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Support for Object.hasOwn narrowing #71

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Support for Object.hasOwn narrowing #71

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8 changes: 8 additions & 0 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -25289,6 +25289,7 @@ namespace ts {
}

function narrowTypeByCallExpression(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
debugger;
if (hasMatchingArgument(callExpression, reference)) {
const signature = assumeTrue || !isCallChain(callExpression) ? getEffectsSignature(callExpression) : undefined;
const predicate = signature && getTypePredicateOfSignature(signature);
Expand All @@ -25306,6 +25307,13 @@ namespace ts {
}
}
}

if (isPropertyAccessExpression(callExpression.expression)) {
const callAccess = callExpression.expression;
if (isIdentifier(callAccess.name) && callAccess.name.escapedText === "hasOwn" && callExpression.arguments.length === 2 && isStringLiteralLike(callExpression.arguments[1])) {
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A downside to special casing the exact names here is that it would break for cases where the function is assigned to a different name:

const hasProp = Object.hasOwn;

if (hasProp(o, 'test')) ...

I think instead of adding this logic within the compiler itself, we might be able to do it by updating the .d.ts definitions that are packaged with tsc. Something like:

interface ObjectConstructor {
    hasOwn<T, P extends string>(o: T, p: P): o is Extract<T, { [p in P]: unknown }>; 
}

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Agreed, it seems like we have the necessary building blocks to express this already.

return narrowByInKeyword(type, callExpression.arguments[1].text as __String, assumeTrue);
}
}
return type;
}

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6 changes: 6 additions & 0 deletions tests/cases/compiler/objectHasOwnNarrowingSimpleUnion.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
// @target: es2022

declare const abcd: { a: string, b: number } | { c: number, d: string };
if (Object.hasOwn(abcd, "a")) {
abcd.b;
}