JS中的循环计时

Question

这段代码是我用 JS 写的：

function startFunction() {
p1 = document.getElementById('p1').innerHTML;
for (var i=1; i<=p1.length; i++) {
        alert(p1.slice(0, i));
    }
}

我在 html 中用 onload 事件调用函数:

    <body onload="startFunction()">

这是带有 p1 ID 的段落：

    <p id="p1">Hi, I'm</p>

我怎样才能使 for 循环延迟。我希望我的程序逐个字母地编写 p1 文本。

Answer 1

var alertEachLetter =function(p1, i){
    setTimeout(function(){
        alert(p1.slice(0, i));
    },1000);
};

 function startFunction() {
 p1 = document.getElementById('p1').innerHTML;
 for (var i=1; i<=p1.length; i++) {
        alertEachLetter(p1, i);
    }
}

为什么要创建这个 alertEachLetter 函数。为此你需要检查这个 link

setTimeout in for-loop does not print consecutive values

Answer 2

你不能也不应该在循环内延迟任何事情，因为这就是无响应页面的制作方式：浏览器不会对用户操作做出反应或做任何可见的事情，直到 JavaScript 代码 returns.

相反，您可以使用一些计时器，例如 setInterval():

function startFunction() {
    var p1 = document.getElementById('p1');
    var txt = p1.innerHTML;
    var i=0;
    var timer = setInterval(function() {
        p1.innerHTML = txt.slice(0,i++);
        if(i>txt.length) {
            clearInterval(timer);
        }
    },500);
}

startFunction();

<p id="p1">Hi, I'm</p>

Answer 3

你不需要循环，你需要 interval。 Javascript 的 interval 功能将在（大约）请求的时间间隔调用您的函数。所以，例如：

function startFunction() {
  var p1 = document.getElementById('p1').innerHTML
  var count = 1
  var finished = p1.length
  var iv = setInterval(function() {
    alert(p1.slice(0,count++))
    if (count > finished) {
      clearInterval(iv) // stops the interval from firing once we finish our task
    }
  }, 1000) // 1000 ms, or every second.
}

Answer 4

以下是我认为可以帮助您实现目标的方法。这种方法使用 setInterval（而不是循环）来多次执行一个函数。看评论理解代码逻辑：

//Grab our DOM elements
var p1 = document.getElementById('p1').innerHTML;
var copy = document.getElementById('copy');

//Execute a function every 250 milliseconds
var intervalId = setInterval(onInterval, 250);

//nextLetter is a function that will return the character at a particular index in the string. The function will increase the index each time it is called. The function will return null once it exceeds the innerHTML length. c is a "private" variable that can't be modified elsewhere in the program.
var nextLetter = (function(i, limit) {
  var c = i;
  return function() {
    var idx = c++;
    if (idx > limit) {
      return null;
    }
    return p1.charAt(idx);
  };
})(0, p1.length);

//The function we will execute at each interval
function onInterval() {
  var letter = nextLetter();
  if (letter) {
    copy.innerHTML += letter;
  } else {
    console.log('End of content reached - removing interval');
    clearTimeout(intervalId);
  }
}

<p id="p1">Make sure to read the in-code comments</p>
<p id="copy"></p>

Answer 5

这是一个使用 setTimeout 而不是 setInterval 的简单示例。除了您不必清除超时外，没有太大区别 - 如果它不满足条件，您根本不需要运行它。

// cache the elements
const p1 = document.getElementById('p1');
const out = document.getElementById('out');

// make the text content from p1 iterable and split it into
// the head (first element), and tail (everything else)
const [head, ...tail] = [...p1.textContent];

const loop = function loop(head, tail) {

  // update the output text content with the result of head
  out.textContent = head;

  // if there's anything left of the tail array
  if (tail.length) {

    // remove the first element of tail and
    // add it to head
    head += tail.shift();

    // call the function again with the new head and tail
    setTimeout(loop, 200, head, tail);
  }

// pass in the head and tail to the function
}(head, tail);

#p1 { display: none; }

<p id="p1">Content written letter by letter</p>
<p id="out"></p>

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