Edit in JSFiddle

window.onload = function () {
    var txt1 = document.getElementById('txt1'),
        txt2 = document.getElementById('txt2'),
        btn1 = document.getElementById('btn1'),
        btn2 = document.getElementById('btn2');
    btn1.onclick = function () {
       return txt1.value ? true : alert('输入不能为空');
    }
    
    btn2.onclick = function () {
       return txt2.value.replace(/^(\s|\u00A0)+|(\s|\u00A0)+$/g,'')
       ? true : alert('输入不能为空');
    } 
}

<h2>不考虑空格</h2>
<input type='text' id='txt1'/>
<button type='button' id='btn1'>click</button>
<h2>过滤空格</h2>
<input type='text' id='txt2'/>
<button type='button' id='btn2'>click</button>