<SCRIPT LANGUAGE="JavaScript">
<!--

function checkInput(element) {
  if (element.value == "") {
    alert("Please enter a value!");
  }
}

// -->
</SCRIPT>
<FORM>
<INPUT TYPE="text" NAME="myField" VALUE="" onBlur="checkInput(this)">
</FORM>

Notice that the dialog box is displayed only if you first access the field and then exits it without entering any value.

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