How to slow down a game, or play it frame by frame

I created a script that does something within a frame that I want to see happen, but it happens so fast that I can’t see it properly.
Is there an easy way to make game playaback in slow motion, or play it frame by frame so I can see what’s going on?
I should probably also mention that I’m working in Unity 2D, & if it’s something I would need to do in script, I’m working in Javascript.

You can goto Edit > Project Settings > Time and change the settings.

Decrease Fixed Timestep and Maximum Allowed Timestep and my game was slowed. It was effective for testing some physics interactions.

Later, you can always reset the settings by clicking on the gear (cog) icon and selecting Reset.

Adjusting the Time Scale value under Edit > Project Settings > Time is actually easier. To set your game speed to half just change the default value from 1.0 to 0.5.