backbone_add_trailing_slash.js

User = Backbone.Model.extend({
    url: function() {
        var origUrl = Backbone.Model.prototype.url.call(this);
        return origUrl += origUrl.endsWith('/') ? '' : '/';
    }
});

To use the endsWith string method, you need the following code:

if (typeof String.prototype.endsWith !== 'function') {
    String.prototype.endsWith = function(suffix) {
        return this.indexOf(suffix, this.length - suffix.length) !== -1;
    };
}