# Compound slot initialization

## Purpose

To explain how to initialise the value of the slots of a compound component and its members.

## Prerequisites

You know how to [create compound components]().

## Introduction

Sometimes, it might be useful to initialise the value of a compound component slot or the slot of a member to assure some behaviour when the component is loaded. You can perform this initialisation in the _manifest_ file associated to your component, using the _inits_ property of a compound component definition. In the following sections, we will explore this process and present some related details.

## Sample case

To clarify the initialization process, we will use the `currency-viewer` compound created in [this tutorial]().

## Initialising slots

You can initialise the slots of a compound component or the slots of its members in the manifest definition using the inits property, which is an array of objects with the following structure:

| Property    | Requirement                              | Description                                                                                                |
| ----------- | ---------------------------------------- | ---------------------------------------------------------------------------------------------------------- |
| description | Optional                                 | A short description of this initialisation, e.g. responsibility.                                           |
| memberIdRef | Required only for members initialization | The _memberId_ of the member that contains the slot. It is only necessary if the slot belongs to a member. |
| slot        | Required                                 | _slotId_ of the slot to be initialised                                                                     |
| value       | Required                                 | The value to init the slot with, it can be an object, an array, a string, a number or a boolean.           |

> If a slot is connected to more than one slot, the initialization will be performed in the order in which the _connections_ were defined in the connections property of the compound component.

### Initialising compound slots

If you want to set the initial value of the slot of a component, you should provide the _slotId_ of the desired slot. Remember that when initialising compounds, the _memberIdRef_ property should not be defined.

### Initialising member slots

Let's say that we want to have the following default values for our compound:

| slot            | memberIdRef       | Init value           |
| --------------- | ----------------- | -------------------- |
| base            | currencyConverter | GBP (Pound sterling) |
| foreignCurrency | currencyConverter | EUR                  |
| date            | currencyConverter | 2016-06-24           |

To aim that, we should extend the _inits_ property of the compound definition as follows:

```javascript
    // ...
    "compoundComponents": [
      {
        "artifactId": "currency-viewer",
        // ...
        "inits": [
          {
            "slot": "base",
            "memberIdRef": "currencyConverter",
            "value": "GBP",
            "description": "Pound sterling would be the default base currency"
          },
          {
            "slot": "foreignCurrency",
            "memberIdRef": "currencyConverter",
            "value": "EUR",
            "description": "Euro would be the default target currency"
          },
          {
            "slot": "base",
            "memberIdRef": "currencyConverter",
            "value": "2018-06-23",
            "description": "Default date for the currency conversion"
          }
        ]
      },
      // ...
    ],
    // ...
```

> Note that if the component of a member has been initialised within its definition, that initialisation will be overwritten by the one you define within the compound that contains it as a member.

#### Result

If you run the demo page of the `currency-viewer` component, you will note that the input texts have the values that you defined as presented below:

![Initialised members demo](https://783451574-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LFauslHG_3YIQtbtVZG%2F-LLb1U6wXABj6rlzYdtf%2F-LLb1Up8unFaxjLOk1D2%2Fcompound_member_init.png?generation=1536105776749517\&alt=media)

You can also check the results at the [online demo](https://cubbles.world/sandbox/my-first-webpackage@0.1.0-SNAPSHOT/currency-viewer-init/demo/index.html).
