javascript - JS 错误 : "undefined is not an object (evaluating ' dataText[i]. 长度')"

Question

尽管这段代码按预期工作，即打印文本，但第 32 行出现控制台错误，内容为:“undefined 不是对象(评估 'dataText[i].length')”

有什么想法吗？

document.addEventListener('DOMContentLoaded',function(event){
  // array with texts to type in typewriter
  var dataText = [ "Hi", "it's Acephala", "Shipping is free for you today",];

  // type one text in the typwriter
  // keeps calling itself until the text is finished
  function typeWriter(text, i, fnCallback) {
    // chekc if text isn't finished yet
    if (i < (text.length)) {
      // add next character to h1
     document.querySelector("h1animation").innerHTML = text.substring(0, i+1) +'<spananimation aria-hidden="true"></spananimation>';

      // wait for a while and call this function again for next character || BK: Speed of text writing
      setTimeout(function() {
        typeWriter(text, i + 1, fnCallback)
      }, 100);
    }
    // text finished, call callback if there is a callback function
    else if (typeof fnCallback == 'function') {
      // call callback after timeout
      setTimeout(fnCallback, 700);
    }
  }
  // start a typewriter animation for a text in the dataText array
   function StartTextAnimation(i) {
     if (typeof dataText[i] == 'undefined'){
        setTimeout(function() {
          StartTextAnimation(0);
        }, 20000);
     }
     // check if dataText[i] exists
    if (i < dataText[i].length) {
      // text exists! start typewriter animation
     typeWriter(dataText[i], 0, function(){
       // after callback (and whole text has been animated), start next text
       StartTextAnimation(i + 1);
     });
    }
  }
  // start the text animation
  StartTextAnimation(0);
});

Answer 1

您的代码执行以下操作:

if (typeof dataText[i] == 'undefined'){
  ...
}
if (i < dataText[i].length) {
  ...
}

这意味着i < dataText[i].length即使 dataText[i] 也会被评估是 undefined 。添加 return第一个结尾处的声明if阻止或使用 else if (i < dataText[i].length) {

<小时/>

除此之外，我相信支票实际上应该是 i < dataText.length 。比较 dataText[i] 的长度似乎不太合理至i .

如果您进行了该更改，则不需要首先提到的更改。

但是，在这种情况下，我宁愿在回调内部进行检查，这意味着 StartTextAnimation总是收到有效的索引:

function StartTextAnimation(i) {
  typeWriter(dataText[i], 0, function(){
    // after callback (and whole text has been animated), start next text
    if (i === dataText.length - 1) {
      setTimeout(function() {
        StartTextAnimation(0);
      }, 20000);
    } else {
      StartTextAnimation(i + 1);
    }
  });
}