react-instantsearch/scripts/jest/matchers/toWarnDev.ts

import { diff } from 'jest-diff';

declare global {
  // eslint-disable-next-line @typescript-eslint/no-namespace
  namespace jest {
    // eslint-disable-next-line @typescript-eslint/no-unused-vars, @typescript-eslint/naming-convention
    interface Matchers<R> {
      toWarnDev(expectedMessage?: string): jest.CustomMatcherResult;
    }
  }
}

export const toWarnDev: jest.CustomMatcher = (
  callback: () => void,
  expectedMessage: string
) => {
  if (expectedMessage !== undefined && typeof expectedMessage !== 'string') {
    throw new Error(
      `toWarnDev() requires a parameter of type string but was given ${typeof expectedMessage}.`
    );
  }

  if (!__DEV__) {
    callback();

    return { pass: true, message: () => '' };
  }

  const originalWarnMethod = console.warn;
  let calledTimes = 0;
  let actualWarning = '';

  console.warn = (message: string) => {
    calledTimes++;
    actualWarning = message;
  };

  callback();

  console.warn = originalWarnMethod;

  // Expectation without any message.
  // We only check that `console.warn` was called.
  if (expectedMessage === undefined && calledTimes === 0) {
    return {
      pass: false,
      message: () => 'No warning recorded.',
    };
  }

  // Expectation with a message.
  if (expectedMessage !== undefined && actualWarning !== expectedMessage) {
    return {
      pass: false,
      message: () => `Unexpected warning recorded.

Difference:

${diff(expectedMessage, actualWarning)}`,
    };
  }

  return { pass: true, message: () => '' };
};