jRespond is no longer supported. It was written to fill the gap before window.matchMedia() was well supported. window.matchMedia() now enjoys support across all modern browsers, rendering jRespond obsolete.

Responsive websites that require JavaScript functionality for some breakpoints and not for others need some type of system for triggering the correct functions at the correct breakpoint and to also be aware of when a browser is resized across breakpoints. Although switching between breakpoints could be seen as an edge case, a few applications for jRespond are:

• Managing functionality for initial page load: Even if the browser is never resized, jRespond can help manage what JavaScript functions are triggered when the page loads for a given breakpoint.
• Development testing: jRespond makes it much easier to test in-browser.
• Borderline device widths: Real user browser resizing and device rotation that crosses breakpoints.

If your project only needs to support modern browsers I highly recommend checking out Rob Tarr's mediaCheck which uses the matchMedia method. But if you're using respond.js as a polyfill to ensure that your site responds on older browsers, jRespond is worth checking out.

jRespond is a script that holds a list of user-defined functions that are fired based on the browser's width compared to a list of customizable breakpoints. Entry and exit functions can be defined so transitions between breakpoints can be managed by removing and unbinding some page elements while creating and binding others. jRespond was built to be independent and browser agnostic. It does NOT sniff for media queries in the stylesheets.

After including jRespond.js, call jRespond and define as many or as few media breakpoints as you need for your project. Labels can be any single-word string:

// call jRespond and add breakpoints
var jRes = jRespond([
	{
		label: 'handheld',
		enter: 0,
		exit: 767
	},{
		label: 'tablet',
		enter: 768,
		exit: 979
	},{
		label: 'laptop',
		enter: 980,
		exit: 1199
	},{
		label: 'desktop',
		enter: 1200,
		exit: 10000
	}
]);

Once running, functions can be registered with jRespond along with a desired breakpoint:

// register enter and exit functions for a single breakpoint
jRes.addFunc({
	breakpoint: 'desktop',
	enter: function() {
		myInitFunc();
	},
	exit: function() {
		myUnInitFunc();
	}
});

Or an array of breakpoints:

// register enter and exit functions for multiple breakpoints
jRes.addFunc({
	breakpoint: ['desktop','laptop'],
	enter: function() {
		myInitFunc();
	},
	exit: function() {
		myUnInitFunc();
	}
});

Or an array of breakpoints and functions:

// register enter and exit functions for multiple breakpoints and functions
jRes.addFunc([
	{
		breakpoint: 'desktop',
		enter: function() {
			myInitFuncDesktop();
		},
		exit: function() {
			myUnInitFuncDesktop();
		}
	},{
		breakpoint: 'laptop',
		enter: function() {
			myInitFuncLaptop();
		},
		exit: function() {
			myUnInitFuncLaptop();
		}
	},{
		breakpoint: 'tablet',
		enter: function() {
			myInitFuncTablet();
		},
		exit: function() {
			myUnInitFuncTablet();
		}
	}
]);

Use '*' to run a function at every breakpoint:

// register enter and exit functions for every breakpoint
jRes.addFunc({
	breakpoint: '*',
	enter: function() {
		myInitFunc();
	},
	exit: function() {
		myUnInitFunc();
	}
});

Ask jRespond what the current breakpoint is at any time:

// get the current breakpoint
jRes.getBreakpoint();

The breakpoint parameter is required but the enter and exit parameters are optional (of course, at least one is required for something to happen).

You can also use jRespond as a browser or Node module.

require(['jRespond'], function (jRes) {
	var mediaQueries = jRes([
		{
			label: 'tablet',
			enter: 768,
			exit: 979
		}
	]);

	mediaQueries.addFunc({
		breakpoint: 'tablet',
		enter: function() {
			myInitFunc();
		},
		exit: function() {
			myUnInitFunc();
		}
	});
});

require(['require', 'jRespond'], function (require) {
	var jRes = require('jRespond');

	var mediaQueries = jRes([
		{
			label: 'tablet',
			enter: 768,
			exit: 979
		}
	]);

	mediaQueries.addFunc({
		breakpoint: 'tablet',
		enter: function() {
			myInitFunc();
		},
		exit: function() {
			myUnInitFunc();
		}
	});
});

jRespond is 1.3kb minified and only polls for the browser width every 500ms. If it detects a change the polling speed is increased to 100ms only until the browser width stops changing.

IE 6+, Safari 5+, Firefox 3+, Chrome 1+

None.

Thanks to Rob Tarr for inspiring the function registration pattern and Markup Boy for helping me with my JavaScript failings.