Upgrade to TypeScript 4.0

[sosukesuzuki]

Closes #8646

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/pr-XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[sosukesuzuki]

Oh, we should remove this warning.

[fisker]

Let's keep these two in typescript-babel-only

[fisker]

Oh, we should remove this warning.

I think we should wait until typescript-eslint officially support v4

[fisker]

There is an option warnOnUnsupportedTypeScriptVersion for this warning? https://github.com/typescript-eslint/typescript-eslint/blob/6f397df42cbcf05c10f304c9bbfdae4803aa0ce2/packages/parser/src/parser.ts#L72

Sorry, should be this one https://github.com/typescript-eslint/typescript-eslint/blob/6f397df42cbcf05c10f304c9bbfdae4803aa0ce2/packages/typescript-estree/src/parser-options.ts#L19

[sosukesuzuki]

Set loggerFn to false then it works fine.

[sosukesuzuki]

According to microsoft/TypeScript#38510, 4.0 will be released on 8/20, so let's wait.

[petejkim]

Current version of prettier can't parse the new "Short-Circuiting Assignment Operators" syntax.

e.g.

foo ??= "default value"

[fisker]

Current version of prettier can't parse the new "Short-Circuiting Assignment Operators" syntax.

e.g.

foo ??= "default value"

@sosukesuzuki Can you check it's tested on typescript parser?

[brodybits]

There will be lint:typecheck errors if you merge with master, due to recently merged PR #8759 (which I tried in my workarea):

• duplicate identifiers from these lines:

  prettier/src/language-js/types/estree.d.ts

  Lines 6 to 12 in cdc250c

  	// WORKAROUND HACK for typescript-eslint issue:
  	// https://github.com/typescript-eslint/typescript-eslint/issues/2388
  	// MUST REMOVE to avoid errors with TypeScript 4.0.0:
  	import * as ts from "typescript";
  	declare module "typescript" {
  	type NamedTupleMember = Node;
  	}

• type issues in src/language-js/utils.js

I really hope we don't have to revert anything major in master.

P.S. I did try typecheck with @typescript-eslint/types@4.0.0-alpha.12 which didn't solve any type issues.

[fisker]

I thought you applied this suggestion #8759 (comment), how to fix it?

[brodybits]

You can just remove the workaround hack lines when you merge from master:

prettier/src/language-js/types/estree.d.ts

Lines 6 to 12 in cdc250c

	// WORKAROUND HACK for typescript-eslint issue:
	// https://github.com/typescript-eslint/typescript-eslint/issues/2388
	// MUST REMOVE to avoid errors with TypeScript 4.0.0:
	import * as ts from "typescript";
	declare module "typescript" {
	type NamedTupleMember = Node;
	}

You will also have to solve the type issues between src/language-js/utils.js and TypeScript 4. I think you will want do this before merging this PR into master. What a mess:)

[sosukesuzuki]

You will also have to solve the type issues between src/language-js/utils.js and TypeScript 4. I think you will want do this before merging this PR into master. What a mess:)

@brodybits How to fix?

[brodybits]

How to fix?

I proposed a quick workaround here: sosukesuzuki#2

This is using any parameter type in 3 places. I recall we discussed elsewhere (@thorn0?) that we should eventually search for any type specifiers and replace them with more specific types at some point in the future. But I would definitely not favor blocking TypeScript 4.0 update with this.

[brodybits]

foo ??= "default value"

seems to be tested with Babel & TypeScript in tests/js/logical-assignment

[sosukesuzuki]

Thank you. I agree with you. For now, let's merge it and consider a fix after the 2.1 release.

[fisker]

Let's merge!

[sosukesuzuki]

Should we add changelog for Short-Circuiting Assignment Operators?