Beware of implicit object conversion

JavaScript performance comparison

Revision 9 of this test case created by XP1

Info

Taken from: http://dev.opera.com/articles/view/efficient-javascript/

Literals, such as strings, numbers, and boolean values, have two representations within ECMAScript. Each of them can be created as either a value or an object. For example, a string value is created simply by saying var oString = 'some content';, while an equivalent string object is created by saying var oString = new String('some content');. Any properties and methods are defined on the string object, not the value. When you reference a property or method of a string value, the ECMAScript engine must implicitly create a new string object with the same value as your string, before running the method. This object is only used for that one request, and will be recreated next time you attempt to use a method of the string value. This example requires the script engine to create 21 new string objects, once for each time the length property is accessed, and once each time the charAt method is called: var s = '0123456789'; for( var i = 0; i < s.length; i++ ) { s.charAt(i); } This equivalent example creates just a single object, and will perform better as a result: var s = new String('0123456789'); for( var i = 0; i < s.length; i++ ) { s.charAt(i); } If your code calls methods of literal values very often, you should consider converting them into objects instead, as in the previous example. Note that although most of the points in this article are relevant to all browsers, this particular optimization is aimed mainly at Opera. It may also affect some other browsers, but can be a little slower in Internet Explorer and Firefox.

Also see http://jsperf.com/string-conversion-speed.

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Testing in unknown unknown
Test Ops/sec
simple string
 
var s = '0123456789';
for (var i = 0; i < s.length; i++) {
  s.charAt(i);
}
 pending…
new String object
 
var s = new String('0123456789');
for (var i = 0; i < s.length; i++) {
  s.charAt(i);
}
 pending…
String object
 
var s = String('0123456789');
for (var i = 0; i < s.length; i++) {
  s.charAt(i);
}
 pending…

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1 comment

Inviz commented : ∞

Be wary that String() does not cast primitive to object, it just makes a type casting:

z = String(1) "1" z.c = 1 1 z.c undefined

z = new String(1) String z.c = 1 1 z.c 1

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