How can I use expect(str).toBeInstanceOf(String) in a Jest assertion for a string that has been created using Buffer#toString()?

Or is the correct thing to do here expect(typeof str).toEqual('string') instead?

Details:

This test case, using typeof, passes:

it('should test a Buffer.toString() - typeof', () => {
  const buf = new Buffer('hello world');
  const str = buf.toString('hex');
  expect(buf).toBeInstanceOf(Buffer);
  expect(typeof str).toEqual('string');
  // expect(str).toBeInstanceOf(String);
});

However, this test case, using .toBeInstanceOf(), fails:

it('should test a Buffer.toString()', () => {
  const buf = new Buffer('hello world');
  const str = buf.toString('hex');
  expect(buf).toBeInstanceOf(Buffer);
  // expect(typeof str).toEqual('string');
  expect(str).toBeInstanceOf(String);
});

Here's the Jest output for it:

 FAIL  ./buffer.jest.js
  ● should test a Buffer.toString()

    expect(value).toBeInstanceOf(constructor)

    Expected value to be an instance of:
      "String"
    Received:
      "68656c6c6f20776f726c64"
    Constructor:
      "String"

      at Object.<anonymous>.it (password.jest.js:11:15)
      at Promise.resolve.then.el (node_modules/p-map/index.js:42:16)
      at process._tickCallback (internal/process/next_tick.js:109:7)

How can I use expect(str).toBeInstanceOf(String) in a Jest assertion for a string that has been created using Buffer#toString()?

Or is the correct thing to do here expect(typeof str).toEqual('string') instead?

Details:

This test case, using typeof, passes:

it('should test a Buffer.toString() - typeof', () => {
  const buf = new Buffer('hello world');
  const str = buf.toString('hex');
  expect(buf).toBeInstanceOf(Buffer);
  expect(typeof str).toEqual('string');
  // expect(str).toBeInstanceOf(String);
});

However, this test case, using .toBeInstanceOf(), fails:

it('should test a Buffer.toString()', () => {
  const buf = new Buffer('hello world');
  const str = buf.toString('hex');
  expect(buf).toBeInstanceOf(Buffer);
  // expect(typeof str).toEqual('string');
  expect(str).toBeInstanceOf(String);
});

Here's the Jest output for it:

 FAIL  ./buffer.jest.js
  ● should test a Buffer.toString()

    expect(value).toBeInstanceOf(constructor)

    Expected value to be an instance of:
      "String"
    Received:
      "68656c6c6f20776f726c64"
    Constructor:
      "String"

      at Object.<anonymous>.it (password.jest.js:11:15)
      at Promise.resolve.then.el (node_modules/p-map/index.js:42:16)
      at process._tickCallback (internal/process/next_tick.js:109:7)

• javascript
• node.js
• testing
• jestjs

1 Answer 1

If you have a look at the toBeInstanceOf implementation, you'll see that instanceof is used for checking, but as you can see it demonstrated in Mozilla docs, string primitive is not the same thing as String derived from Object.

Your first variant is the correct way to check:

expect(typeof str).toEqual('string');