这段代码,为何弹出

function binarySearch1(arr,val,leftIndex,rightIndex){
if(leftIndex>rightIndex){
return -1;
}
var midIndex=Math.floor((leftIndex+rightIndex)/2);
var midVal=arr[midIndex];
if(midVal>val){
binarySearch1(arr,val,leftIndex,midIndex-1);
}else if(midVal<val){
binarySearch1(arr,val,midIndex+1,rightIndex);
}else{
return midIndex;
}
}
var arr=[1,2,3,4,5];
Array.prototype.find=function(val){
return binarySearch1(this,val,0,this.length-1);
};
alert(arr.find(-1));