TextField: For uncontrolled scenarios, update value when defaultValue changes

[JasonGore]

Pull request checklist

• Addresses an existing issue: Fixes Textfield: Updating defaultValue doesnot update the value in input. #1564
• Include a change request file using $ npm run change

Description of changes

For uncontrolled scenarios (when value prop is undefined), update value when defaultValue prop changes.

Focus areas to test

Add tests to make sure value is updated (or not) depending on controlled and uncontrolled scenarios.

Microsoft Reviewers: Open in CodeFlow

[Markionium]

Few comments on the tests. I'll approve anyhow, so you can decide what you want to do with them ;)

[Markionium]

I know pulling out values that are used multiple times as a dev seems to do the right thing to do. Personally when it comes to test code i find it easier to read when just repeating the values.

    textField.setProps({ value: 'John', defaultValue: 'Your name' });

    expect(textField.getDOMNode().querySelector('input')).toBeTruthy();
    expect(textField.getDOMNode().querySelector('input')!.value).toEqual('John');

Additionally it improves readability a bit more even when you put a new line between the stuff you do and the things you assert.
In the case above the setProps and expect.

[JasonGore]

Related to my other comment, I feel this is basically a test to say input is non-null && input is John before accessing input, not two separate tests.

[Markionium]

I think we can probably remove these .toBeTruthy() asserts. I'm not sure how they add value? We're basically testing querySelector in JSDom, which should not be our responsibility :) The test would fail on the next line anyhow if it does not exist.

[JasonGore]

This was the convention throughout this file and I copied a test, but I'm all for standardizing on test methodology. However, it's not testing for the existence of querySelector, it's testing for the existence of the input field. Since the next line is assuming input is non-null, this seems it would generate clearer failure output if things are not as expected.