When you add a new optional parameter to a function, there is no guarantee that you won't break code that calls the function. Here is one notorious example:

When you add a new optional parameter to a function, there is no guarantee that you won't break code that calls the function. Here is one notorious example: [ '1', '1', '1', '1' ].map( s => parseInt(s) ) This returns the expected result: [ 1, 1, 1, 1 ] Then you think, "I don't need that little wrapper, I can just pass parseInt as the callback directly": [ '1', '1', '1', '1' ].map( parseInt ) And this returns something that you may not expect: [ 1, NaN, 1, 1 ] That happens because parseInt() takes an optional second parameter with the number base, and map() passes a second parameter with the array index. Oops! A similar situation can arise when you add a new optional parameter. You still have to check all the call sites, and for a public API you won't be able to do that.