<div id="box" style="background: #334;width: 100px;height: 100px;position: absolute;cursor: move;z-index: 999;"></div>
<div id="box2" style="background: green;width: 100px;height: 100px;position: absolute;top: 200px;left: 500px;"></div>

(function () {
	var dragging = false
	var boxX, boxY, mouseX, mouseY, offsetX, offsetY
	var box = document.getElementById('box')
	var box2 = document.getElementById('box2')
	var box2X, box2Y

	// 鼠标按下的动作
	box.onmousedown = down
	// 鼠标的移动动作
	document.onmousemove = move
	// 释放鼠标的动作
	document.onmouseup = up

	// 鼠标按下后的函数,e为事件对象
	function down(e) {
		dragging = true
		
		// 获取元素所在的坐标
		boxX = box.offsetLeft
		boxY = box.offsetTop

		// 获取元素box2所在的坐标
		box2X = box2.offsetLeft
		box2Y = box2.offsetTop

		// 获取鼠标所在的坐标
		mouseX = parseInt(getMouseXY(e).x)
		mouseY = parseInt(getMouseXY(e).y)

		// 鼠标相对元素左和上边缘的坐标
		offsetX = mouseX - boxX
		offsetY = mouseY - boxY
	}

	// 鼠标移动调用的函数
	function move(e){
		if (dragging) {
			// 获取移动后的元素的坐标
			var x = getMouseXY(e).x - offsetX
			var y = getMouseXY(e).y - offsetY

			// 计算可移动位置的大小， 保证元素不会超过可移动范围
			var width = document.documentElement.clientWidth - box.offsetWidth
			var height = document.documentElement.clientHeight - box.offsetHeight

			// min方法保证不会超过右边界，max保证不会超过左边界
			x = Math.min(Math.max(0, x), width)
			y = Math.min(Math.max(0, y), height)

			// 给元素及时定位
			box.style.left = x + 'px'
			box.style.top = y + 'px'

			// 碰撞检测
			// x坐标值的范围判断，y坐标值的范围判断
			var judge_x = (x >= box2X - box2.offsetWidth) && (x <= box2X + box2.offsetWidth)
			var judge_y = (y >= box2Y - box2.offsetHeight) && (y <= box2Y + box2.offsetHeight)
			if (judge_x && judge_y) {
				console.log("碰撞到")
			}
		}
	}

	// 释放鼠标的函数
	function up(e){
		dragging = false
	}

	// 函数用于获取鼠标的位置
	function getMouseXY(e){
		var x = 0, y = 0
		e = e || window.event
		
		if (e.pageX) {
			x = e.pageX
			y = e.pageY
		} else {
			x = e.clientX + document.body.scrollLeft - document.body.clientLeft
			y = e.clientY + document.body.scrollTop - document.body.clientTop
		}
		return {
			x: x,
			y: y
		}
	}
})()

// 碰撞检测
// x坐标值的范围判断，y坐标值的范围判断
var judge_x = (x >= box2X - box2.offsetWidth) && (x <= box2X + box2.offsetWidth)
var judge_y = (y >= box2Y - box2.offsetHeight) && (y <= box2Y + box2.offsetHeight)
if (judge_x && judge_y) {
	console.log("碰撞到")
}