[Flow Analysis] throw statements in nested function calls

[buschtoens]

Search Terms

flow analysis throw error exception undefined non-null assertion optional nested function call

Suggestion

Teach flow analysis to understand throw statements in nested function calls.

Use Cases

Many frameworks, even Node's standard library, come with a range of assertion helpers. In its most basic form, they take a condition, and an error message that will be thrown, when the condition evaluates to a false / falsy value.

TypeScript currently does not understand throw statements in nested function calls, which makes using assert helpers annoying, since you have either have to repeat the same conditional throw statement right after the assertion, use a non-null assertion, or a type cast, to tell TypeScript, that this value is present / fulfills some further criteria.

Examples

function assert(message: string, condition: boolean) {
    if (!condition) throw new Error(message);
}

function foo<T>(someArg?: T): T {
    assert('Argument missing.', Boolean(someArg));

    // Toggle this line to fix:
    // if (!someArg) throw new Error('Argument missing.');
    
    return someArg;
}

TypeScript REPL

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Possible Duplicates / Related

• Possibly undefined but it's not #19415: Possibly undefined but it's not
• feature request: always-throw functions detect in null check #16546: always-throw functions detect in null check
• Trade-offs in Control Flow Analysis #9998: Trade-offs in Control Flow Analysis
• feature request: always-throw functions detect in null check #16546 (comment):

  But it's not currently possible to signal that a function invocation completely changes the control flow graph, and I don't know if we ever will be able to.

[MartinJohns]

Duplicate of #32130, #10421, #31512, #31879, #17760.

[yortus]

Sounds like #8655

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.