# Patching code

Sometimes, we need to customize the way the UI works. Many common needs are
covered by some supported API. For example, all registries are good extension
points: the field registry allows adding/removing specialized field components,
or the main component registry allows adding components that should be displayed
all the time.

However, there are situations for which it is not sufficient. In those cases, we
may need to modify an object or a class in place. To achieve that, Odoo
provides the utility function `patch`. It is mostly useful to override/update
the behavior of some other component/piece of code that one does not control.

## Description

The patch function is located in `@web/core/utils/patch`:

```{eval-rst}
.. js:function:: patch(objToPatch, extension)

    :param object objToPatch: the object that should be patched
    :param object extension: an object mapping each key to an extension
    :returns: a function to remove the patch

    The `patch` function modifies in place the `objToPatch` object (or class) and
    applies all key/value described in the `extension` object.  An unpatch
    function is returned, so it can be used to remove the patch later if necessary.

    Most patch operations provide access to the parent value by using the
    native `super` keyword (see below in the examples).
```

## Patching a simple object

Here is a simple example of how an object can be patched:

```javascript
import { patch } from "@web/core/utils/patch";

const object = {
  field: "a field",
  fn() {
    // do something
  },
};

patch(object, {
  fn() {
    // do things
  },
});
```

When patching functions, we usually want to be able to access the `parent`
function. To do so, we can simply use the native `super` keyword:

```javascript
patch(object, {
  fn() {
    super.fn(...arguments);
    // do other things
  },
});
```

:::{warning}
`super` can only be used in a method not a function. This means that the
following constructs are invalid for javascript.

```javascript
const obj = {
  a: function () {
    // Throws: "Uncaught SyntaxError: 'super' keyword unexpected here"
    super.a();
  },
  b: () => {
    // Throws: "Uncaught SyntaxError: 'super' keyword unexpected here"
    super.b();
  },
};
```
:::

Getters and setters are supported too:

```javascript
patch(object, {
  get number() {
    return super.number / 2;
  },
  set number(value) {
    super.number = value;
  },
});
```

(frontend-patching-class)=

## Patching a javascript class

The `patch` function is designed to work with anything: object or ES6 class.

However, since javascript classes work with the prototypal inheritance, when
one wishes to patch a standard method from a class, then we actually need to patch
the `prototype`:

```javascript
class MyClass {
  static myStaticFn() {...}
  myPrototypeFn() {...}
}

// this will patch static properties!!!
patch(MyClass, {
  myStaticFn() {...},
});

// this is probably the usual case: patching a class method
patch(MyClass.prototype, {
  myPrototypeFn() {...},
});
```

Also, Javascript handles the constructor in a special native way which makes it
impossible to be patched. The only workaround is to call a method in the original
constructor and patch that method instead:

```javascript
class MyClass {
  constructor() {
    this.setup();
  }
  setup() {
    this.number = 1;
  }
}

patch(MyClass.prototype, {
  setup() {
    super.setup(...arguments);
    this.doubleNumber = this.number * 2;
  },
});
```

:::{warning}
It is impossible to patch directly the `constructor` of a class!
:::

## Patching a component

Components are defined by javascript classes, so all the information above still
holds. For these reasons, Owl components should use the `setup` method, so they
can easily be patched as well (see the section on {ref}`best practices<frontend/owl/best_practices>`).

```javascript
patch(MyComponent.prototype, {
  setup() {
    useMyHook();
  },
});
```

## Removing a patch

The `patch` function returns its counterpart. This is mostly useful for
testing purposes, when we patch something at the beginning of a test, and
unpatch it at the end.

```javascript
const unpatch = patch(object, { ... });
// test stuff here
unpatch();
```

## Applying the same patch to multiple objects

It could happen that one wants to apply the same patch to multiple objects but
because of the way the `super` keyword works, the `extension` can only be used
for patching once and cannot be copied/cloned ([check the documentation of the keyword](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/super#description)).
A function returning the object used to patch can be used to make it unique.

```javascript
const obj1 = {
  method() {
    doSomething();
  },
};

const obj2 = {
  method() {
    doThings();
  },
};

function createExtensionObj() {
  return {
    method() {
      super.method();
      doCommonThings();
    },
  };
}

patch(obj1, createExtensionObj());
patch(obj2, createExtensionObj());
```

:::{warning}
If an `extension` is based on another then the two extensions should
be applied separately. Do not copy/clone an extension.

```javascript
const object = {
  method1() {
    doSomething();
  },
  method2() {
    doAnotherThing();
  },
};

const ext1 = {
  method1() {
    super.method1();
    doThings();
  },
};

const invalid_ext2 = {
  ...ext1, // this will not work: super will not refer to the correct object in methods coming from ext1
  method2() {
    super.method2();
    doOtherThings();
  },
};

patch(object, invalid_ext2);
object.method1(); // throws: Uncaught TypeError: (intermediate value).method1 is not a function

const valid_ext2 = {
  method2() {
    super.method2();
    doOtherThings();
  },
};

patch(object, ext1); // first patch base extension
patch(object, valid_ext2); // then the new one
object.method1(); // works as expected
```
:::