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import { renderHook } from '@testing-library/react-hooks';
import usePreviousDistinct, { Predicate } from '../src/usePreviousDistinct';
describe('usePreviousDistinct', () => {
it('should be defined', () => {
expect(usePreviousDistinct).toBeDefined();
});
function getHook<T>(initialValue?: T, predicate?: Predicate<T>) {
return renderHook(({ val, cmp }) => usePreviousDistinct<T>(val as T, cmp), {
initialProps: {
val: initialValue || 0,
cmp: predicate,
} as { val?: T; cmp?: Predicate<T> },
});
}
it('should return undefined on init', () => {
expect(getHook().result.current).toBeUndefined();
});
it('should not invoke predicate on first render', () => {
const spy = jest.fn();
getHook(0, spy);
expect(spy).not.toHaveBeenCalled();
});
it('should update previous value only after render with different value', () => {
const hook = getHook();
expect(hook.result.current).toBeUndefined();
hook.rerender({ val: 0 });
expect(hook.result.current).toBeUndefined();
hook.rerender({ val: 1 });
expect(hook.result.current).toBe(0);
hook.rerender({ val: 2 });
expect(hook.result.current).toBe(1);
hook.rerender({ val: 2 });
expect(hook.result.current).toBe(1);
hook.rerender({ val: 3 });
expect(hook.result.current).toBe(2);
});
it('should work fine with `undefined` values', () => {
const hook = renderHook(({ value }) => usePreviousDistinct(value), {
initialProps: { value: undefined as undefined | number },
});
expect(hook.result.current).toBeUndefined();
hook.rerender({ value: 1 });
expect(hook.result.current).toBeUndefined();
hook.rerender({ value: undefined });
expect(hook.result.current).toBe(1);
hook.rerender({ value: 2 });
expect(hook.result.current).toBeUndefined();
});
it('should receive a predicate as a second parameter that will compare prev and current', () => {
const obj1 = { label: 'John', value: 'john' };
const obj2 = { label: 'Jonny', value: 'john' };
const obj3 = { label: 'Kate', value: 'kate' };
const predicate = (a, b) => !!a && a.value === b.value;
const hook = getHook(obj1 as { label: string; value: string }, predicate);
expect(hook.result.current).toBeUndefined();
hook.rerender({ val: obj2, cmp: predicate });
expect(hook.result.current).toBeUndefined();
hook.rerender({ val: obj3, cmp: predicate });
expect(hook.result.current).toBe(obj1);
});
});
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