if (document.getElementById('j1').checked){
if(document.form1.text1.value == ''){
alert('请输入') ;
}
}
<input type="checkbox" name="checkbox" value="checkbox" id="j1">
<input type="text" name="text1">
if(document.form1.text1.value == ''){
alert('请输入') ;
}
}
<input type="checkbox" name="checkbox" value="checkbox" id="j1">
<input type="text" name="text1">
if(text.value == ""){
alert("errmessage");
}
}