## Custom Code

The **Custom Code** tool allows you to add complex logic to a workflow that might otherwise be difficult with MESA's built-in tools. You can create custom integrations to third-party services, even those that MESA does not yet integrate, and code custom business logic using JavaScript.

## Configure

### To open the code editor

Click the **</\> Edit Code** button to edit your custom code.

### Anatomy of a Custom Code step

```javascript
const Mesa = require('vendor/Mesa.js');

/**
 * A MESA Script exports a class with a script() method.
 */
module.exports = new class {

/**
   * MESA Script
   *
   * @param {object} prevResponse The response from the previous step
   * @param {object} context Additional context about this task
   */
  script = (prevResponse, context) => {

// Retrieve the Variables Available to this step
    const vars = context.steps;

// Add your custom code here
    let response = {}

// Call the next step in this workflow
    // response will be the Variables Available from this step
    Mesa.output.next(response);
  }

}
```

Code is executed in a V8 environment and can be written in standard JavaScript or ES6. When this step is run, the `script()` method will be called with two parameters: `prevResponse` and `context`. In order for your workflow to continue its subsequent steps, be sure to include a call to `Mesa.output.next(response);` somewhere within your `script()` method.

#### The `context` object

| Property  | Type    | Description |
|-----------|---------|-------------|
| steps     | `Object`| An object containing the variables for each step that has been run so far, named with the step's unique key.|

For example, to access a step with a key of "shopify" use: `context.steps['shopify']`.

| Property  | Type | Description |
|-----------|------|-------------|
| trigger   | [Trigger](https://docs.getmesa.com/tools/custom-code#the-trigger-object) | This object is the configuration of the current step being run.|
| task      | [Task](https://docs.getmesa.com/tools/custom-code#the-task-object) | This object is information about the current task's execution.|

#### The `Trigger` object

| Property  | Type    | Description |
|-----------|---------|-------------|
| metadata  | `Object`| The values entered in configuration fields, with variable replacements.|
| raw_metadata | `Object`| The values entered in configuration fields, without variable replacements.|
| fields    | `Object` | An object of the configuration fields from the UI.|

#### The `Task` object

| Property  | Type    | Description |
|-----------|---------|-------------|
| context.headers | `Object` | A key-value object of the HTTP headers passed to this task. |
| is_test   | `Object` | Is the current task being run as a test? |

### Using Libraries

Libraries offer built-in functionality that extends the power of your code. The [MESA SDK library](https://docs.getmesa.com/tools/custom-code/libraries/sdk) is included by default and [other libraries](https://docs.getmesa.com/tools/custom-code/libraries) can be imported as needed.

## Going Further

### Logging information

To debug your script, it can be helpful to log data at a particular step in your code. This can be done by calling [`Mesa.log.info()`](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.log.info-message-meta). This is equivalent to `console.log()` in browser-based JavaScript. When the workflow runs, the info will be available in your workflow's [Logs tab](https://docs.getmesa.com/workflow-activity/logs).

### Throwing errors

Throw a JavaScript error to halt the execution of your workflow:

```javascript
throw new Error("your error description")
```

### Interacting with external APIs

Use [`Mesa.request.get()`](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.request.get-path-options) to make HTTP requests to external APIs. This is the equivalent to `fetch()` in browser-based JavaScript.

## Technical Notes

### Timezones

All system timezones use your Shopify Store's timezone. You can view and update your timezone from the Shopify Dashboard: Admin > Settings > General, under "Standards and formats". If you need to get dates in a different timezone, use the [Mesa.date.setTimezone(timezone)](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.date.settimezone-timezone) method.

### Unsupported NodeJS methods

Scripts are run in a stock V8 environment. This means that some common NodeJS methods are not available:

- `request()`: Alternative: `Mesa.request.*()`[Documentation](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.request.get-path-options).

- `console.log()`: Alternative: `Mesa.log.*()`[Documentation](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.log.info-message-meta).

- `async` or `await`: All `Mesa.request.*()` methods are synchronous, so `async` and `await` are not necessary.

- DOM manipulation methods: Alternative: `Mesa.xml.decode()`[Documentation](https://docs.getmesa.com/tools/custom-code/libraries/sdk#mesa.xml.decode-xmlstring-namespacesep).