“index” vs. function parameter: what’s the difference?

[mathiasbynens]

Question about https://github.com/TechQuery/array-unique-proposal#core-features:

While Array.prototype.unique() invoked with:

1. no parameter, it'll work as [...new Set(array)];

2. one index-key parameter (Number, String or Symbol), it'll get values from each array element with the key, and then deduplicates the origin array based on these values;

3. one function parameter, it'll call this function for each array element, and then deduplicates the origin array based on these returned values.

How is 2 different from 3?

[Jack-Works]

2 is an abbreviated version of 3.

"key" equals to (x) => x.key

Same for number and symbols.

[ljharb]

I think I'd be pretty confused if I learned that shortcut pattern on unique and found it wasn't available on all the other predicate-taking array methods.

[Jack-Works]

I think I'd be pretty confused if I learned that shortcut pattern on unique and found it wasn't available on all the other predicate-taking array methods.

maybe we can add that

[ljharb]

That seems like it'd be better as a separate cross-cutting proposal, if so

[jridgewell]

Agreed with @ljharb here. I think we should drop 2 for now, and possibly add it as its own proposal with all of the other array methods (particularly map).

[hax]

I think unique key is very common use case and I hope it could be included in this proposal.

[ljharb]

@hax I agree it's common; but it's already common for every array method that takes a function, not just this new one.

[hax]

@ljharb I think it's possible to add key param for other array methods, but consider unique() potentially could support multiple keys ( #7 ) which unlikely fit for other methods, I suppose the key usage in unique is different than other array methods.

[ljharb]

Anything with a predicate seems like it’d apply the same to single or multiple keys.

[hax]

Maybe, but I feel arr.filter(["a", "b"]), arr.find("a"), arr.some(['a', 'b']) are confusing.

[ljharb]

I do too, but I’d think the same about unique with an array.

[Jack-Works]

in the last meeting, the committee requires to remove the key: string | symbol signature. it should be a separate proposal to add this kind of feature to all Array methods. @TechQuery

[TechQuery]

@ljharb @jridgewell
Only map() may be possible to add a String-parameter signature, for example objects.map('name'), adding a string parameter to other Array methods with a callback parameter may confuse developers:

• filter(): keep the elements which has an Equal string value or Truthy key-value?
• findIndex(): find an element index which has an Equal string value or Truthy key-value?
• some(): detect an element which has an Equal string value or Truthy key-value?
• every(): detect every element has an Equal string value or Truthy key-value?
• sort(): sort elements by a string key, but Ascending or Descending?

[ljharb]

given the similarity between .map(() => 'name') and .map('name') i'd say the same potential confusion applies anywhere - or, if not, doesn't apply anywhere.

[hax]

I suggest we remove index parameter, and make non-function param throw TypeError so we could reconsider it in the future.