Detecting active breakpoints in JavaScript

During the recent responsive redesign of we had the requirement to execute different JavaScript dependant on the active media query breakpoint. Our use case was specific to the smartphone experience of Google Maps, but similar requirements for more basic navigation interactions probably exist.

The easy solution is to simply check something like window.innerWidth e.g.

if(window.innerWidth <= 480) {
    // Do something for small screens
} else if (window.innerWidth <= 1024) {
    // Do something for medium screens

However this requires breakpoint dimensions being defined in 2 places, both in SASS/CSS and also JS. Not great separation of concerns and definitely increasing the risk of bugs. Another solution was needed...

Since we're using the excellent Breakpoint SASS library to create our breakpoints, they are already given a name e.g.

$small: 600px;
.content {
    @include breakpoint($small) {
        content: 'Phone stuff';

What we really want is to be able to test if $small-screen is active by name not by dimensions. If we decided to tweak the breakpoint width's in SASS, the JS code will just continue to work. The solution (all be it hacky), is to encode the name of the breakpoint within the font-family style of a class that is then queried from JS e.g.

.media-query-info {
    display: none;
    font-family: '{"enabled" : "0"}';

    @media all and (max-width: 600px) and (min-width: 0px) {
        font-family: '{"enabled" : "1", "breakpoint" : "small"}';

    @media all and (max-width: 800px) and (min-width: 600px) {
        font-family: '{"enabled" : "1", "breakpoint" : "medium"}';

    @media all and (min-width: 800px) {
        font-family: '{"enabled" : "1", "breakpoint" : "large"}';

The above goes a step further and encodes the breakpoint as JSON. Using the following, it's now possible to write conditional JS based on the active media query. Job done!

// Get MQ info stored in font-family
var infoJSON = $(".media-query-info").css("font-family");

// Remove the leading/trailing quote and remove backslashes
infoJSON = infoJSON.substring(1, infoJSON.length-1).replace(/\134/g,"");

// Parse
var info = JSON.parse(infoJSON);

// Output active breakpoint

Check out this pen for a full example.

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