Y=MX+B

Click twice to draw a line.

The HTML file:

<head>
 <script src="https://cdn.rawgit.com/google/code-prettify/master/loader/run_prettify.js"></script>
  <script type="text/javascript" async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-MML-AM_CHTML"> </script>
 <script type="text/x-mathjax-config">
  MathJax.Hub.Config({tex2jax: {inlineMath: [['$','$'], ['\\(','\\)']]}});
</script>

 <link rel="stylesheet" href="http://mirkwood.cs.edinboro.edu/~bennett/style/hw.css">
</head>

<body>
<h1>Y=MX+B</h1>
Click twice to draw a line.
<p>
<canvas id="canvas" width="400" height="400" style="border:1px solid #000000;"></canvas>
<p>

<script type="text/javascript" src="common.js"></script>
<script type="text/javascript" src="badLine.js"></script>

<script type="text/javascript">

canvas = document.getElementById("canvas");
ctx = canvas.getContext('2d'); 
canvas.tabIndex = 0;

canvas.addEventListener("click", MyHandler);
</script>

badLine.js

"use strict"

function OtherLine(x0, y0, x1, y1) {
    let m = (y0-y1)/(x0-x1);

        if (x0 > x1) {
            let tmp = x0;
            x0 = x1;
            x1 = tmp;
            tmp = y0;
            y0 = y1;
            y1 = tmp;
        }

        let b = y0 - m*x0;

        for(let x = x0; x <= x1; x++) {
           let y = m*x + b; 
           SetPixel(x,y);
        }
    return;
}

common.js

"use strict"

let x0,y0;
let x1, y1;
let state = 0;


function SetPixel(x, y, color = "black") {

    x = Math.round(x);
    y = Math.round(y);

    ctx.beginPath();
    ctx.fillStyle = color;
    ctx.fillRect(x,y,1,1);
    ctx.fill()

    return;
}

function HorizontalLine(x, y0, y1){
    let tmp;

    if (y0 > y1) {
       tmp = y1;
       y1 = y0;
       y0 = tmp;
    }

    for(let y = y0; y <= y1; y++) {
       SetPixel(x,y);
    }

    return;
}

function VerticalLine(y, x0, x1) {
    let tmp;

    if (x0 > x1) {
       tmp = x1;
       x1 = x0;
       x0 = tmp;
    }

    for(let x = x0; x <= x1; x++) {
       SetPixel(x,y);
    }

    return;
}

function Line(x0, y0, x1, y1) {
    let dx, dy;

    if (x1 == x0) {
       HorizontalLine(x0, y0, y1);
    } else if (y0 == y1) {
       VerticalLine(y0, x0, x1);
    } else {
       OtherLine(x0,y0,x1,y1); 
    }

    return;
}

function MyHandler(evnt) {
    switch(state) {
      case 0:
           x0= evnt.offsetX;
           y0 = evnt.offsetY;
           state = 1;
           break;
      case 1:
           x1= evnt.offsetX;
           y1 = evnt.offsetY;
           state = 0;
           Line(x0, y0, x1, y1);
    }

    return;
}