Question

While experimenting with some different methods for generating JavaScript arrays, I stumbled on a weird result. Using map to push an array of self-references (DEMO):

a=[1,1,1,1,1,1,1,1,1,1];
a=a.map(a.push,a);

I get the following result (in Chrome):

[13,16,19,22,25,28,31,34,37,40]

Can anyone explain why?

Answer 1

For each element in a, push is being called with that element, the index of that element, and the array being traversed. For each element in the array, then, we add these three additional elements. This accounts for the length increasing by three for each element in the original array. The result of push is the length of the array after the elements are added, thus the resulting array (from map) is an array holding the lengths of the a array after each push callback is completed.

See the documentation for map and push.