如何在javascript中的以下代码中优化使用数组？

Question

Is there a way to make the following code faster? It's becoming too slow, when length of array is more than 1000 records, especially in IE6.

  dbusers = data.split(";");
  $("#users").html("");
  for (i = 0; i < dbusers.length; i++) {
     if ($("#username").val() != "") {
        if (dbusers[i].indexOf($("#username").val()) != -1) {
           $("#users").append(dbusers[i] + "<br>");
        }
     } else {
        $("#users").append(dbusers[i] + "<br>");
     }
  }

Answer 1

Minimize the amount of work you do in the loop. Don't add stuff to the DOM in the loop, create a string.

var dbusers = data.split(";");
var username = $("#username").val();
var userlist = "";
if (username == "") {
    for (i = 0; i < dbusers.length; i++) {
        userlist += dbusers[i] + "<br>";
    }
} else {
    for (i = 0; i < dbusers.length; i++) {
        if (dbusers[i].indexOf(username) != -1) {
            userlist += dbusers[i] + "<br>";
        }
    }   
}   
$("#users").html(userlist);

Answer 2

Faster than those by far ( especially in IE!) is to build your string as an array (yes, really) and then concatenate it at the end:

var dbusers = data.split(";"), username = $('#username').val();
$("#users").html($.map(dbusers, function(_, dbuser) {
  if (username == '' || dbuser.indexOf(username) > 0)
    return dbuser + '<br>';
  return '';
}).get().join(''));

The $.map() routine will build an array from the return values of the function you pass. Here, my function is returning the user string followed by the <br> . The resulting array is then turned into a string by calling the native join() routine. Especially when you've got like 1000 things to work with, this will be much faster than building a string with repeated calls to += ! Try the two versions and compare!

Answer 3

Use a document fragment.

You can perform more optimizations, too, like removing that nasty if and creating the nodes yourself.

var frag = document.createDocumentFragment(),
    dbUsers = data.split(';'),
    dbUsersLength = dbUsers.length,
    curDbUser,
    usernameVal = $('#username').val();

for(i = 0; i < dbUsersLength; ++i) {
    curDbUser = dbUsers[i];

    if(curDbUser.indexOf(usernameVal) !== -1) {
        frag.appendChild(document.createTextNode(curDbUser));
        frag.appendChild(document.createElement('br'));
    }
}

$('#users').empty().append(frag);

I made a tool to benchmark all the current answers: http://dev.liranuna.com/strager/stee1rat.html

ghoppe's and mine seem to be the fastest.

Answer 4

IE6 doesn't support querySelector , so lookups can be particularly slow. Keep HTML manipulation within loops to a minimum by reducing the number of appends you do, each one has a regular expression run on it to extract the HTML and convert it to a DOM object. Also work in some micro optimisations where you can, might improve performance a little especially over thousands of iterations.

var usersEl = $("#users"); // reduce lookups to the #users element
var result  = "";          // create a variable for the HTML string
var unameVal = $("#username").val(); // do the username value lookup only once
dbusers = data.split(";");
usersEl.html(""); 

// Store the length of the array in a var in your loop to prevent multiple lookups
for (var i = 0, max = dbusers.length; i < max; i++) { 
  if (unameVal !== "") { 
    if (dbusers[i].indexOf(unameVal) != -1) { 
      result += dbusers[i] + "<br>"; 
    } 
  } else { 
    result += dbusers[i] + "<br>"; 
  }
} 
usersEl.html(result);  // Set the HTML only once, saves multiple regexes