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.