An (almost perfect) zoom function that zooms into the mouse cursor pointer. Assumes that the canvas is full screen - adjusting for this shouldn't be too tough.

zoom.js

/*
 * An (almost perfect) zoom function that zooms into the mouse cursor pointer.
 * Assumes that the canvas is full screen - adjusting for this shouldn't be too
 * tough.
 * 
 * Attaches a zoom level update function to the mouse wheel event of
 * the document. Calling this activates zooming via the mouse wheel.
 *
 * Assumes that it is part of a class of sorts.
 *
 * this.canvas      - The canvas
 * this.offset      - The panning offset that we should update
 * this.zoom        - The zoom level that we should update.
 * this.zoomDelta   - The amount that we should zoom by each time.
 ******************************************************************************
 * Changelog
 ******************************************************************************
 * 20th December 2015:
	* Added this changelog.
 */
function trackMouseWheel() {
	// Track the *vertical* mouse wheel movement
	this.canvas.addEventListener("wheel", (function(event) {
		// Calculate how much to change the zoom level by
		var scrollDelta = 0;
		if(event.deltaY < 0)
			scrollDelta = this.zoomDelta;
		else if(event.deltaY > 0)
			scrollDelta = -this.zoomDelta;
		
		// Calculate the new offset
		this.offset = new Vector(
			event.clientX + ((this.offset.x / (this.zoom - scrollDelta)) - (event.clientX / (this.zoom - scrollDelta))) * this.zoom,
			event.clientY + ((this.offset.y / (this.zoom - scrollDelta)) - (event.clientY / (this.zoom - scrollDelta))) * this.zoom
		);
		
		// Update the zoom level
		this.zoom += scrollDelta;
	}).bind(this));
}