Examples of multiple dispatch in JavaScript

repl.js

widgetize.method(Error, Object, function(err) {
    return err;
});
render.method(Error, Function, function(err, append) {
    append($('<span/>').addClass('error').text(err));
});

// A special value to represent values we're waiting for
function Waiting() {}
widgetize.method(Waiting, Object, function(wait) {
    return wait;
});
render.method(Waiting, Function, function(_, append) {
    append($('<img/>').attr('src', 'ajax-loader.gif'));
});

types.js

function decodeValue(json) {
    if (typeof(json) === 'object' && json) {
        if (json.hasOwnProperty('!')) {
            return decodeSpecial(json['!'], json);
        }
        return decodeObject(json);
    }
    return json;
}

var decodeObject = procedure('decodeObject');
decodeObject.method(Array, function(arr) {
    var len = arr.length;
    var vals = new Array(len);
    for (var i = 0; i < len; i++) {
        vals[i] = decodeValue(arr[i]);
    }
    return vals;
});
decodeObject.method(Object, function(obj) {
    var res = {};
    for (var k in obj) {
        if (obj.hasOwnProperty(k)) res[k] = decodeValue(obj[k]);
    }
    return res;
});

var decodeSpecial = procedure('decodeSpecial');
decodeSpecial.method('undefined', Object, function() {
    return undefined;
});
decodeSpecial.method('table', Object, function(_type, json) {
    return new Table(json.data, json.columns);
});

widgetize.js

    var widgetize = procedure('widgetize');

    var render = Widget.render;

    // A superclass and API
    function Presentation() {
        Widget.call(this);
    }
    Presentation.prototype = inheritFrom(Widget);

    // API
    Presentation.widgetize = widgetize;

    // Sometimes this is called with just one argument; so we don't
    // have to specialise for each type twice, this transforms
    // undefined (not a subtype of Object) into null.
    widgetize.method(Object, undefined, function(val, _parent) {
        return widgetize(val, null);
    });

    function primitive(val, parent) { return val; }
    widgetize.method(String, Object, primitive);
    widgetize.method(Boolean, Object, primitive);
    widgetize.method(Number, Object, primitive);
    widgetize.method(null, Object, primitive);
    widgetize.method(undefined, Object, primitive);
    widgetize.method(undefined, undefined, primitive);

    // primitives are given as the value rather than a widget. Use this as a fallback.
    function renderPrimitive(val, appender, type) {
        appender($('<code/>').addClass(type || typeof(val)).text(String(val)));
    }

    render.method(Object, Function, renderPrimitive);

    render.method(null, Function, function(val, app) {
        return renderPrimitive('null', app, 'null');
    });

    render.method(String, Function, function(val, app) {
        return renderPrimitive('"' + val + '"', app);
    });