JavaScript MouseEvent mozInputSource Property

What You’ll Learn

MouseEvent.mozInputSource is a non-standard read-only number that reports the kind of input device that created the mouse event. Learn the MOZ_SOURCE_* values, why device type matters for accuracy, how to feature-detect safely, and how Pointer Events replace it.

Introduction

Not every “mouse” event comes from a physical mouse. Touch screens, pens, and even keyboards can produce mouse-like events. Firefox exposes mozInputSource so you can tell which device was involved—handy when touch coordinates need larger hit targets than a precise mouse.

element.addEventListener("mousedown", (event) => {
  console.log(event.mozInputSource); // number in Firefox; often undefined elsewhere
});

Understanding the Property

MDN: MouseEvent.mozInputSource provides information indicating the type of device that generated the event. That lets you, for example, decide whether coordinates came from an actual mouse or from a touch event (which can affect how accurately you interpret those coordinates).

• Instance — read event.mozInputSource in a handler.
• Number — one of the MOZ_SOURCE_* values (0–6).
• Non-standard — not in any official specification.
• Firefox-first — usually missing in Chromium and Safari.

📝 Syntax

const source = mouseEvent.mozInputSource;

Value

A number describing the input device when the property is supported; otherwise the property is typically undefined.

Safe read with fallback

function getInputKind(event) {
  if (typeof event.mozInputSource === "number") {
    return event.mozInputSource; // Firefox / non-standard
  }
  if (typeof event.pointerType === "string" && event.pointerType) {
    return event.pointerType; // standard Pointer Events
  }
  return "unknown";
}

📚 MOZ_SOURCE_* Values

MDN documents these constants and numeric values. In Firefox they may also appear as MouseEvent.MOZ_SOURCE_MOUSE, MouseEvent.MOZ_SOURCE_TOUCH, and so on.

⚖️ mozInputSource vs pointerType

Use Pointer Events for new apps. Keep mozInputSource only when you must understand or maintain older Firefox-specific code.

Examples Gallery

Examples follow MDN MouseEvent.mozInputSource. Best results are in Firefox. Other browsers usually show undefined—that is expected for a non-standard API.

const log = document.querySelector("#log");

document.addEventListener("click", (e) => {
  log.textContent = `mozInputSource: ${e.mozInputSource}`;
});

mozInputSource: 1

const LABELS = {
  0: "unknown",
  1: "mouse",
  2: "pen",
  3: "eraser",
  4: "cursor",
  5: "touch",
  6: "keyboard"
};

document.addEventListener("click", (e) => {
  const code = e.mozInputSource;
  const label = LABELS[code] || String(code);
  console.log(code, label);
});

1 mouse

document.addEventListener("click", (e) => {
  if (typeof e.mozInputSource !== "number") {
    console.log("mozInputSource not supported in this browser");
    return;
  }
  console.log("Supported. Source code:", e.mozInputSource);
});

mozInputSource not supported in this browser

const MOZ_SOURCE_TOUCH = 5;
const MOZ_SOURCE_MOUSE = 1;

box.addEventListener("mousedown", (e) => {
  if (typeof e.mozInputSource !== "number") {
    console.log("Use pointerType instead on this browser");
    return;
  }
  if (e.mozInputSource === MOZ_SOURCE_TOUCH) {
    console.log("Touch source — prefer larger hit targets");
  } else if (e.mozInputSource === MOZ_SOURCE_MOUSE) {
    console.log("Mouse source — fine-grained coordinates OK");
  } else {
    console.log("Other source:", e.mozInputSource);
  }
});

Mouse source — fine-grained coordinates OK

target.addEventListener("pointerdown", (e) => {
  // Standard: "mouse" | "pen" | "touch" | ""
  console.log("pointerType:", e.pointerType);

  // Optional legacy peek (Firefox only)
  if (typeof e.mozInputSource === "number") {
    console.log("mozInputSource:", e.mozInputSource);
  }
});

pointerType: mouse

🚀 Common Use Cases

• Reading Firefox-only device type while debugging legacy code.
• Understanding why touch vs mouse accuracy might differ (MDN use case).
• Comparing Mozilla constants with modern pointerType strings.
• Teaching non-standard APIs and why feature detection matters.
• Migrating old Firefox scripts toward Pointer Events.

🔧 How It Works

📝 Notes

• MDN: Non-standard; not part of any specification.
• Not labeled Deprecated or Experimental on the MDN page—but still unsafe for production cross-browser use.
• Primarily a Firefox / Gecko extension; often undefined elsewhere.
• Prefer PointerEvent.pointerType for new code.
• Related: movementY, movementX, button, MouseEvent(), Window, JavaScript hub.

💡 Best Practices