Adding Custom Functions Into Array.prototype
Answer : Modifying the built-in object prototypes is a bad idea in general, because it always has the potential to clash with code from other vendors or libraries that loads on the same page. In the case of the Array object prototype, it is an especially bad idea, because it has the potential to interfere with any piece of code that iterates over the members of any array, for instance with for .. in . To illustrate using an example (borrowed from here): Array.prototype.foo = 1; // somewhere deep in other javascript code... var a = [1,2,3,4,5]; for (x in a){ // Now foo is a part of EVERY array and // will show up here as a value of 'x' } Unfortunately, the existence of questionable code that does this has made it necessary to also avoid using plain for..in for array iteration, at least if you want maximum portability, just to guard against cases where some other nuisance code has modified the Array prototype. So you really need to do both: you should avoid...