sp15_searchsettings.js

var ctx = new SP.ClientContext.get_current();
var web = ctx.get_site().get_rootWeb();
var props =  web.get_allProperties();

//Search Center Url
props.set_item("SRCH_ENH_FTR_URL","/sites/search/pages");

//Search Scope Drop down option
props.set_item("SRCH_SITE_DROPDOWN_MODE","HideScopeDD_DefaultContextual");

//Target Search Results page
props.set_item("SRCH_TRAGET_RESULTS_PAGE","/sites/Search/Pages/results.aspx");

web.update();

ctx.load(web);

ctx.executeQueryAsync(function () {
    alert("Search Settings Modified");
 },
 function() {
  alert("failed");
});