The neon effect with javascript P5 JavaScript library.

The P5.js is a JavaScript library for creative coding.
You can use by adding this link on your HTML area using the script tag:

https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.9.0/p5.min.js

You can use it like any JavaScript library.
You have two functions: setup and a draw for setup and drawing.

function setup() {
  createCanvas(window.innerWidth, window.innerHeight);
}

function draw() {
  ellipse(50, 50, 80, 80);
}

Then you can fill these functions with any javascript code.
You can see a neon effect with two bezier functions.
The example uses the mouse movement to change the bezier lines.
You can see many examples created by me at codepen website.