Detect unreachable code in more scenarios

Search Terms

logical assignment binary expression dead never truthy falsy

Suggestion

Raise compiler errors when the left side of an expression using a logical operator (&& or ||) is unrechable.

Also detect unreachable branches of ternary expressions.

Use Cases

Code that is statically verifiable to be unreahable is almost always a programmer error so TypeScript should detect unreachable code in more scenarios.

Examples

declare const truthy: true
declare const falsy: false;
declare function doSomething(): number

const a = true ? 5 : doSomething(); // error: doSomething is unreachable
const b = truthy ? 5 : doSomething(); // error: doSomething is unreachable

const c = false ? doSomething() : 5; // error: doSomething is unreachable
const d = falsy ? doSomething() : 5; // error: doSomething is unreachable

if (!truthy) {
    doSomething(); // error: doSomething is unreachable
}

if (falsy) {
    doSomething(); // error: doSomething is unreachable
}

Playground link.

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It would also be nice to expand unreachable code checks to other expressions that are known to be truthy or falsy eg. object/array literals, NaN eg.

declare function doSomething(): number

const a = [1,2,3] || doSomething() // error: doSomething is unreachable
const b = { count: 1 } && doSomething() // error: doSomething is unreachable

if ([1,2,3]) {
    doSomething() // error: doSomething is unreachable
}

if ({ count: 1 }) {
    doSomething() // error: doSomething is unreachable
}

Playground link.

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Finally, detecting unreachable branches of a ternary expression

declare const truthy: true
declare const falsy: false;
declare function doSomething(): number

const a = true ? 5 : doSomething(); // error: doSomething is unreachable
const b = truthy ? 5 : doSomething(); // error: doSomething is unreachable

const c = false ? doSomething() : 5; // error: doSomething is unreachable
const d = falsy ? doSomething() : 5; // error: doSomething is unreachable

Playground link

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.

This wouldn't be a breaking change in existing TypeScript/JavaScript code

There would be new type errors. The extra safety could be opted into by extending the allowUnreachableCode flag or creating a new flag.

Commencez par reproduire les exemples d’opérateurs logiques, d’opérateur ternaire et de littéraux dans les liens TypeScript Playground fournis, puis suivez les vérifications existantes du compilateur concernant le code inatteignable ainsi que l’option allowUnreachableCode. Le travail est terminé lorsque les cas truthy et falsy convenus produisent des erreurs du compilateur sans modifier le JavaScript généré.