Catch browser's “zoom” event in JavaScript

Question

Is it possible to detect, using JavaScript, when the user changes the zoom in a page? I simply want to catch a \"zoom\" event and respond to it (similar to window.onresize e

Answer 1

According to MDN, "matchMedia" is the proper way to do this https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio#Monitoring_screen_resolution_or_zoom_level_changes

it's a bit finicky because each instance can only watch one MQ at a time, so if you're interested in any zoom level change you need to make a bunch of matchers.. but since the browser is in charge to emitting the events it's probably still more performant than polling, and you could throttle or debounce the callback or pin it to an animation frame or something - here's an implementation that seems pretty snappy, feel free to swap in _throttle or whatever if you're already depending on that.

Run the code snippet and zoom in and out in your browser, note the updated value in the markup - I only tested this in Firefox! lemme know if you see any issues.

const el = document.querySelector('#dppx')

if ('matchMedia' in window) {
  function observeZoom(cb, opts) {
    opts = {
      // first pass for defaults - range and granularity to capture all the zoom levels in desktop firefox
      ceiling: 3,
      floor: 0.3,
      granularity: 0.05,
      ...opts
    }
    const precision = `${opts.granularity}`.split('.')[1].length

    let val = opts.floor
    const vals = []
    while (val <= opts.ceiling) {
      vals.push(val)
      val = parseFloat((val + opts.granularity).toFixed(precision))
    }

    // construct a number of mediamatchers and assign CB to all of them
    const mqls = vals.map(v => matchMedia(`(min-resolution: ${v}dppx)`))

    // poor person's throttle
    const throttle = 3
    let last = performance.now()
    mqls.forEach(mql => mql.addListener(function() {
      console.debug(this, arguments)
      const now = performance.now()
      if (now - last > throttle) {
        cb()
        last = now
      }
    }))
  }

  observeZoom(function() {
    el.innerText = window.devicePixelRatio
  })
} else {
  el.innerText = 'unable to observe zoom level changes, matchMedia is not supported'
}

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