JavaScript Screen mozEnabled Property

What You’ll Learn

Screen.mozEnabled is a deprecated, non-standard instance property: a Boolean that controlled the device screen. Setting it to false was meant to turn the screen off. Learn the meaning of the flag, how it paired with mozBrightness, how to feature-detect safely, and why modern pages must not control screen power—with five examples and try-it labs.

Introduction

In some older Mozilla environments (including Firefox OS era APIs), privileged code could turn the device screen on or off through screen.mozEnabled. MDN describes it simply: this Boolean attribute controls the device’s screen. Setting it to false turns the screen off.

That kind of power does not belong on the open web. Modern browsers do not expose a standard way for a random site to blank the physical display. This tutorial covers the legacy property so you can read old snippets—and avoid writing new ones.

console.log(window.screen.mozEnabled);

Understanding the Property

MDN: this Boolean attribute controls the device’s screen. Setting it to false will turn off the screen.

• Instance — on window.screen where supported.
• Historically read/write — not a modern cross-browser API.
• Boolean — screen on (true) vs off (false).
• Deprecated & Non-standard — not in any specification.

📝 Syntax

// Read (legacy only)
const on = window.screen.mozEnabled;

// Write (legacy only — do not use in new apps)
// window.screen.mozEnabled = false; // would turn the screen off

Value

A boolean. When the API existed, false meant turn the screen off. Pair it mentally with mozBrightness (backlight level)—both were Mozilla screen-power helpers, both are deprecated.

🔄 mozEnabled vs mozBrightness

🎯 What To Do Instead

🛡️ Feature Detection Pattern

Wrap every access. Unsupported browsers should log a clear message and skip writes.

function getMozEnabled() {
  if (!("mozEnabled" in window.screen)) {
    return { supported: false, value: null };
  }
  return { supported: true, value: Boolean(window.screen.mozEnabled) };
}

console.log(getMozEnabled());

Examples Gallery

Examples follow MDN Screen.mozEnabled. On modern browsers the demos print “not supported”—that is the correct teaching outcome. No example turns the display off.

if ("mozEnabled" in window.screen) {
  console.log("mozEnabled is present (legacy)");
  console.log(typeof screen.mozEnabled);
} else {
  console.log("mozEnabled not supported");
}

mozEnabled not supported

function readMozEnabled() {
  if (!("mozEnabled" in screen)) {
    return "(unsupported)";
  }
  return screen.mozEnabled;
}

console.log(readMozEnabled());
console.log(typeof screen.mozEnabled);

(unsupported)
undefined

function labelMozEnabled() {
  if (!("mozEnabled" in screen)) {
    return "unsupported (do not use in new apps)";
  }
  return screen.mozEnabled
    ? "screen reported on (legacy)"
    : "screen reported off (legacy)";
}

console.log(labelMozEnabled());

unsupported (do not use in new apps)

// MDN: setting false would turn the screen off (legacy only)
const legacyIdea = "screen.mozEnabled = false;";

if ("mozEnabled" in screen) {
  console.log("API present; write skipped in this tutorial demo");
  console.log("would have run:", legacyIdea);
} else {
  console.log("API missing; no write attempted");
  console.log("legacy idea only:", legacyIdea);
}

API missing; no write attempted
legacy idea only: screen.mozEnabled = false;

console.log({
  mozEnabled: "mozEnabled" in screen
    ? screen.mozEnabled
    : "(unsupported — deprecated & non-standard)",
  mozBrightness: "mozBrightness" in screen
    ? screen.mozBrightness
    : "(unsupported — deprecated & non-standard)",
  width: screen.width,
  height: screen.height,
  colorDepth: screen.colorDepth
});

{
  "mozEnabled": "(unsupported — deprecated & non-standard)",
  "mozBrightness": "(unsupported — deprecated & non-standard)",
  "width": 1920,
  "height": 1080,
  "colorDepth": 24
}

🚀 Common Use Cases

• Reading / migrating old Firefox OS or Mozilla-privileged code.
• Interview context: why websites must not power off the display.
• Feature-detection drills for moz* prefixed Screen APIs.
• Auditing a codebase for deprecated Screen power properties to remove.
• Pairing with mozBrightness when documenting legacy device APIs.

🔧 How It Works

📝 Notes

• Deprecated and Non-standard (MDN)—both banners shown above.
• Not Experimental on MDN.
• Not part of any specification.
• This tutorial never assigns false in live demos.
• Related: mozBrightness, isExtended, Window hub, JavaScript hub.

💡 Best Practices