Initialise lunr.js search on demand when focusing on the search input field.

on-demand-search.js

(function() {
  // execute the given function only once
  function once(fn) { 
    var done = false;
    return function () {
      return done ? void 0 : ((done = true), fn.apply(this, arguments))
    }; 
  }
  
  // load a .js script on demand, executing the on loaded function after downloaded and parsed
  var demand = function(src, onLoaded) {
    var onLoadOnce = once(onLoaded);
    var domscript = document.createElement('script'); 
    domscript.src = src;
    
    domscript.onload = onLoadOnce;

    domscript.onreadystatechange = function() {
      if ('loaded' === domscript.readyState) {
        onLoadOnce();
      }
    };

    document.getElementsByTagName('head')[0].appendChild(domscript);
  };

  // add an event handler to the focus of the given element input
  var focused = function(id, onFocus) {
    var elem = document.getElementById(id);

    if (elem.addEventListener) {
      elem.addEventListener('focus', onFocus, false);
    } else if (elem.attachEvent) {
    	elem.attachEvent('onfocus', onFocus);
    }
  };

  // load the search .js file when the search query input gains focused (once)
  var loadOnDemand = function(loaded) {
    focused('search-query', once(function() {
      demand('/js/search.min.js', loaded);
    }));
  };

  // initialize lunr.js search after loading dependencies on demand
  loadOnDemand(function() {
    (function($) {
      $('#search-query').lunrSearch({
        index:    '/search.json',
        results:  '#search-results',
        entries:  '.entries',
        template: '#search-results-template'
      });
    })(jQuery);
  });
})();

on-demand-search.min.js

(function(){function e(a){var b=!1;return function(){return b?void 0:(b=!0,a.apply(this,arguments))}}var f=function(a,b){var c=e(b),d=document.createElement("script");d.src=a;d.onloadDone=!1;d.onload=c;d.onreadystatechange=function(){"loaded"===d.readyState&&!d.onloadDone&&c()};document.getElementsByTagName("head")[0].appendChild(d)},g=function(a,b){var c=document.getElementById(a);c.addEventListener?c.addEventListener("focus",b,!1):c.attachEvent&&c.attachEvent("onfocus",b)};(function(a){g("search-query",e(function(){f("/js/search.min.js",a)}))})(function(){jQuery("#search-query").lunrSearch({index:"/search.json",results:"#search-results",entries:".entries",template:"#search-results-template"})})})();