Extension Migration

Quick Start

htmx 4 replaces the callback-based extension API with event-based hooks. Extensions register handlers for lifecycle events instead of implementing callback methods.

The simplest migration: rename defineExtension to registerExtension and map your callbacks to hooks.

// htmx 2.x
htmx.defineExtension('my-ext', {
    onEvent: function(name, evt) {
        if (name === 'htmx:beforeRequest') { /* ... */ }
    }
});

// htmx 4
htmx.registerExtension('my-ext', {
    htmx_before_request: (elt, detail) => { /* ... */ }
});

What Changed

No hx-ext attribute

Extensions load by including the script. No attribute needed:

<script src="/path/to/htmx.js"></script>
<script src="/path/to/ext/my-extension.js"></script>

Restrict which extensions can load:

<meta name="htmx-config" content='{"extensions": "sse, ws"}'>

Event hooks replace callbacks

Instead of a single onEvent callback that switches on event names, each event gets its own hook method. Hook names use underscores where events use colons:

All hooks receive (elt, detail). Return false to cancel.

handle_swap is special

Unlike other hooks, handle_swap is called directly with positional parameters (no htmx_ prefix, no detail object):

handle_swap: (swapStyle, target, fragment, swapSpec) => {
    if (swapStyle === 'my-swap') {
        target.appendChild(fragment);
        return true;
    }
    return false;
}

Detail object replaces event properties

All hooks receive detail.ctx with full request/response context:

• detail.ctx.request.body (FormData in htmx_config_request)
• detail.ctx.request.headers
• detail.ctx.response.status
• detail.ctx.text (response body, modifiable in htmx_after_request)
• detail.ctx.target

OOB swap stripping

OOB swaps automatically strip the wrapper element for non-outer swap styles. Name custom swap styles starting with “outer” (e.g., outerMorph) to preserve the wrapper.

Callback Migration Map

init

// htmx 2.x
init: function(api) {
    return null;
}

// htmx 4
init: (internalAPI) => {
    api = internalAPI;
}

Store the internalAPI reference for use in other hooks. No return value needed.

getSelectors

Removed. Use htmx_after_init to check for attributes:

// htmx 2.x
getSelectors: function() {
    return ['[my-custom-attr]'];
},
onEvent: function(name, evt) {
    if (name === 'htmx:afterProcessNode') {
        initializeCustomBehavior(evt.target);
    }
}

// htmx 4
htmx_after_init: (elt) => {
    if (api.attributeValue(elt, 'my-custom-attr')) {
        initializeCustomBehavior(elt);
    }
}

onEvent

Replace with individual hooks:

// htmx 2.x
onEvent: function(name, evt) {
    if (name === 'htmx:beforeSwap' && evt.detail.xhr.status !== 200) {
        var target = getRespCodeTarget(evt.detail.requestConfig.elt, evt.detail.xhr.status);
        if (target) {
            evt.detail.shouldSwap = true;
            evt.detail.target = target;
        }
    }
}

// htmx 4
htmx_before_swap: (elt, detail) => {
    if (detail.ctx.response.status !== 200) {
        var target = getRespCodeTarget(elt, detail.ctx.response.status);
        if (target) {
            detail.ctx.target = target;
        }
    }
}

transformResponse

Removed. Modify detail.ctx.text in htmx_after_request:

// htmx 2.x
transformResponse: function(text, xhr, elt) {
    var tpl = htmx.closest(elt, '[mustache-template]');
    if (tpl) {
        var data = JSON.parse(text);
        var template = htmx.find('#' + tpl.getAttribute('mustache-template'));
        return Mustache.render(template.innerHTML, data);
    }
    return text;
}

// htmx 4
htmx_after_request: (elt, detail) => {
    var tpl = elt.closest('[mustache-template]');
    if (tpl) {
        var data = JSON.parse(detail.ctx.text);
        var template = document.querySelector('#' + tpl.getAttribute('mustache-template'));
        detail.ctx.text = Mustache.render(template.innerHTML, data);
    }
}

Event flow: response received, ctx.text set, htmx:after:request fires, ctx.text consumed into fragment, htmx:before:swap.

encodeParameters

Removed. Modify detail.ctx.request.body in htmx_config_request:

// htmx 2.x
onEvent: function(name, evt) {
    if (name === 'htmx:configRequest') {
        evt.detail.headers['Content-Type'] = 'application/json';
    }
},
encodeParameters: function(xhr, parameters, elt) {
    var object = {};
    parameters.forEach(function(value, key) {
        if (Object.hasOwn(object, key)) {
            if (!Array.isArray(object[key])) object[key] = [object[key]];
            object[key].push(value);
        } else {
            object[key] = value;
        }
    });
    return JSON.stringify(object);
}

// htmx 4
htmx_config_request: (elt, detail) => {
    detail.ctx.request.headers['Content-Type'] = 'application/json';
    var object = {};
    detail.ctx.request.body.forEach(function(value, key) {
        if (Object.hasOwn(object, key)) {
            if (!Array.isArray(object[key])) object[key] = [object[key]];
            object[key].push(value);
        } else {
            object[key] = value;
        }
    });
    detail.ctx.request.body = JSON.stringify(object);
}

ctx.request.body is FormData in htmx_config_request. It can be replaced with any value (string, JSON, URLSearchParams). For GET/DELETE, body becomes query parameters. For POST/PUT/PATCH, body becomes URLSearchParams (unless multipart).

isInlineSwap and handleSwap

Both replaced by handle_swap:

// htmx 2.x
isInlineSwap: function(swapStyle) {
    return swapStyle === 'morphdom';
},
handleSwap: function(swapStyle, target, fragment) {
    if (swapStyle === 'morphdom') {
        morphdom(target, fragment.firstElementChild || fragment.firstChild);
        return [target];
    }
}

// htmx 4
handle_swap: (swapStyle, target, fragment) => {
    if (swapStyle === 'morphdom') {
        morphdom(target, fragment.firstElementChild || fragment.firstChild);
        return true;
    }
    return false;
}

Return truthy if handled, falsy otherwise. Can return an array of elements for settle tracking.

Removed Callbacks

Checklist

1. Rename defineExtension to registerExtension
2. Replace onEvent with individual htmx_* hooks
3. Replace transformResponse with htmx_after_request
4. Replace encodeParameters with htmx_config_request
5. Merge isInlineSwap and handleSwap into handle_swap
6. Replace getSelectors with htmx_after_init
7. Remove hx-ext attributes from HTML
8. Update event names (colons to underscores in hook names)
9. Test custom swap styles with OOB swaps