What is the best way to learn After Effects Expression

Next exercise, draw a line 400px long oriented at 45° using createPath() in the path property of a shape layer with a stroke.

HELP: Use JavaScript to select point on a circle 

path expression:

lineLength = 400;
angle = 45;
p1 = [0, 0]; // first point poistion
pts = [p1]; // array of point coordinates

x = // Calcultate X position;
y = // Calcultate Y position;
pts.push(p1 + [x, y]);

createPath(pts, [], [], false)

The Youtube Channel, Animoplex, has a well-structured AE Expressions Course - 

https://youtu.be/SFgWa5G0VAE?list=PLvr5U5ZSt6IzHyvSL9fo0M9NRPsTvra31

I recommend it to anyone who's keen to learn AE Expressions. And if you reach a stage where you want to go deeper, I've got a niche Advanced AE Course that uses Advanced Expressions. 

https://www.broadcastgems.com/consulting-training

We can give you small exercises to do:

1/ Create a square with a stroke
2/ Edit size and rotation expressions
3/ Duplicate the layer n times

size expression:

baseSize = 760;  // Base size of the square
layerIndex = index - 1;  // Adjusted layer index
reduction = 15;  // Size reduction per layer
s = // Calculate the size using the values mentioned above
[s, s]

rotation expression:

layerIndex = index - 1;
// Calculate the rotation using the value mentioned above