Use of the functionality provided by the
new keyword has several advantages over building each object from scratch:
- Performance. This is a side-effect of #1: if I want to add 10 methods to every object I create, I could just write a creation function that manually assigns each method to each new object... Or, I could assign them to the creation function's
prototype and use
new to stamp out new objects. Not only is this faster (no code needed for each and every method on the prototype), it avoids ballooning each object with separate properties for each method. On slower machines (or especially, slower JS interpreters) when many objects are being created this can mean a significant savings in time and memory.
new has one crucial disadvantage, ably described by other answers: if you forget to use it, your code will break without warning. Fortunately, that disadvantage is easily mitigated - simply add a bit of code to the function itself:
// if user accidentally omits the new keyword, this will
// silently correct the problem...
if ( !(this instanceof foo) )
return new foo();
// constructor logic follows...
Now you can have the advantages of
new without having to worry about problems caused by accidentally misuse. You could even add an assertion to the check if the thought of broken code silently working bothers you. Or, as some commented, use the check to introduce a runtime exception:
if ( !(this instanceof arguments.callee) )
throw new Error("Constructor called as a function");
(Note that this snippet is able to avoid hard-coding the constructor function name, as unlike the previous example it has no need to actually instantiate the object - therefore, it can be copied into each target function without modification.)
with is especially enlightening for those of us who initially dismissed this much-maligned feature as a gimmick).