Javascript Modules

Flectra supports three different kinds of javascript files:

As described in the assets management page, all javascript files are bundled together and served to the browser. Note that native javascript files are processed by the Flectra server and transformed into Flectra custom modules.

Let us briefly explain the purpose behind each kind of javascript file. Plain javascript files should be reserved only for external libraries and some small specific low level purposes. All new javascript files should be created in the native javascript module system. The custom module system is only useful for old, not yet converted files.

Plain Javascript files

Plain javascript files can contain arbitrary content. It is advised to use the iife immediately invoked function execution style when writing such a file:

(function () {
  // some code here
  let a = 1;
  console.log(a);
})();

The advantages of such files is that we avoid leaking local variables to the global scope.

Clearly, plain javascript files do not offer the benefits of a module system, so one needs to be careful about the order in the bundle (since the browser will execute them precisely in that order).

Note

In Flectra, all external libraries are loaded as plain javascript files.

Native Javascript Modules

Most new Flectra javascript code should use the native javascript module system. This is simpler, and brings the benefits of a better developer experience with a better integration with the IDE.

There is a very important point to know: Flectra needs to know which files should be translated into Flectra modules and which files should not be translated. This is an opt-in system: Flectra will look at the first line of a JS file and check if it contains the string @flectra-module. If so, it will automatically be converted to an Flectra module.

For example, let us consider the following module, located in web/static/src/file_a.js:

/** @flectra-module **/
import { someFunction } from './file_b';

export function otherFunction(val) {
    return someFunction(val + 3);
}

Note the comment in the first line: it describes that this file should be converted. Any file without this comment will be kept as-is (which will most likely be an error). This file will then be translated into an Flectra module that look like this:

flectra.define('@web/file_a', function (require) {
'use strict';
let __exports = {};

const { someFunction } = require("@web/file_b");

__exports.otherFunction = function otherFunction(val) {
    return someFunction(val + 3);
};

return __exports;
)};

So, as you can see, the transformation is basically adding flectra.define on top, and updating the import/export statements.

Another important point is that the translated module has an official name: @web/file_a. This is the actual name of the module. Every relative imports will be converted as well. Every file located in an Flectra addon some_addon/static/src/path/to/file.js will be assigned a name prefixed by the addon name like this: @some_addon/path/to/file.

Relative imports work, but only if the modules are in the same Flectra addon. So, imagine that we have the following file structure:

addons/
    web/
        static/
            src/
                file_a.js
                file_b.js
    stock/
        static/
            src/
                file_c.js

The file file_b can import file_a like this:

/** @flectra-module **/
import {something} from `./file_a`

But file_c need to use the full name:

/** @flectra-module **/
import {something} from `@web/file_a`

Aliased modules

Because Flectra modules follow a different module naming pattern, a system exists to allow a smooth transition towards the new system. Currently, if a file is converted to a module (and therefore follow the new naming convention), other files not yet converted to ES6-like syntax in the project won’t be able to require the module. Aliases are here to map old names with new ones by creating a small proxy function. The module can then be called by its new and old name.

To add such alias, the comment tag on top of the file should look like this:

/** @flectra-module alias=web.someName**/
import { someFunction } from './file_b';

export default function otherFunction(val) {
    return someFunction(val + 3);
}

Then, the translated module will also create an alias with the requested name:

flectra.define(`web.someName`, function(require) {
    return require('@web/file_a')[Symbol.for("default")];
});

The default behaviour of aliases is to re-export the default value of the module they alias. This is because “classic” modules generally export a single value which would be used directly, roughly matching the semantics of default exports. However it is also possible to delegate more directly, and follow the exact behaviour of the aliased module:

/** @flectra-module alias=web.someName default=0**/
import { someFunction } from './file_b';

export function otherFunction(val) {
    return someFunction(val + 3);
}

In that case, this will define an alias with exactly the values exported by the original module:

flectra.define(`web.someName`, function(require) {
    return require('@web/file_a');
});

Note

Only one alias can be defined using this method. If you were to need another one to have, for example, three names to call the same module, you would have to add a proxy manually. This is not good practice and should be avoided unless there is no other options.

Limitations

For performance reasons, Flectra does not use a full javascript parser to transform native modules. There are, therefore, a number of limitations including but not limited to:

  • an import or export keyword cannot be preceded by a non-space character,

  • a multiline comment or string cannot have a line starting by import or export

    // supported
import X from "xxx";
export X;
  export default X;
    import X from "xxx";

/*
 * import X ...
 */

/*
 * export X
 */


// not supported

var a= 1;import X from "xxx";
/*
  import X ...
*/

  • when you export an object, it can’t contain a comment

    // supported
export {
  a as b,
  c,
  d,
}

export {
  a
} from "./file_a"


// not supported
export {
  a as b, // this is a comment
  c,
  d,
}

export {
  a /* this is a comment */
} from "./file_a"

  • Flectra needs a way to determine if a module is described by a path (like ./views/form_view) or a name (like web.FormView). It has to use a heuristic to do just that: if there is a / in the name, it is considered a path. This means that Flectra does not really support module names with a / anymore.

As “classic” modules are not deprecated and there is currently no plan to remove them, you can and should keep using them if you encounter issues with, or are constrained by the limitations of, native modules. Both styles can coexist within the same Flectra addon.

Flectra Module System

Flectra has defined a small module system (located in the file addons/web/static/src/js/boot.js, which needs to be loaded first). The Flectra module system, inspired by AMD, works by defining the function define on the global flectra object. We then define each javascript module by calling that function. In the Flectra framework, a module is a piece of code that will be executed as soon as possible. It has a name and potentially some dependencies. When its dependencies are loaded, a module will then be loaded as well. The value of the module is then the return value of the function defining the module.

As an example, it may look like this:

// in file a.js
flectra.define('module.A', function (require) {
    "use strict";

    var A = ...;

    return A;
});

// in file b.js
flectra.define('module.B', function (require) {
    "use strict";

    var A = require('module.A');

    var B = ...; // something that involves A

    return B;
});

An alternative way to define a module is to give explicitly a list of dependencies in the second argument.

flectra.define('module.Something', ['module.A', 'module.B'], function (require) {
    "use strict";

    var A = require('module.A');
    var B = require('module.B');

    // some code
});

If some dependencies are missing/non ready, then the module will simply not be loaded. There will be a warning in the console after a few seconds.

Note that circular dependencies are not supported. It makes sense, but it means that one needs to be careful.

Defining a module

The flectra.define method is given three arguments:

  • moduleName: the name of the javascript module. It should be a unique string. The convention is to have the name of the flectra addon followed by a specific description. For example, web.Widget describes a module defined in the web addon, which exports a Widget class (because the first letter is capitalized)

    If the name is not unique, an exception will be thrown and displayed in the console.

  • dependencies: the second argument is optional. If given, it should be a list of strings, each corresponding to a javascript module. This describes the dependencies that are required to be loaded before the module is executed. If the dependencies are not explicitly given here, then the module system will extract them from the function by calling toString on it, then using a regexp to find all the require statements.

    flectra.define('module.Something', ['web.ajax'], function (require) {
    "use strict";

    var ajax = require('web.ajax');

    // some code here
    return something;
});

  • finally, the last argument is a function which defines the module. Its return value is the value of the module, which may be passed to other modules requiring it. Note that there is a small exception for asynchronous modules, see the next section.

If an error happens, it will be logged (in debug mode) in the console:

  • Missing dependencies: These modules do not appear in the page. It is possible that the JavaScript file is not in the page or that the module name is wrong

  • Failed modules: A javascript error is detected

  • Rejected modules: The module returns a rejected Promise. It (and its dependent modules) is not loaded.

  • Rejected linked modules: Modules who depend on a rejected module

  • Non loaded modules: Modules who depend on a missing or a failed module

Asynchronous modules

It can happen that a module needs to perform some work before it is ready. For example, it could do an rpc to load some data. In that case, the module can simply return a promise. The module system will simply wait for the promise to complete before registering the module.

flectra.define('module.Something', function (require) {
    "use strict";

    var ajax = require('web.ajax');

    return ajax.rpc(...).then(function (result) {
        // some code here
        return something;
    });
});