Javascript不警告输入值

Question

The code below isn't working as expected. I have excluded the rest of the tags. Whenever I execute the html, the alert alerts nothing at all. Has anyone encountered this problem before?

<p><input type="text" id = "earth"> Enter your weight on Earth.<BR><BR></p>

    <script type="text/javascript">

    function calculate() 
    {
        var weight = input.earth.value;

        window.alert(weight);




    }
    </script>

Answer 1

You're not getting the element correctly - input.earth means nothing. You should use getElementById() :

var weight = document.getElementById('earth').value;

Answer 2

you may try also:

<input type="text" id ="earth"> Enter your weight on Earth
<button onclick="alert(document.getElementById('earth').value)">alert</button>