Proposal: Do expression and async do expression

[Jack-Works]

This is an experimental PR to support ECMAScript proposal do expression.

Proposal: Do expression and Async do expression

Spec: Do expression and Async do expression

PR preview Playground

Known problems / TODOs:

• Completion type analysis should be based on control flow analysis.
• There're many false positives on the Not all code paths return a value check.
• break or continue inside the for head doesn't lead to a type error (there is no spec yet, only a memo so I don't know how to implement this.)
• The following code will report type error:

function f(param: number) {
    const double = do {
        if (param === 0) return null;
        param * param;
    }
    return double
}

Type evaluation rule of the do expression:

I made a change to the checker of typescript. Here are the new evaluation rules:

• Every statement might return void or Type as a type checking result. If it returns void, it is treated as not contribute to the type of the statement.

New type evaluation rules

New Expressions

• DoExpression: type of its containing block statement. if there is none, fallback to voidType.

Statements

Statement:

• BlockStatement: type is last type of it's inner statement.
• VariableStatement: SyntaxError
• EmptyStatement: Does not contribute. See https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation
• ExpressionStatement: type of it's containing expression
• IfStatement: union type of all branches. if there is no else block, it will treated as voidType
• BreakableStatement:
• • IterationStatement: SyntaxError
• • SwitchStatement: union type of all clauses
• ContinueStatement: never
• BreakStatement: never
• ReturnStatement: never
• WithStatement: any
• LabelledStatement: type of it's inner statement
• ThrowStatement: never
• TryStatement: union type of try clause and catch clause. finally clause is ignored.
• DebuggerStatement: Does not contribute. See https://tc39.es/ecma262/#sec-block-runtime-semantics-evaluation

Extra checks

A var declaration cannot be used within a do expression unless the target is ESNext

function a() {
    (do {
        var x = 1;
        x;
    });
    x; // 1
}

Not all code paths return a value

Requires noImplicitReturns to be true

Inlay hints

Show all "exit points" of the do expression to make it easier to identify.

[Kingwl]

Cool!
Let me (us) know if you want to make a tarball package and a playground.

[Kingwl]

@typescript-bot pack this.

[typescript-bot]

Heya @Kingwl, I've started to run the tarball bundle task on this PR at 4fad104. You can monitor the build here.

[typescript-bot]

Hey @Kingwl, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/94041/artifacts?artifactName=tgz&fileId=A7FBEB57B050AF55EBAE4A4B0E554BE4F005E2844E9C42011E87081414201BDE02&fileName=/typescript-4.2.0-insiders.20210123.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@4.2.0-pr-42437-4".;

[Kingwl]

@typescript-bot pack this

[typescript-bot]

Heya @Kingwl, I've started to run the tarball bundle task on this PR at 4277519. You can monitor the build here.

[typescript-bot]

Hey @Kingwl, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/94082/artifacts?artifactName=tgz&fileId=03AB43DA1756835BDCCA79CD1B8F1C9915A59B169C2BEF71FBB9D0BC482BED2F02&fileName=/typescript-4.2.0-insiders.20210124.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@4.2.0-pr-42437-7".;

[Jack-Works]

@Kingwl hi can you pack this thanks!

[Kingwl]

@typescript-bot pack this.

[Kingwl]

Oops, seems I cannot trigger ts bot anymore.

@orta might can give some help to you.

[orta]

@typescript-bot pack this.

[typescript-bot]

Heya @orta, I've started to run the tarball bundle task on this PR at 860f554. You can monitor the build here.

[typescript-bot]

Hey @orta, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/109194/artifacts?artifactName=tgz&fileId=B9DA44164CBE2153BFD07BB1D81F452609651B215184EE0682AF92B3C5E96F7202&fileName=/typescript-4.5.0-insiders.20210824.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@4.5.0-pr-42437-39".;

[Jack-Works]

Updates:

• Transform: fix return inside parameter of a function
• Transform: fix var declared variable hoisting
• Transform: support break through switch statement
• Transform: fix in class fields
• LSP: support inlay hint for all exit points of the do expression

@orta Hi can you let the bot pack this?

[Jack-Works]

Got some problems with break analysis...

[Jack-Works]

close for now. I'll re-open and rebase this once the proposal gets advanced.