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<!DOCTYPE html>
<html>
<body>
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<p>在本例中，我们省略了最后一个参数，并在循环内增加了计数器变量。</p>
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<p>单击这个按钮可按升序循环遍历数组的索引。</p>
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<button onclick="myFunction()">试一试</button>
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<p id="demo"></p>
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<script>
function myFunction() {
  var cars = ["BMW", "Volvo", "Saab", "Ford"];
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  var i = 0;
  var len = cars.length;
  var text = "";
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  for (; i < len;) {
    text += cars[i] + "<br>";
    i++;
  }
  document.getElementById("demo").innerHTML = text;
}
</script>
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</body>
</html>
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