I was under the impression that this code
let x: (a: { b: number }) => void = (a: { b: number, c: string }) => { alert(a.c) };
x({ b: 123 });
would result in an error because the lambda function requires an additional property on the a argument, making the signatures incompatible. However, testing it in the most recent TypeScript playground revealed no errors! What could be the reason for this unexpected outcome?
When expressing
let x: (a: { b: number }) => voidYou have essentially defined that "x is a function that necessitates an object with a b property."
Later, you assigned it a lambda function that takes an object with both b and c properties. This remains valid as any input object must contain a b key.
Passing an object with only a b key does not trigger any errors.
If the aim is for it to fail because of the absence of the c key, adjust the type of x as shown below:
let x: (a: { b: number, c: string }) => void$ npm install -S <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="8dffe8eceef9a0ffe2f8f9e8ffa0f9ffece3fee4f9e4e2e3cdbca3b9a3bd">[email protected]</a> npm ERROR! code ERESOLVE npm ERROR! Unable to r ...
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