1. Does NumberFormatOptions & DateTimeFormatOptions cover everything? I see that BigInt also has toLocaleString. Could other types have it too?
2. since this adds an overload, locales should not be optional. Otherwise a zero-argument call might resolve to this when it should resolve to the es5 overload.

Thanks for the comments!

1. When I checked, it looks like there are only NumberFormatOptions and DateTimeFormatOptions which are accepted by JS in all types with this method (Object, Array, Number, BigInt, TypedArray). BigInt arrays are covered by the current definitions upon testing, but I will still need to update the toLocaleString signature of TypedArrays. I'll send a commit for that

2. I tried not making it optional, but it seems to stop working with single argument calls based on my test run

arrayToLocaleStringES2015.ts(4,11): error TS2575: No overload expects 1 arguments, but overloads do exist that expect either 0 or 2 arguments.
arrayToLocaleStringES2015.ts(10,14): error TS2575: No overload expects 1 arguments, but overloads do exist that expect either 0 or 2 arguments.

I think they should be kept optional, what do you think? In my baselines errors, it was showing the expected behaviour for es5

1. 👍🏼
2. The error makes it look like both locales and options are both required. But only locales should be required; options should stay optional. When I did a quick local test with a test project using es2015, all 3 calls below worked:

const a = [1,2,3]
a.toLocaleString()
a.toLocaleString('en-US')
a.toLocaleString('en-US', {style: 'currency', currency: 'USD'}) // thanks copilot

Hello! I applied the changes to es2015

1. 👍🏼
2. The error makes it look like both locales and options are both required. But only locales should be required; options should stay optional. When I did a quick local test with a test project using es2015, all 3 calls below worked:

const a = [1,2,3]
a.toLocaleString()
a.toLocaleString('en-US')
a.toLocaleString('en-US', {style: 'currency', currency: 'USD'}) // thanks copilot