如何在JavaScript中记录一个ArrayIndex？

Question

我有阵列 var john = ['asas','gggg','ggg'];

如果我john在索引3处访问,即.john[3], 它失败.

如何显示消息或警告,说明该索引没有值？

Answer 1

if (typeof yourArray[undefinedIndex] === "undefined") {
  // It's undefined
  console.log("Undefined index: " + undefinedIndex;
}

Answer 2

function checkIndex(arrayVal, index){
    if(arrayVal[index] == undefined){
        alert('index '+index+' is undefined!');
        return false;
    }
    return true;
}

//use it like so:
if(checkIndex(john, 3)) {/*index exists and do something with it*/}
else {/*index DOES NOT EXIST*/}