<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>

An alert dialog box informs the user that he or she must enter a value in the text box if the box is left empty. The dialog box is displayed only if the user first accesses the field and then exits it without entering any value.

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