Function def

packages/expect/src/jest-expect.ts:42–62

(
    name: keyof Assertion | (keyof Assertion)[],
    fn: (this: Chai.AssertionStatic & Assertion, ...args: any[]) => any,
  )

  const customTesters = getCustomEqualityTesters()

  function def(
    name: keyof Assertion | (keyof Assertion)[],
    fn: (this: Chai.AssertionStatic & Assertion, ...args: any[]) => any,
  ) {
    const addMethod = (n: keyof Assertion) => {
      const softWrapper = wrapAssertion(utils, n, fn)
      utils.addMethod(chai.Assertion.prototype, n, softWrapper)
      utils.addMethod(
        (globalThis as any)[JEST_MATCHERS_OBJECT].matchers,
        n,
        softWrapper,
      )
    }

    if (Array.isArray(name)) {
      name.forEach(n => addMethod(n))
    }
    else {
      addMethod(name)
    }
  }

  (['throw', 'throws', 'Throw'] as const).forEach((m) => {
    utils.overwriteMethod(chai.Assertion.prototype, m, (_super: any) => {