# 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

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']`.
trigger	Trigger	This object is the configuration of the current step being run.
task	Task	This object is information about the current task's execution.

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'].

trigger Trigger This object is the configuration of the current step being run.

task Task 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](/tools/custom-code/libraries/sdk.md) is included by default and [other libraries](/tools/custom-code/libraries.md) 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/pages/OI2cEwPTTQFLMOoElwYt#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](/workflow-activity/logs.md).

### Throwing errors Throw a JavaScript error to halt the execution of your workflow: ```javascript throw new Error("your error description") ``` In the User Interface, this will look like:

### Interacting with external APIs Use [`Mesa.request.get()`](https://docs.getmesa.com/tools/pages/OI2cEwPTTQFLMOoElwYt#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/pages/OI2cEwPTTQFLMOoElwYt#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/pages/OI2cEwPTTQFLMOoElwYt#mesa.request.get-path-options). * `console.log()`: Alternative: `Mesa.log.*()` [Documentation](https://docs.getmesa.com/tools/pages/OI2cEwPTTQFLMOoElwYt#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/pages/OI2cEwPTTQFLMOoElwYt#mesa.xml.decode-xmlstring-namespacesep). --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.getmesa.com/tools/custom-code.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.