'esnext.array' doesn't enable access to Array.prototype.flat()

[AbakumovAlexandr]

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms: esnext, esnext.array, Array.prototype.flat()

Code

// greeter.ts

function greeter(persons: string[]) {
    return "Hello, " + persons.flat().join();
}

var user: string[] = ["Jane User"];

document.body.innerHTML = greeter(user);


// tsconfig.json
{
  "files": [
    "greeter.ts"
  ],
  "compilerOptions": {
    "target": "esnext",
    "lib": [
      "es2017",
      "esnext.array",
      "dom"
    ]
  }
}

Expected behavior:
Should compile successfully. According to MDN, it's an experimental API supported by multiple browsers already.

Actual behavior:
Compilation error:

greeter.ts:2:32 - error TS2339: Property 'flat' does not exist on type 'string[]'.

2 return "Hello, " + persons.flat().join(); ~~~~

Playground Link

[kitsonk]

.flat is specified in lib.esnext.array.d.ts:

https://github.com/Microsoft/TypeScript/blob/ef7997cb73ce46081cbfebc0edd2a0295ba30c33/lib/lib.esnext.array.d.ts#L152-L222

It feels like something else is going on. It won't work in the playground because it doesn't support changing things like the libs and target.

[kitsonk]

Your tsconfig.json doesn't contain any files or include properties. How are you compiling greeter.ts? At the moment, the config file would not apply to the greeter.ts in something like Visual Studio code.

[AbakumovAlexandr]

@kitsonk , yes, I've made sure flat() is there in lib.esnext.array.d.ts before creating this.

I compiled it with tsc greeter.ts hoping that tsc would pick up tsconfig.json. I've just tried again with files section added (see updated initial post) and the error is the same.

[kitsonk]

@AbakumovAlexandr try tsc -p . instead...

[AbakumovAlexandr]

@kitsonk just tried tsc -p tsconfig.json with the same error.

Does esnext.array + flat() work for you when you're reproducing this project?

[kitsonk]

$ tsc --version
Version 3.0.1
$ tsc -p .

No errors output, greeter.js that is emitted:

function greeter(persons) {
    return "Hello, " + persons.flat().join();
}
var user = ["Jane User"];
document.body.innerHTML = greeter(user);

I even with typescript@next:

$ tsc --version
Version 3.1.0-dev.20180810
$ tsc -p .

Again, no errors... 🤷‍♂️

[RyanCavanaugh]

@AbakumovAlexandr what's the output of tsc -v ?

[AbakumovAlexandr]

@kitsonk I think I've figured out the source of the issue thanks to the feedback from your side!

I've switched local TypeScript versions only, while my global one stayed 2.9.2. So it's always compliled with TS 2.9.2. But in 2.9.2, flat() was called flatten(). That's why it couldn't find it.

Unfortunately, tsc doesn't print its version to the console leaving a room for confusion. If you, guys, feel like you'd like to implement it, let me know and I'll create a different issue for that.

[AbakumovAlexandr]

@RyanCavanaugh see my above comment, please.

[kitsonk]

It does print to the console with --version and most compilers are implicitly silent.

[favna]

@AbakumovAlexandr I thought I was facing the same issue just now (which is why I even found this open issue to begin with). I have TypeScript v3.2.2 and my lib property in tsconfig was set to "lib": ["es2018", "dom", "esnext.array"], being that I just added the esnext.array value. When I moved esnext.array to the front however ("lib": ["esnext.array", "es2018", "dom"],) the error went away and the code compiled normally.

Just reporting my findings as this may be the resolution for your problem as well, in which case this issue could maybe be closed, or if the order shouldn't matter (not too sure, never faced it before) then changed.

[AbakumovAlexandr]

@favna Thank you for reporting this! This is really interesting. Since I don't think the order of libs should matter, this also could be a clue for TS team to figure the issue out.