How to be notified once a web font has loaded

Question

Google's Web Fonts API offers a way to define callback functions to be executed if a font has finished loading, or couldn't be loaded etc. Is there a way to achieve something similar using CSS3 web fonts (@font-face)?

Answer 1

2015 Update

Chrome 35+ and Firefox 41+ implement the CSS font loading API (MDN, W3C). Call document.fonts to get a FontFaceSet object, which has a few useful APIs for detecting the load status of fonts:

• check(fontSpec) - returns whether all fonts in the given font list have been loaded and are available. The fontSpec uses the CSS shorthand syntax for fonts.
  Example: document.fonts.check('bold 16px Roboto'); // true or false
• document.fonts.ready - returns a Promise indicating that font loading and layout operations are done.
  Example: document.fonts.ready.then(function () { /*... all fonts loaded...*/ });

Here's a snippet showing these APIs, plus document.fonts.onloadingdone, which offers extra information about the font faces.

alert('Roboto loaded? ' + document.fonts.check('1em Roboto'));  // false

document.fonts.ready.then(function () {
  alert('All fonts in use by visible text have loaded.');
   alert('Roboto loaded? ' + document.fonts.check('1em Roboto'));  // true
});

document.fonts.onloadingdone = function (fontFaceSetEvent) {
   alert('onloadingdone we have ' + fontFaceSetEvent.fontfaces.length + ' font faces loaded');
};

<link href='https://fonts.googleapis.com/css?family=Roboto:400,700' rel='stylesheet' type='text/css'>
<p style="font-family: Roboto">
  We need some text using the font, for the font to be loaded.
  So far one font face was loaded.
  Let's add some <strong>strong</strong> text to trigger loading the second one,
    with weight: 700.
</p>

IE 11 doesn't support the API. Look at available polyfills or support libraries if you need to support IE:

• Web Font Loader - developed by Google and Adobe
• FontFaceOnload - lighter, similar approach to Web Font Loader
• FontLoader - polyfill

Answer 2

Tested in Safari, Chrome, Firefox, Opera, IE7, IE8, IE9:

function waitForWebfonts(fonts, callback) {
    var loadedFonts = 0;
    for(var i = 0, l = fonts.length; i < l; ++i) {
        (function(font) {
            var node = document.createElement('span');
            // Characters that vary significantly among different fonts
            node.innerHTML = 'giItT1WQy@!-/#';
            // Visible - so we can measure it - but not on the screen
            node.style.position      = 'absolute';
            node.style.left          = '-10000px';
            node.style.top           = '-10000px';
            // Large font size makes even subtle changes obvious
            node.style.fontSize      = '300px';
            // Reset any font properties
            node.style.fontFamily    = 'sans-serif';
            node.style.fontVariant   = 'normal';
            node.style.fontStyle     = 'normal';
            node.style.fontWeight    = 'normal';
            node.style.letterSpacing = '0';
            document.body.appendChild(node);

            // Remember width with no applied web font
            var width = node.offsetWidth;

            node.style.fontFamily = font;

            var interval;
            function checkFont() {
                // Compare current width with original width
                if(node && node.offsetWidth != width) {
                    ++loadedFonts;
                    node.parentNode.removeChild(node);
                    node = null;
                }

                // If all fonts have been loaded
                if(loadedFonts >= fonts.length) {
                    if(interval) {
                        clearInterval(interval);
                    }
                    if(loadedFonts == fonts.length) {
                        callback();
                        return true;
                    }
                }
            };

            if(!checkFont()) {
                interval = setInterval(checkFont, 50);
            }
        })(fonts[i]);
    }
};

Use it like:

waitForWebfonts(['MyFont1', 'MyFont2'], function() {
    // Will be called as soon as ALL specified fonts are available
});

Answer 3

The JS Library used by Google Web Fonts API (and Typekit) can be used without the service: WebFont Loader.

It defines callbacks for what you ask, and many more.

Answer 4

2017 Update

The JS library FontFaceObserver is definitely the best, most lightweight, cross-browser solution as of 2017. It also exposes a Promise-based .load() interface.

Answer 5

The document.fonts.ready field is unreliable on Safari. I have found the only reliable cross-browser way (modern browsers) to be repeatedly checking for document.fonts.status === 'loaded'. Here's an example with exponential back off:

 const waitForFontsLoaded = document.fonts?.ready.then(() => {
    if (document.fonts?.status === 'loaded') {
        return null;
    }
    console.warn('Browser reported fonts ready but something is still loading...');
    return new Promise((resolve) => {
        let waitTimeMs = 5;
        const checkFontsLoaded = () => {
            if (document.fonts?.status === 'loaded') {
                return resolve();
            }
            waitTimeMs *= 2;
            return setTimeout(checkFontsLoaded, waitTimeMs);
        };
        setTimeout(checkFontsLoaded, 5);
    });
});

await waitForFontsLoaded

Answer 6

I created two methods to check for a specific font. The first method is the best one as it uses the 'fonts' interface directly with the 'check' method. The second method is not as good, but still functional, as it detects the difference directly in the DOM by comparing the size of the text with the default font to the text with the new font. It's possible, although rare, for the fonts to be so close in size that the event wouldn't fire, but I think it's highly unlikely. If that happens, you can add another span to check the difference between the serif font as well.

(Although it is pure javascript it works with React)

METHOD 1

const fontName = "Fira Sans Condensed",
    maxTime = 2500 // 2.5s

// EXAMPLE 1
fontOnload(fontName).then(() => {
    console.log("success")
})

// EXAMPLE 2
fontOnload(fontName, maxTime).then(() => {
    console.log("success")
}).catch(() => {
    console.log("timeout")
})

async function fontOnload(fontName, maxTime = Infinity, timeInterval = 10) {
    const startTime = performance.now()

    return new Promise((resolve, reject) => {
        setInterval(() => {
            const currentTime = performance.now(),
                elapsedTime = currentTime - startTime
            if (document.fonts.check("12px " + fontName)) {
                resolve(true)
            } else if (elapsedTime >= maxTime) {
                reject(false)
            }
        }, timeInterval)
    })
}

METHOD 2

const fontName = "Fira Sans Condensed",
    maxTime = 2500 // 2.5s

// EXAMPLE 1
fontOnloadDOM(fontName).then(() => {
    console.log("success")
})

// EXAMPLE 2
fontOnloadDOM(fontName, maxTime).then(() => {
    console.log("success")
}).catch(() => {
    console.log("timeout")
})

async function fontOnloadDOM(fontName, maxTime = Infinity, timeInterval = 10) {
    return new Promise((resolve, reject) => {
        const startTime = performance.now(),
            abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789",
            mainStyle = "font-size:24px!important;display:inline!important;font-family:",
            body = document.body,
            container = document.createElement("div"),
            span1 = document.createElement("span"),
            span2 = document.createElement("span")

        container.classList.add("font-on-load")
        container.setAttribute("style", "display:block!important;position:absolute!important;top:-9999px!important;left:-9999px!important;opacity:0!important;")

        span1.setAttribute("style", mainStyle + "sans-serif!important;")
        span2.setAttribute("style", mainStyle + "\"" + fontName + "\",sans-serif!important;")

        span1.innerText = abc.repeat(3)
        span2.innerText = abc.repeat(3)

        container.append(span1, span2)
        body.append(container)

        const interval = setInterval(() => {
            const currentTime = performance.now(),
                elapsedTime = currentTime - startTime,
                width1 = span1.clientWidth || span1.getBoundingClientRect().width,
                width2 = span1.clientWidth || span2.getBoundingClientRect().width,
                diffWidths = Math.abs(width1 - width2)

            if (diffWidths > 9) {
                clearInterval(interval)
                resolve(true)
            } else if (elapsedTime >= maxTime) {
                clearInterval(interval)
                reject(false)
            }
        }, timeInterval)
    })
}

Answer 7

The window.load event will fire when everything has loaded - that should include fonts So you could use that as the call back. However I don't think you have to is you decide to use the web font loader as

In addition to the google, typekit, ascender and monotype options, there is also a custom module that can load a stylesheet from any web-font provider.

WebFontConfig = { custom: { families: ['OneFont', 'AnotherFont'], urls: [ 'http://myotherwebfontprovider.com/stylesheet1.css', 'http://yetanotherwebfontprovider.com/stylesheet2.css' ] } };

The library sends the same events regardless of which provider you specify.