Wednesday 7 July 2021

TIL: two things about reduce operations


When working through yesterday's article ("TDD, code puzzle, recursing reductions"), I learned two new things about how reduce operations work.

The first thing was that I was having problems with defaulting the initial value of my callback. I had something like this:

array = ["tahi", "rua", "toru", "wha"]

list = array.reduce((result="", value) => {
    return result.listAppend(value.ucase())

When I was running this on Lucee I was getting an error: can't call method [listAppend] on object, object is null. The callback was not receiving its default value.

After a while I clocked that I had Lucee's "Complete Null Support" switched on, and this was ruining things for everyone. Why?

Because the method signature for Lucee's Array.reduce has two forms:


Array.reduce(callback, initialValue)

I was using the first form, but Lucee still needs to cater for the second form. What's happening is that when Lucee encounters the first form, it's saying "OK so no initialValue. But… null support is switched on, so I'll just pass null for that". So when the callback is called, the initial value is null, so the default in the function signature for the callback is "not needed". Harrumph. I don't like this much, but I understand why it is the way it is I s'pose.

The easy fix is to just use the second form all the time.

Brad Wood pointed out another pitfall when using the callback's default value for that first argument. Consider this variant of the code from above:

array = []

list = array.reduce((result="", value) => {
    return result.listAppend(value.ucase())

There's no elements in the array, so the callback is never called, so the initial value for the result is never set. So on Lucee with null support set, the result is null. On ColdFusion, and on Lucee with null support switched off, it errors along the lines of variable [list] doesn't exist. It's worthwhile remembering that the default value on the callback parameter is only for the callback, and it simply "coincidentally" works as a default value for the entire operation if the callback is used. To give the operation a default starting value, you need to set it on the reduce call, not the callback signature.

Cheers for pointing this out to me, Brad. It seems obvious once you mentioned it, but it had never occurred to me.

That's it. Two things I learned. Cool.