ES2019 Object.fromEntries uses PropertyKeys as mapped key type

[AWare]

TypeScript Version: 3.5.0-dev.20190514

Search Terms:
fromEntries
Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
type T = {}

const a: { [key: string]: T } = { "key": {} }

const b: [string, T][] = Object.entries(a)

const c : {[key: number]: T; [key: string]: T } = Object.fromEntries(b)

const d : { [key: string]: T } = Object.fromEntries(b)

Expected behavior:

Compilation should fail because number is not a key of a.

edit: The return type of Object.fromEntries(b) should be inferred as d: { [key: string]: T } .

Actual behavior:

edit: The return type of Object.fromEntries(b) is {[key: number]: T; [key: string]: T } Adding an erroneous [key: number].

Object.fromEntries has type:

 fromEntries<T = any>(entries: Iterable<readonly [PropertyKey, T]>): { [k in PropertyKey]: T };

However if I make a second type parameter for fromEntries which extends PropertyKey and use that for the key type in forEntries:

 fromEntries<K extends PropertyKey, T = any>(entries: Iterable<readonly [K, T]>): { [k in K]: T };

I'm willing to make a PR for this if this sounds like a reasonable approach.

Playground Link:

Apologies, but I could not use es2019 in the playground.

Related Issues:

#30934

#30933 #25999

[RyanCavanaugh]

Which line are you expecting to fail? c? Why?

[AWare]

Sorry @RyanCavanaugh I didn't explain my test case correctly.

Even with the desired behaviour I think c would compile. I've edited my case to show what I expect to happen more clearly. (Namely the return type of Object.fromEntries taking the key type information from the tuple array instead of PropertyKey)

[AWare]

@RyanCavanaugh Would you mind if I attempted this and raised a PR please?

[donkeytronkilla]

I believe this is also causing an error in our project. I have distilled the error down to the following simple repo which I think is the same issue @AWare is describing here.

package.json:

{
  "name": "tserror",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "build-good": "rimraf index.js && tsc --build tsconfig.good.json",
    "build-bad": "rimraf index.js && tsc --build tsconfig.bad.json"
  },
  "devDependencies": {
    "typescript": "^3.5.2",
    "rimraf": "^2.6.3"
  }
}

tsconfig.good.json:

{
    "compilerOptions": {
        "lib": ["esnext"],
        "strict": true,
        "keyofStringsOnly": false,
    },
    "include": ["index.ts"],
    "exclude": ["node_modules"]
}

tsconfig.bad.json:

{
    "compilerOptions": {
        "lib": ["esnext"],
        "strict": true,
        "keyofStringsOnly": true,
    },
    "include": ["index.ts"],
    "exclude": ["node_modules"]
}

index.ts:

const moo: string = "oh oh" 

When I run npm run build-good this simple example builds as expected. However, if I then go and run npm run build-bad I get the following error:

The only difference in the "bad" build is that i've set "keyofStringsOnly": true instead of false. This is a stripped down version of my actual configuration but I believe it is just the interaction of "keyofStringsOnly":true and lib:["esnext"] that is relevant to this issue.

[ipanasenko]

Having same issue on my side, same configuration ("keyofStringsOnly": true)

[matt-usurp]

Got the same issue randomly, not sure what triggered it as things were compiling fine. But using a target of either esnext or es2019 will cause this compilation issue. Also confirmed this by updating to typescript@next and running the same code.

Replication details are as follows, I did this in my /tmp directory and you can pretty much copy and paste these steps to reproduce.

tsconfig.json

{
  "compilerOptions": {
    "target": "es2019",
    "keyofStringsOnly": true,
  },
}

Running tsc will complain that it has nothing to do, so touch index.ts.
Then running the tsc command will result in the following error:

*chop*/typescript/lib/lib.es2019.object.d.ts:28:81 - error TS2322: Type 'string | number | symbol' is not assignable to type 'string'.
  Type 'number' is not assignable to type 'string'.

28     fromEntries<T = any>(entries: Iterable<readonly [PropertyKey, T]>): { [k in PropertyKey]: T };
                                                                                   ~~~~~~~~~~~

Found 1 error.

This is specifically part of es2019 (currently esnext aliases this I believe).
The fix, for now, is to "target": "es2018" instead in your tsconfig.json.

{
  "compilerOptions": {
    "target": "es2018",
    "keyofStringsOnly": true,
  },
}

[jpandl19]

I changed target to "target": "es2018", but I am still getting the following error:

node_modules/typescript/lib/lib.es2019.object.d.ts:28:81 - error TS2322: Type 'string | number | symbol' is not assignable to type 'string'.
  Type 'number' is not assignable to type 'string'.

28     fromEntries<T = any>(entries: Iterable<readonly [PropertyKey, T]>): { [k in PropertyKey]: T };

Here is my tsconfig.json

{
  "compilerOptions": {
    "target": "es2018",
    "moduleResolution": "node",
    "module": "commonjs",
    "sourceMap": true,
    "rootDir": "src",
    "outDir": "dist",
    "keyofStringsOnly": true,
    "lib": ["esnext", "dom"],
    "esModuleInterop": true,
  },
  "exclude": ["node_modules", "**/*.spec.ts"]
}

The only thing that works is removing "keyofStringsOnly": true,.

I am not sure why this is still happening. Has anyone else experienced this/ any suggestions on a fix?