with-fallback.ts
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with-fallback.ts
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	// use this to more concisely define values with one or more fallbacks to use when they are null.

	// If called with two args, the second one has to be non-null
	// if called with three args, the third one has to be non-null
	//
	// in two-arg mode, if the first argument is not null, then it's returned and fallback is not used.
	// if the first argument is null, then the second arg is returned
	//
	// in three-arg mode, the first value is never returned. If the first arg is not-null, then the second arg is returned (if it's not null). Otherwise the third arg is returned

	export function withFallback<T1, T2 extends NonNullable<unknown>>(
	value1: T1,
	value2: T2
	): NonNullable<T1> \| T2;
	export function withFallback<T1, T2, T3 extends NonNullable<unknown>>(
	value1: T1,
	value2: T2,
	value3?: T3
	): NonNullable<T2> \| T3;
	export function withFallback<T1, T2, T3 extends NonNullable<unknown>>(
	value1: T1,
	value2: T2,
	value3?: T3
	): NonNullable<T1> \| NonNullable<T2> \| T3 {
	if (value3 !== undefined) {
	// three arg mode
	if (
	value1 !== null &&
	value1 !== undefined &&
	value2 !== null &&
	value2 !== undefined
	) {
	return value2;
	} else {
	return value3;
	}
	} else {
	// two arg mode
	if (value1 !== null && value1 !== undefined) {
	return value1;
	} else if (value2 !== undefined && value2 !== null) {
	return value2;
	} else {
	throw new Error("No suitable fallback: ${arguments}");
	}
	}
	}