Thank you @j0pgrm for making a contribution! The approach you took is not quite right, as it breaks functionality. I'll help you find a better solution.

I suggest you take the following steps:

• Inside the root directory of your local clone of the Underscore project, on your own double-chaining branch, run npm install. This will install all the tools we use during Underscore development.
• Run npm run test. You will see that six tests fail, due to your changes.
• Run git revert HEAD. This will create a new commit that restores the situation before your change.
• Run npm run test again. Now, all the tests pass (although we know there is a bug, which is Double chaining leads to a stack overflow #2853 and which you are trying to fix).
• Add a new test which exposes the bug. It should do something like var testValue = _.chain(1).chain() and then assert that testValue.value() is equal to 1. Since the bug is not solved yet, we expect this test to fail initially. Commit this change.
• Run npm run test again and verify that your new test fails, while all other tests pass.
• Open test/index.html in your browser. This will run the tests in the browser and should give you the same result.
• Use the developer tools of your browser to set a breakpoint at the start of your new test. If you have not done this before, here is a tutorial for Firefox and here is one for Chrome. Step through the code to see what is causing the bug. Do this repeatedly until you fully understand what is going on.
• Post a comment here to explain what you observed and what you think is causing the bug.

After the above steps, we'll discuss your findings and possible solutions.