It's a feature and more consistent as other "Array" category methods like Underscore's _first, _.last, _.initial, and _.zipObject don't have fail-safes for not passing a required argument. The fail-safes are implemented in the "Collections" category methods though I'm not sure if they are really needed either. Underscore is tied to its _.each (a "Collections" method) when it needs to iterate and so has the side effect of its behavior bleeding into other non collections methods.

Did you run into an issue in your code where you expected an empty array?

yes, after drop-in replacement I found that this code fails because match returns array or null.

var result = _.flatten(string.match(/regexp/))[0] //result or undefined, but not throw Error;

I understand that this is not best idea to use _.flatten in this way, but anyway. In this commits I missed that underscore also flattens objects:

_.flatten({'a': 1}); //[1]

I can do it in this PR if you want.

underscore flatten(for in loop):

_.flatten({'a': [1, 2, [3]], 'b': [1,[4],10]}); // [1,2,3,1,4,10] 

lodash flatten (for loop):

_.flatten({'a': [1, 2, [3]], 'b': [1,[4],10]}); // [] 

In this commits I missed that underscore also flattens objects:

_.flatten({'a': 1}); //[1]
I can do it in this PR if you want.

Naw, no need. The flattening of objects is undocumented/untested and an unintended side effect of Underscore's _.each overuse/dependency.

yes, after drop-in replacement I found that this code fails because match returns array or null.

var result = _.flatten(string.match(/regexp/))[0] //result or undefined, but not throw Error;

The string.match(/regexp/) case is a great example of when a fail safe would be nice, because it's annoying to always do (string.match(/regexp/) || 0). However, I think you juuuust happened to not run into issues as I pointed out other "Arrays" methods would have thrown errors.

I will close this for now, but will consider adding a fail safe to all "Arrays" methods.

Resolved by 0a2e5fd2.

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