JavaScript实现文本框placeholder提示文字功能示例

<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>www.jb51.net JS文本框placeholder提示</title>
</head>
<body>
  <input id="input" type="text" value="请输入关键词">
</body>
<script>
    window.onload = function() {
      var defaultValue = "请输入关键词";
      var input = document.getElementById("input");
      input.style.color = "grey";
      input.onfocus = function() {
        if (this.value == defaultValue) {
          input.value="";
          setCursorPosition(this, 0);
        }
      };
      input.onblur = function() {
        if (this.value == "") {
          this.value = defaultValue;
        }
      };
      input.onkeypress = function(e) {
        e = e || window.event;
        var key = e.charCode || e.keyCode || e.which;
        if (this.value == defaultValue) {
          this.value = "";
          this.style.color = "black";
        }
        if (this.value.length == 1 && key == 8) {
          this.value = defaultValue;
          this.style.color = "grey";
          setCursorPosition(this, 0);
        }
      };
    };
    function setCursorPosition(elem, index) {
      if (elem.setSelectionRange) {
        elem.focus();
        elem.setSelectionRange(index, index);
      }
      else if (elem.createTextRange) {
        var range = elem.createTextRange();
        range.collapse(true);
        range.moveEnd('character', index);
        range.moveStart('character', index);
        range.select();
      }
    }
</script>
</html>