * Convert an array of little-endian words to a string
 */
function binl2rstr(input) {
  var output = "";
  for (var i = 0; i < input.length * 32; i += 8)
    output += String.fromCharCode((input[i >> 5] >>> (i % 32)) & 0xFF);
  return output;
}

/*
 * Calculate the MD5 of an array of little-endian words, and a bit length.
 */
function binl_md5(x, len) {
  /* append padding */
  x[len >> 5] |= 0x80 << ((len) % 32);
  x[(((len + 64) >>> 9) << 4) + 14] = len;

  var a = 1732584193;
  var b = -271733879;
  var c = -1732584194;
  var d = 271733878;

  for (var i = 0; i < x.length; i += 16) {
    var olda = a;
    var oldb = b;
    var oldc = c;
    var oldd = d;

    a = md5_ff(a, b, c, d, x[i + 0], 7, -680876936);
    d = md5_ff(d, a, b, c, x[i + 1], 12, -389564586);
    c = md5_ff(c, d, a, b, x[i + 2], 17, 606105819);
    b = md5_ff(b, c, d, a, x[i + 3], 22, -1044525330);
    a = md5_ff(a, b, c, d, x[i + 4], 7, -176418897);
    d = md5_ff(d, a, b, c, x[i + 5], 12, 1200080426);
    c = md5_ff(c, d, a, b, x[i + 6], 17, -1473231341);
    b = md5_ff(b, c, d, a, x[i + 7], 22, -45705983);
    a = md5_ff(a, b, c, d, x[i + 8], 7, 1770035416);
    d = md5_ff(d, a, b, c, x[i + 9], 12, -1958414417);
    c = md5_ff(c, d, a, b, x[i + 10], 17, -42063);
    b = md5_ff(b, c, d, a, x[i + 11], 22, -1990404162);
    a = md5_ff(a, b, c, d, x[i + 12], 7, 1804603682);
    d = md5_ff(d, a, b, c, x[i + 13], 12, -40341101);
    c = md5_ff(c, d, a, b, x[i + 14], 17, -1502002290);
    b = md5_ff(b, c, d, a, x[i + 15], 22, 1236535329);

    a = md5_gg(a, b, c, d, x[i + 1], 5, -165796510);
    d = md5_gg(d, a, b, c, x[i + 6], 9, -1069501632);
    c = md5_gg(c, d, a, b, x[i + 11], 14, 643717713);
    b = md5_gg(b, c, d, a, x[i + 0], 20, -373897302);
    a = md5_gg(a, b, c, d, x[i + 5], 5, -701558691);
    d = md5_gg(d, a, b, c, x[i + 10], 9, 38016083);
    c = md5_gg(c, d, a, b, x[i + 15], 14, -660478335);
    b = md5_gg(b, c, d, a, x[i + 4], 20, -405537848);
    a = md5_gg(a, b, c, d, x[i + 9], 5, 568446438);
    d = md5_gg(d, a, b, c, x[i + 14], 9, -1019803690);
    c = md5_gg(c, d, a, b, x[i + 3], 14, -187363961);
    b = md5_gg(b, c, d, a, x[i + 8], 20, 1163531501);
    a = md5_gg(a, b, c, d, x[i + 13], 5, -1444681467);
    d = md5_gg(d, a, b, c, x[i + 2], 9, -51403784);
    c = md5_gg(c, d, a, b, x[i + 7], 14, 1735328473);
    b = md5_gg(b, c, d, a, x[i + 12], 20, -1926607734);

    a = md5_hh(a, b, c, d, x[i + 5], 4, -378558);
    d = md5_hh(d, a, b, c, x[i + 8], 11, -2022574463);
    c = md5_hh(c, d, a, b, x[i + 11], 16, 1839030562);
    b = md5_hh(b, c, d, a, x[i + 14], 23, -35309556);
    a = md5_hh(a, b, c, d, x[i + 1], 4, -1530992060);
    d = md5_hh(d, a, b, c, x[i + 4], 11, 1272893353);
    c = md5_hh(c, d, a, b, x[i + 7], 16, -155497632);
    b = md5_hh(b, c, d, a, x[i + 10], 23, -1094730640);
    a = md5_hh(a, b, c, d, x[i + 13], 4, 681279174);
    d = md5_hh(d, a, b, c, x[i + 0], 11, -358537222);
    c = md5_hh(c, d, a, b, x[i + 3], 16, -722521979);
    b = md5_hh(b, c, d, a, x[i + 6], 23, 76029189);
    a = md5_hh(a, b, c, d, x[i + 9], 4, -640364487);
    d = md5_hh(d, a, b, c, x[i + 12], 11, -421815835);
    c = md5_hh(c, d, a, b, x[i + 15], 16, 530742520);
    b = md5_hh(b, c, d, a, x[i + 2], 23, -995338651);

    a = md5_ii(a, b, c, d, x[i + 0], 6, -198630844);
    d = md5_ii(d, a, b, c, x[i + 7], 10, 1126891415);
    c = md5_ii(c, d, a, b, x[i + 14], 15, -1416354905);
    b = md5_ii(b, c, d, a, x[i + 5], 21, -57434055);
    a = md5_ii(a, b, c, d, x[i + 12], 6, 1700485571);
    d = md5_ii(d, a, b, c, x[i + 3], 10, -1894986606);
    c = md5_ii(c, d, a, b, x[i + 10], 15, -1051523);
    b = md5_ii(b, c, d, a, x[i + 1], 21, -2054922799);
    a = md5_ii(a, b, c, d, x[i + 8], 6, 1873313359);
    d = md5_ii(d, a, b, c, x[i + 15], 10, -30611744);
    c = md5_ii(c, d, a, b, x[i + 6], 15, -1560198380);
    b = md5_ii(b, c, d, a, x[i + 13], 21, 1309151649);
    a = md5_ii(a, b, c, d, x[i + 4], 6, -145523070);
    d = md5_ii(d, a, b, c, x[i + 11], 10, -1120210379);
    c = md5_ii(c, d, a, b, x[i + 2], 15, 718787259);
    b = md5_ii(b, c, d, a, x[i + 9], 21, -343485551);

    a = safe_add(a, olda);
    b = safe_add(b, oldb);
    c = safe_add(c, oldc);
    d = safe_add(d, oldd);
  }
  return Array(a, b, c, d);
}

/*
 * These functions implement the four basic operations the algorithm uses.
 */
function md5_cmn(q, a, b, x, s, t) {
  return safe_add(bit_rol(safe_add(safe_add(a, q), safe_add(x, t)), s), b);
}
function md5_ff(a, b, c, d, x, s, t) {
  return md5_cmn((b & c) | ((~b) & d), a, b, x, s, t);
}
function md5_gg(a, b, c, d, x, s, t) {
  return md5_cmn((b & d) | (c & (~d)), a, b, x, s, t);
}
function md5_hh(a, b, c, d, x, s, t) {
  return md5_cmn(b ^ c ^ d, a, b, x, s, t);
}
function md5_ii(a, b, c, d, x, s, t) {
  return md5_cmn(c ^ (b | (~d)), a, b, x, s, t);
}

/*
 * Add integers, wrapping at 2^32. This uses 16-bit operations internally
 * to work around bugs in some JS interpreters.
 */
function safe_add(x, y) {
  var lsw = (x & 0xFFFF) + (y & 0xFFFF);
  var msw = (x >> 16) + (y >> 16) + (lsw >> 16);
  return (msw << 16) | (lsw & 0xFFFF);
}

/*
 * Bitwise rotate a 32-bit number to the left.
 */
function bit_rol(num, cnt) {
  return (num << cnt) | (num >>> (32 - cnt));
}


module.exports = {
  hex_md5: hex_md5
}

④通过上述步骤获得了sign以及统一下单的所有必须参数，后分装xml格式，把xmlFormatData发送到上面的接口地址，返回需要的prepay_id，进行（三）发起支付请求

官方示例：

    <xml>
   <appid>wx2421b1c4370ec43b</appid>
   <attach>支付测试</attach>
   <body>JSAPI支付测试</body>
   <mch_id>10000100</mch_id>
   <detail><![CDATA[{ "goods_detail":[ { "goods_id":"iphone6s_16G", "wxpay_goods_id":"1001", "goods_name":"iPhone6s 16G", "quantity":1, "price":528800, "goods_category":"123456", "body":"苹果手机" }, { "goods_id":"iphone6s_32G", "wxpay_goods_id":"1002", "goods_name":"iPhone6s 32G", "quantity":1, "price":608800, "goods_category":"123789", "body":"苹果手机" } ] }]]></detail>
   <nonce_str>1add1a30ac87aa2db72f57a2375d8fec</nonce_str>
   <notify_url>http://wxpay.wxutil.com/pub_v2/pay/notify.v2.php</notify_url>
   <openid>oUpF8uMuAJO_M2pxb1Q9zNjWeS6o</openid>
   <out_trade_no>1415659990</out_trade_no>
   <spbill_create_ip>14.23.150.211</spbill_create_ip>
   <total_fee>1</total_fee>
   <trade_type>JSAPI</trade_type>
   <sign>0CB01533B8C1EF103065174F50BCA001</sign>
</xml>

 wx.request({
          url: 'https://api.mch.weixin.qq.com/pay/unifiedorder',
          method: "POST",
          dataType:'json',
          data: formData,//formData为上述分装后的xml格式字符串
          success: (res) => {//返回值为xml格式数据，可以从xml格式中解析出prepay_id

            }

以下提供统一下单获取签名后并格式化xml的代码

/*
  功能：生成统一下单的签名sign并格式化为xml格式，返回官方文档说明中，需要发送到接口的的xml格式的数据
  @para：
      obj：生成统一签名需要的参数和值构成的对象，对象的属性名必须与官方文档中需要传入的参数名完全一致。如下示例

              var obj = {
                  appid: appid,
                  body: body,
                  mch_id: mch_id,
                  nonce_str: nonce_str,
                  notify_url: notify_url,
                  openid: openid,
                  out_trade_no: out_trade_no,
                  spbill_create_ip: spbill_create_ip,
                  total_fee: total_fee,
                  trade_type: 'JSAPI',
                  .....:""
              };
      key：商户的key
*/
function getSignAndXmlFormat(obj,key) {
  var keys = Object.keys(obj);
  keys = keys.sort();
  var newArgs = {};
  keys.forEach(function (key) {
    newArgs[key] = obj[key];
  });

  var str = '';
  for (var k in newArgs) {
    str += '&' + k + '=' + newArgs[k];
  }
  str = str.substr(1);
  str = str + '&key=' + key;
  var sign = MD5Util.hex_md5(str).toUpperCase();

  var formData = "<xml>";
  for (var k in newArgs) {
    formData += "<" + k + ">" + newArgs[k] + "</" + k + ">";

  }
  formData += "<sign>" + sign + "</sign>";
  formData += "</xml>"
  return formData;
};

（三）发起支付请求

wx.requestPayment(OBJECT)发起微信支付。
参数说明：

    appId: appid,
    nonceStr: nonceStr,
    package: package1,//
    signType: signType,
    timeStamp: timeStamp，
    paySign:paySign

以上参数只有一个package
,以及paySign是不知道的，而经过第（二）步，我们已经获得了prepay_id，所以上述的参数我们只需要再得到paySign。paySign的获得其实和第（二）步中统一下单的签名获取是一样的，先把上述支付请求的参数除了paySign的参数，按照字典序排序拼接，后再拼接上key，最后进过md5加密得到paySign。特别注意这个package 参数的拼接。

得到paySign的代码

// 生成支付请求的签名
function paysignjs(appid, nonceStr, package1, signType, timeStamp,key) {
  var ret = {
    appId: appid,
    nonceStr: nonceStr,
    package: package1,
    signType: signType,
    timeStamp: timeStamp
  };
  var str = raw(ret);//raw函数是第（二）步中的raw
  str = str + '&key=' + key;
  return MD5Util.hex_md5(str).toUpperCase();//md5加密转化为大写
};

最后所有的参数都知道了，发起支付请求：

 wx.requestPayment({
        'timeStamp': timeStamp,
         'nonceStr': nonceStr,
         'package': package1,
         'signType': 'MD5',
         'paySign': paySign,
          'success': function (res) {
                console.log(res)

           },
              'fail': function (res) {
                console.log(res)
            }
      })

经过以上步骤即可通过测试发起支付了，效果如下：

具体所有步骤参考官方文档。。