Auto-complete in JavaScript

I recently got interviewed in a company for Senior level position. They asked me to develop auto-complete component (2 hours time limit) using plain JavaScript similar to this.

I wrote below code, which works as expected, but they responded saying that the solution doesn't match expectations. Please help me figure out what I could improve in this.

Here is the link to working example.

function customAutoComplete(input,sourceList){

if(!input){
    console.error('No input box provided');
}    
if(!sourceList ||   sourceList.length=0) { 
            var listItem=document.createElement("option");               
            listItem.appendChild(document.createTextNode(itm));
            suggList.appendChild(listItem);
            size++;
        }
    });      
    suggList.setAttribute('size',size);
    if(size>1){
        return suggList;
    }
    return null;
} 

function getSuggestList(){
    var acListId=input.id + '_autoComplete';         
    var acList= document.getElementById(acListId)
    if(!acList){
        acList = document.createElement("select");
        acList.setAttribute('class','suggestion-list');
        acList.setAttribute('id',acListId);
        var stl='left:' + input.offsetLeft + 'px;top:' + (input.offsetTop + input.offsetHeight) + 'px;min-width:' + input.offsetWidth + 'px';
        acList.setAttribute('style',stl);
        acList.addEventListener('keydown',function(e){autoCompleteItemSelected(e,acList)});
        acList.addEventListener('blur', function(){clearSuggestionList(suggList)} );
    }

    else{
        acList.innerHTML='';
    }

    return acList;
}
function clearSuggestionList(suggList){
    if(!suggList){
        suggList=getSuggestList();
    }
    suggList.outerHTML='';
}
function autoCompleteItemSelected(e,list){
    if(e.keyCode==13){ 
        input.value=list.selectedOptions[0].text;
        clearSuggestionList();
    } 
}
}

There's a few things that can be reviewed:

Your indentation and spacing are incorrect, use JSFiddle's TidyUp function to clean it up.

All of the e.keyCode == matches shouldn't be matched to magic numbers, but moved to a kind of enum or something similar:

var KEYS = {
    Enter: 13,
    DownArrow: 40,
    UpArrow: 38
};
// ...
if (e.keyCode == KEYS.Enter){

suggList:

Don't sacrifice readability for a few characters: suggestedList is more than fine.

console.error:

Instead of console.error, use throw instead: console.error('No sourceList provided for customAutoComplete') into: throw 'ACError: No sourceList provided for customAutoComplete'

StackExchange Code Review Q#108214, answer score: 3