How do you know if a point falls on a diagonal line?

Question

JavaScript

So far I've come up with this, but it doesn't work correctly.

var x1 = 10
var y1 = 10
var x2 = 500
var y2 = 400
// ...
var a = (x2 - x1) / (y2 - y1)
// ...
var x = mouseX - x1
var y = mouseY - y1
var isLine = (x > 0 && y > 0) 
           && Math.abs(a - x / y) / < 0.5
            && x < (x2 - x1)
            && y < (y2 - y1);

Unfortunately, I can't find the answer through search.
Tell me please.