If your javascript is in a webpage hosted on a different domain, you have the usual cross-site request restriction.

So first, how to get the response from the endpoint to your page:

1. check if the endpoint is CORS-enabled - if so, you can use AJAX
2. check if the endpoint supports JSONP - is so you can use JSONP (not all endpoints support JSON)
3. if neither, you can use a proxy webservice like http://morph.talis.com which can take the URI of a SPARQL query and serve JSONP.

If the answer is 3:

1. Write/generate your SPARQL query and assign it to a variable in your javascript: var query = '{your query here}';.
2. create a variable for the URI of your query: var queryUri = endpointUri+'?query='+encodeURIComponent(query);
3. create the URI of the request to the Morph service: var jsonpUri = 'http://morph.talis.com/?data-uri[]='+encodeURIComponent(queryUri)+'&output=jsonp&callback=processSparqlResults';
4. create a script tag: var newScript = document.createElement('script');
5. give it a src attribute: newScript.src=jsonpUri;
6. append the script tag to the document body: document.body.appendChild(newScript);
7. Make sure you have defined the callback function: function processSparqlResults(data){ console.log(data); }
8. If you did a SELECT or ASK query, the JSON format used will be http://www.w3.org/TR/rdf-sparql-json-res/ ; if DESCRIBE or CONSTRUCT, it will be http://n2.talis.com/wiki/RDF_JSON_Specification

Here is a nice tutorial from DailyJS.com: A Simple Linked Data and JavaScript Tutorial

I think some SPARQL endpoints can be configured to return JSON, which can be pulled into a web page without cross-domain security restrictions getting in the way.

One appraoch I have used it to run a SPARQL query using YQL and then grab the JSON response. For example, try a query of the form: select * from sparql.search where query=”YOUR_SPARQL_QUERY” and service=”SPARQL_ENDPOINT_URL” in the Yahoo Developer console [http://developer.yahoo.com/yql/console/] (See http://blog.ouseful.info/2010/11/02/accessing-linked-data-in-scraperwiki/ for a couple of examples.)

Another tool I've used is SPARQLproxy [http://data-gov.tw.rpi.edu/ws/sparqlproxy.php] which lets you enter a query and endpoint, and get the result in a variety of formats, inclusing three different JSON formats (SPARQL JSON, MIT Simile/Exhibit JSON and Google Viz API JSON).