Check if field content is in an array.

testField.js

<script type="text/javascript">
    var contentArray = [
      ...
    ];
    var testField = document.getElementById('test_field');
    testField.onkeyup = function(){
        if(contentArray.indexOf(testField.value) != -1) {
            alert("It's in the array !");
        } else {
            alert("It's not in the array !");
        }
    };
</script>