JavaScript驗證輸入是否爲空

HTML元素:

<input type="text" name="title" id="title" value="${myContent.title}"
			                  οnkeyup="validateNotEmptyField(document.getElementById('title'))"
			                  οnsubmit="validateNotEmptyField(document.getElementById('title'))"
			                  οnchange="validateNotEmptyField(document.getElementById('title'))">

JavaScript代碼:

function validateNotEmptyField(field) {
    if (field.value == '' || field.value == null) {
        field.setCustomValidity('不能爲空, 請輸入');
    } else {
        field.setCustomValidity('');
    }
}