<button onclick="mere()">Tryk</button>

<form name="holder">

<input type="text" class="challenge" name="challenge[]" size="10"><br/>

</form>

var a = [1, 2, 3, 4, 5, 6]

function mere() {
var inputs = document.getElementsByClassName( 'challenge' ),
names  = [].map.call(inputs, function( input ) {
    return input.value;
}).join( '|' );


if (inputs > 0){
a.push.apply(a, inputs);
console.log(inputs);
}
else {
alert("empty")
}
}