Passing an id through a function parameter

Question

Trying to get a value of a textbox by passing the ID into a function parameter. Should be easy but can't wrap my head around it.

JavaScript:

function checkfield(id) 
    {
        var field = document.getElementById(id);

        if (isNaN(field) == true) 
            {
                alert('You must enter a valid number!');
                field.value = "0";
                field.focus(textbox);
                field.select(textbox);
                return false;
            }
        return true;

    }

HTML:

<input type="text" id="numbox19" name="customerZip" style="width: 240px" value=" " onchange="checkfield('numbox19')"/>

Error Message

Error: Unable to get property 'value' of undefined or null reference

Answer 1

您输入的ID是“ 19”而不是“ numberbox19”

Answer 2

Your ID is 19 , but you're passing numberbox19 to the Javascript function. There are also several syntax errors.

Try:

<input type="text" id="numberbox19" name="customerZip" style="width: 240px" value=" " onchange="checkfield(this)"/>

And Javascript:

function checkfield(field) // <-- we passed the field in directly from the HTML, using 'this'
{
    alert(field.value);

    if (isNaN(field.value)) // check the value, not the field itself!
    {
        alert('You must enter a valid number!');
        field.value = "0";
        field.focus(); // not "field.focus(textbox);"
        return false;
    }
    return true;

}

The good thing here is, if you ever decide to change the ID for any reason, you only have to change it in one place instead of two.