Global mode versus instance mode for namespace isolation in multi-sketch pages

Direct answer

Instance mode provides near-complete — but not total — isolation of p5 properties from window. In current stable releases (p5.js 1.x line), instance mode keeps every API function (createCanvas, draw, mouseX, etc.) as a method on your sketch instance, so two sketches on one page cannot collide through p5's own symbols. The one thing that stays global is the p5 constructor itself (window.p5), which must exist for new p5() to work. So: instance mode isolates the sketch API, not the constructor.

If global mode is ever deprecated, the expected migration is mechanical: wrap your sketch in a callback, prefix every p5 call with the instance parameter, and move sketch-level globals inside the closure. There is no announced removal date, so this is a future-proofing exercise, not an urgent one.

Libraries that assume global p5 symbols (p5.sound, p5.play, and many community addons) are the main reason to stay on global mode — or to plan a compatibility shim.

What is confirmed vs. likely

Confirmed behavior (stable 1.x): Global mode attaches the full p5 API to window as a side effect of loading the script. Instance mode attaches nothing except the constructor. Both modes expose identical rendering APIs and lifecycle hooks.

Likely, but verify: The deprecation discussion in the p5.js repository has been open for years without a committed timeline, and no console deprecation warning ships in stable builds. Treat "global mode is going away soon" as unconfirmed; check the current issue tracker and release notes before planning around a removal date.

Minimal instance-mode sketch

const sketchA = (p) => {
  let x = 0; // private to this sketch
  p.setup = () => { p.createCanvas(200, 200); };
  p.draw = () => { p.background(x++ % 255); };
};
new p5(sketchA, document.getElementById('a'));
new p5(sketchA, document.getElementById('b')); // fully independent

Verify isolation in the console after loading:

'createCanvas' in window  // false in pure instance mode
typeof window.p5          // 'function' — the constructor only

Library compatibility: the real constraint

Many addons check for window.p5 or call global p5 functions at load time. In a pure instance-mode page they may fail silently or throw p5 is not defined. Two practical options:

• Stay in global mode for pages that depend on those libraries, and accept the shared namespace — but then only run one global-mode sketch per page.
• Shim the global: assign window.p5 = p5; before loading the addon. This satisfies load-time checks but does not guarantee the addon binds to your specific instance, so test each library individually.

Note also that ES module usage (import p5 from 'p5') effectively requires instance mode, since global mode depends on script-tag side effects that module builds avoid.

Migration expectations if deprecation lands

1. Wrap the sketch body in (p) => { ... } and pass it to new p5().
2. Prefix every p5 API call and constant with the instance (p.createCanvas, p.width, p.PI).
3. Move top-level sketch variables into the closure.
4. Audit third-party addons first — they, not your own code, are the likely blocker.

No official codemod exists, so large sketches require manual refactoring. The one diagnostic detail that would change this recommendation: which specific addons your page loads, since that determines whether global mode is a hard requirement today.