Add unit test for easing function

[MasatoMakino]

Related Issues

• Help add new easing functions Help add new easing functions #507
• tween jumps (jitters) to the final value at the end of animation tween jumps (jitters) to the final value at the end of animation #116

Purpose of this Pull Request

This PR is for adding unit tests to TWEEN.Easing functions.

Details of the changes

'Test TWEEN.Easing should starts at 0.0, ends at 1.0. TWEEN.Easing.InOut() should be 0.5 at midpoint'

Based on #507, this test checks that all easing functions satisfy conditions f(0) = 0.0, f(1.0) = 1.0. This test also checks that Easing.InOut(0.5) = 0.5.

'Test TWEEN.Easing should pass a specific value'

This test verifies that a result of f(Math.LOG10E) is not broken. The purpose of this test is to check if results of the function changes depending on the version of node.js.

toBeCloseTo are based on jest's functions.

fix Sinusoidal easing

tween.js/src/Easing.ts

Line 73 in d8b4f09

return 1 - Math.cos((amount * Math.PI) / 2)

Because 1 - Math.cos(1.0 * Math.PI / 2) = 0.99999999999 , it does not snap to 1.0.
By replacing Math.cos with Math.sin, this function snap to 1.0.

fix Back easing

Added truncation of 1.0 and 0.0.

[trusktr]

Does this fix a jump?

[MasatoMakino]

Yes.
Back.In(1.0) = 0.9999999999999998
This jump is not visually problematic, but it is not enough to pass the test.
As the jump is very small, I did not modify the process and added a truncation.

[trusktr]

Hey, nice stuff! What do you mean by "The purpose of this test is to check if results of the function changes depending on the version of node.js."?

[trusktr]

Cool. I suppose this can be moved to the definition of the function groups in https://github.com/tweenjs/tween.js/blob/master/src/Easing.ts, then imported here.

[MasatoMakino]

During the writing of this test, I considered how to add the definition to Easing.ts.

In order to add the EasingFunctionGroup type definition, we had to modify the export of index.ts.
I thought it would be excessive to modify index.ts to add tests, so I added a type definition to test.ts.
In the future, I also think that adding a type definition to Easing.ts is the best way to go.

[MasatoMakino]

Hey, nice stuff! What do you mean by "The purpose of this test is to check if results of the function changes depending on the version of node.js."?

If node.js is updated and the decimal point handling or Math class handling changes, this test will detect the change.

CI of tween.js supports multiple versions of node.js. Therefore, it is easy to determine which version the change occurred in.

A possibility of such a disruptive change in node.js is very low. However, tracking with CI does not waste developers' time. I think it's a good deal to add a few dozen lines of test code to save our collaborators' valuable time.