Question

I want to check if an input tag named “field2” exists when the user is filling out input name “field1”. I do that by executing a JavaScript function using the onchange event on field1’s input tag. (I’m testing using alert boxes.) If field2 does not exist, JavaScript clicks a button and the form is updated to have both field1 and field2. But the alert box pops up even when field2 exists no matter which of the 3 methods I use. I tried all sorts of combinations using if with null and 'undefined', etc.

Why do the alert boxes pop up if field2 exists ?

function foobar(){

if(!document.getElementsByName("field2"){
alert("foobar");
}

if(!document.forms[0].field2){
alert("foobar");
}

if(!document.forms[0].elements.namedItem("field2"){
alert("foobar");
}
}

Answer 1

Actually, the problem was that the page had various forms and therefore forms[0] was not referring to the form I wanted. So I think the best way is to use this and refer to the input field directly. Also, it is clearer to compare to undefined rather than !.

This works:

function foobar(fooform){
    if (fooform.field2 === undefined) {
        alert("foobar");
    }
}

Called like this:

foobar(this.form);