Custom functions are a powerful feature in Power Query (M Language) that allow you to reuse code and improve the maintainability of your data transformations. Here's how to use them effectively:

Creating Custom Functions:

There are two main ways to create custom functions:

1. From Scratch (Advanced Editor):
* Go to "Home" > "New Source" > "Blank Query".
* Rename the query to reflect your function's purpose (e.g., "FormatDate").
* Click "Transform" > "Advanced Editor".
* Here you write your M code using the lambda function syntax. The basic structure is:

```
(parameter1, parameter2) =>
Let,
variable1 = expression1,
variable2 = expression2,
In
result expression
```

* Replace `parameter1` and `parameter2` with the names of your function's inputs (if any).
* Define any intermediate variables (`variable1`, `variable2`) using `Let` statements.
* The `In` clause specifies the final result your function will return.

2. From Existing Queries:
* Right-click on a query that performs a specific transformation you want to reuse.
* Select "Create Function". Power Query creates a new function based on that query's steps.

Invoking Custom Functions:

Once you have a custom function, you can use it in two ways:

1. New Column:
* Go to the "Add Column" tab.
* Select "Invoke Custom Function".
* Enter a name for the new column.
* Choose your function from the "Function query" dropdown.
* Select the column(s) to be passed as arguments to your function (if applicable).
* Click "OK".

2. Within Another M Formula:
* Directly call your function by name in your M code, passing any required arguments within parentheses.

Benefits of Using Custom Functions:

* Reduced Code Duplication: Write complex logic once and reuse it throughout your queries.
* Improved Maintainability: Changes to a function are reflected everywhere it's used.
* Enhanced Readability: Complex transformations become easier to understand with clear function names.

Tips for Effective Use:

* Descriptive Names: Choose clear and concise names that reflect the function's purpose.
* Parameterization: Allow your functions to accept different inputs for flexibility.
* Modular Design: Break down complex transformations into smaller, reusable functions.
* Documentation: Add comments within your function code to explain its logic.

By following these guidelines, you can leverage custom functions to streamline your Power Query workflows, improve code quality, and make your data transformations more maintainable.