How does this code pass an argument to a callback that takes no arguments?

Question

I'm sifting through some code and I don't get how this piece of code is able to pass arguments to a callback that takes no arguments.

We start with this object:

const oracleEndpoints = {
  pancake: () => fetchPancake(),
  lps: () => fetchLP(endpoints.lps), <-- endpoints.lps is an API url
};

And this map:

const oracleToIds = new Map()
oracleToIds.set("lps", ["png-snob-avax", "png-png-avax"])
oracleToIds.set("pancake", ["WBNB", "BREW"])

Now this is the part I don't get:

const promises = oracleToIds.keys().map(key => oracleEndpoints[key](oracleToIds.get(key)));

oracleEndpoints[key] is an anonymous callback (() => fetchPancake() or () => fetchLP(endpoints.lps)) and oracleToIds.get(key) is an array of strings.

What's going on in this expression oracleEndpoints[key](oracleToIds.get(key)) then where an unparametised callback is passed an array?

How is it able to do this?

Could you give more detail as to what the code does, and how you know that the argument - `oracleToIds.get(key)` - is being used? From glancing at the code I agree with you, this should do nothing. — Robin Zigmond, Mar 21 '21 at 17:08

score 0 · Answer 1 · answered Mar 21 '21 at 17:08

I'd double check if the array argument is actually being used by anything meaningful. It could easily be a mistake in the code caused by confusion of the writer, given the high level of nesting mashed into a single line of the const promises code that you showed.

This would silently cause an error that would only manifest in application usage -- no exception is thrown in JavaScript if you are passing extra params to a function.

How does this code pass an argument to a callback that takes no arguments?

1 Answers1