Always throw user configuration parse failed, will use default configuration about elasticlunr.js HOT 1 OPEN

since you say based EXACTLY on the example
The default content of keyup would do this assuming that $(#configuration).val() is blank
If you hack it in app.js to read

$('input').bind('keyup', debounce(function () {
if ($(this).val().length < 2) return
// var config = $('#configuration').val(); THIS WILL BE EMPTY SO...
var config = '{ "fields": { "title": {"boost": 2}, "tags": {"boost": 2}, "body": {"boost": 1} }, "boolean": "OR"}';
var json_config = null;
json_config = new elasticlunr.Configuration(config, window.idx.getFields()).get();
var query = $(this).val()
var results = null;
results = window.idx.search(query, json_config).map(function (result) {
return questions.filter(function (q) { return q.id === parseInt(result.ref, 10) })[0]
})
// console.log(results); // FOR OPTIONAL MATCH OUTPUTS (can be handy to debug)
renderQuestionList(results)
}))

And amend the config var to be something you want., that might do it?
Hope that helps.

from elasticlunr.js.