I need the TypeScript compiler to throw an 'Object is possibly 'undefined'' error if attempting to directly access any element of an array without first checking if the array is empty. This way, I would always be required to check for undefined before accessing any element, potentially using optional chaining.
If it has been confirmed that the array is not empty, then I should be able to access its elements without needing to verify for undefined values.
This requirement ensures that I can be certain the array is not empty, as trying to access any element in an empty array will immediately return undefined and prevent further chaining, avoiding errors such as trying to read a property of undefined.
How can I achieve this?
Here is a code example to clarify my question:
interface Element {
a: {
aa: string;
bb: string;
};
b: {
aa: string;
bb: string;
};
}
const element: Element = {
a: { aa: "aa", bb: "bb" },
b: { aa: "aa", bb: "bb" },
};
type ElementArray = Element[];
const array: ElementArray = [element, element];
const emptyArray: ElementArray = [];
const getFirstAWithoutLengthCheck = (array: ElementArray) => {
return array[0].a; // I want the TypeScript compiler to throw an 'Object is possibly 'undefined'' error here
};
const getFirstAWithLengthCheck = (array: ElementArray) => {
if (array.length) {
return array[0].a; // No errors should be present here
}
return null;
};
const getFirstAOptChaining = (array: ElementArray) => {
return array[0]?.a; // No errors should occur with optional chaining
};
// The following line will result in error "cannot read property a of undefined", so we must use optional chaining or length check in this function, even though TypeScript does not require it
console.log(getFirstAWithoutLengthCheck(array)); // aa
console.log(getFirstAWithoutLengthCheck(emptyArray)); // crash!
// By checking the array length, access to the first element should work normally without errors
console.log(getFirstAWithLengthCheck(array)); // aa
console.log(getFirstAWithLengthCheck(emptyArray)); // null
// Using optional chaining, no errors should occur
console.log(getFirstAOptChaining(array)); // aa
console.log(getFirstAOptChaining(emptyArray)); // undefined