<div>
  <span>请选择颜色</span> 
  <select id="selColor">
    <option value="roseGold">玫瑰金</option>
    <option value="luxuryGold">土豪金</option>
  </select>
</div>
<div>
  <span>请输入购买数量：</span>
  <input type="text" id="selNum" /> 
</div>
<div>
  <span>您选择的颜色为：</span><strong id="conColor"></strong>
  <span>您选择的数量为：</span><strong id="conNum"></strong>
</div>

<button id="nextBtn">加入购物车</button>

// 库存量
var goods = {
  roseGold: 10,
  luxuryGold: 10
};

var colorSelect = document.getElementById("selColor"),
  numberInput = document.getElementById("selNum"),
  colorInfo = document.getElementById("conColor"),
  numberInfo = document.getElementById("conNum"),
  nextBtn = document.getElementById("nextBtn");

colorSelect.onchange = function() {
  var color = colorSelect.value, // 颜色
    number = +numberInput.value, // 数量
    stock = goods[color];    // 对应颜色的库存量

  colorInfo.innerHTML = color;
  if(!color) {
    nextBtn.disabled = true;
    nextBtn.innerHTML = "请选择手机颜色";
    return;
  }

  if(!number || (((number - 0) | 0) !== (number - 0))) {
    nextBtn.disabled = true;
    nextBtn.innerHTML = "请选择手机数量";
    return;
  }

  if( number > stock) {
    nextBtn.disabled = true;
    nextBtn.innerHTML = "库存不足";
    return;
  }

  nextBtn.disabled = false;
  nextBtn.innerHTML = "加入购物车";

};

/* 购买数量做同样处理 */

<div>
  <span>请选择颜色：</span> 
  <select id="selColor">
    <option value="roseGold">玫瑰金</option>
    <option value="luxuryGold">土豪金</option>
  </select>
</div>
<div>
  <span>请选择内存：</span>
  <select id="selMemory">
    <option value="16G">16G</option>
    <option value="64G">64G</option>
  </select>
</div>
<div>
  <span>请输入购买数量：</span>
  <input type="text" id="selNum" /> 
</div>
<div>
  <span>您选择的颜色为：</span><strong id="conColor"></strong>
  <span>您选择的内存为：</span><strong id="conMemory"></strong>
  <span>您选择的数量为：</span><strong id="conNum"></strong>
</div>

<button id="nextBtn">加入购物车</button>

// 库存量
var goods = {
  "roseGold|16G": 10,
  "roseGold|32G": 10,
  "luxuryGold|16G": 10,
  "luxuryGold|32G": 10
};

var colorSelect = document.getElementById("selColor"),
    memorySelect = document.getElementById("selMemory"),
    numberInput = document.getElementById("selNum"),
    colorInfo = document.getElementById("conColor"),
    memeryInfo = document.getElementById("conMemory"),
    numberInfo = document.getElementById("conNum"),
    nextBtn = document.getElementById("nextBtn");

var mediator = (function() {
  return {
    changed: function(obj) {
      var color = colorSelect.value, // 颜色
      memory = memorySelect.value,// 内存
      number = +numberInput.value, // 数量
      stock = goods[color + '|' + memory];    // 对应颜色的库存量

      if(obj === colorSelect) {
        colorInfo.innerHTML = color;
      }else if(obj === memorySelect) {
        memeryInfo.innerHTML = memory;
      }else if(obj === numberInput) {
        numberInfo.innerHTML = number;
      }

      if(!color) {
        nextBtn.disabled = true;
        nextBtn.innerHTML = "请选择手机颜色";
        return;
      }

      if(!memory) {
        nextBtn.disabled = true;
        nextBtn.innerHTML = "请选择手机内存";
        return;
      }

      if(!number || (((number - 0) | 0) !== (number - 0))) {
        nextBtn.disabled = true;
        nextBtn.innerHTML = "请选择手机数量";
        return;
      }

      if( number > stock) {
        nextBtn.disabled = true;
        nextBtn.innerHTML = "库存不足";
        return;
      }

      nextBtn.disabled = false;
      nextBtn.innerHTML = "加入购物车";
    }
  };
})();

// 事件函数
colorSelect.onchange = function() {
  mediator.changed(this);
};
memorySelect.onchange = function() {
  mediator.changed(this);
};
numberInput.oninput = function() {
  mediator.changed(this);
}