js 如何在多个按钮中获取点击按钮的id值

window.onload = function(){
				var arr = document.getElementsByTagName('button');
				for(var i = 0;i<arr.length;i++){
					arr[i].onclick = function(){
						alert(this.id);
					}
				}
			}

<button id="b1">b1</button>
		<button id="b2">b2</button>
		<button id="b3">b3</button>
		<button id="b4">b4</button>