The “index.html” file can be written as short as this: 
<script src=https://cdn.JsDelivr.net/npm/p5></script>
<script src=https://po4599.GitHub.io/javascript/grid.js></script>
<script src=sketch.js></script>
Take another look at the short “index.html” above and you won’t spot any quotes or anything like it! 
How about instead coding your “grid.js” library in such a way that it seamlessly injects itself into the p5js library as part of its API? 
The best moment to draw your grid is right after the draw() callback returns. 
p5js got an undocumented method exclusive for 3rd-party libraries named registerMethod() which allows us to register our own callback functions: 
https://github.com/processing/p5.js/blob/1.0.1/src/core/main.js#L593-L599
Which are later on called back on 4 specific moments: 
https://github.com/processing/p5.js/blob/1.0.1/src/core/main.js#L715-L716
For example, the init[] registered functions are called back when the sketch is still being initialized:
https://github.com/processing/p5.js/blob/1.0.1/src/core/main.js#L494-L499
While pre[] & post[] respectively happen just before & after draw() is called back:
https://github.com/processing/p5.js/blob/1.0.1/src/core/structure.js#L392-L399
So basically it’s just a matter of invoking registerMethod() to register grid() as a “post” category callback. 
In the version I’ve come up w/ your grid() function was rewritten in such a way it’s compatible to both global & instance mode sketches: 
And while I was at it, I’ve also added 3 new methods (toggleGrid(), showGrid() & isGridVisible()) to p5js’ API so the grid display can be toggled on/off. 
You can see an online demo along its full source on this link below: 
And that same demo sketch running now on full page: 
Register-Method-Post-Draw.Glitch.me
I’m also gonna leave all the 3 source code files posted here as well: 
“index.html”:
<script defer src=https://cdn.JsDelivr.net/npm/p5></script>
<script defer src=grid.js></script>
<!-- <script defer src=https://Register-Method-Post-Draw.Glitch.me/grid.js></script> -->
<script defer src=sketch.js></script>
<!-- <script defer src=https://Register-Method-Post-Draw.Glitch.me/sketch.js></script> -->
“sketch.js”:
'use strict';
const FRAMES_TO_TOGGLE = 5,
ALL_COLORS = 0x1000,
RGB_COMPONENTS = 3;
function setup() {
createCanvas(1200, 600).mousePressed(redraw);
noLoop();
}
function draw() {
background('#' + hex(~~random(ALL_COLORS), RGB_COMPONENTS));
frameCount % FRAMES_TO_TOGGLE || print(toggleGrid().isGridVisible());
}
“grid.js”:
/**
* Register Method Post Draw (v1.2.3)
* GoToLoop (2019-Mar-05)
*
* Discourse.Processing.org/t/shared-library-for-simple-functions/18374/4
* Glitch.com/~matter-js-bouncing-colorful-balls
*/
'use strict';
p5.prototype.registerMethod('init', function initGrid() {
this._gridIsVisible = true;
this._gridStyles = {
'FILL': 'yellow', // 0o200
'STROKE': 0o250,
'BOLD': 1,
'GRID': 10,
'OFF_X': 2,
'OFF_Y': 12,
'TEXT': {
'FILL': 0o350, // 0
'SIZE': 11,
'OFFSET_X': 60,
'OFFSET_Y': 2,
'FONT': 'Courier',
'STYLE': this.BOLD,
'HOR': this.LEFT,
'VER': this.BASELINE
}
};
});
p5.prototype.registerMethod('post', function grid() {
const ctx = this == window && p5.instance || this;
if (!ctx._gridIsVisible | ctx._renderer.isP3D) return;
const { width: w, height: h, mouseX, mouseY, _gridStyles } = ctx,
{ FILL, STROKE, BOLD, GRID, OFF_X, OFF_Y, TEXT } = _gridStyles,
stepX = w / GRID, stepY = h / GRID,
loc = mouseX + ',' + mouseY;
ctx.push();
ctx.resetMatrix().fill(FILL).stroke(STROKE).strokeWeight(BOLD);
ctx.textFont(TEXT.FONT).textStyle(TEXT.STYLE);
ctx.textSize(TEXT.SIZE).textAlign(TEXT.HOR, TEXT.VER);
for (var y = 0; y < h; y += stepY) for (var x = 0; x < w; x += stepX) {
ctx.line(x, 0, x, h).line(0, y, w, y);
x || ctx.text(Math.round(y), OFF_X, y - OFF_X);
y || ctx.text(Math.round(x), x + OFF_X, OFF_Y);
}
ctx.fill(TEXT.FILL).text(loc, w - TEXT.OFFSET_X, h - TEXT.OFFSET_Y);
ctx.pop();
});
p5.prototype.toggleGrid = function () {
this._gridIsVisible = !this._gridIsVisible;
return this;
};
p5.prototype.showGrid = function (visible=true) {
this._gridIsVisible = !!visible;
return this;
};
p5.prototype.isGridVisible = function () {
return this._gridIsVisible;
};