# Google Apps Script Patterns: Writing custom formulas/functions and handling single and range inputs
Custom functions have been in Google Apps Script since it’s public launch in 2009. These let you extend [Google Sheets function list](https://support.google.com/docs/table/25273?hl=en&ref_topic=3105474) allowing you to create and share your own spreadsheet formula. The official [Google Developer docs for Custom Functions](https://developers.google.com/apps-script/guides/sheets/functions#optimization) are very comprehensive and in this post I just want to highlight one pattern for detecting and handling user inputs.
Similar to built-in functions like `FIND`, [`TRIM`](https://support.google.com/docs/answer/3094140), and [`VLOOKUP`](https://support.google.com/docs/answer/3093318) you will probably need to handle both single values and ranges. For example, even a function like `TRIM`, whilst using a string or reference to a cell containing a string to be trimmed, when combined with `ARRAYFORMULA` can be used to use array references in non-array functions e.g. `=ARRAYFORMULA(TRIM(A1:A3))`.
The documentation highlights using [`Array.map()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) as a way that you can detect if the input is an array and if so recursively iterate over that array. Below (also [available in this gist](https://gist.github.com/mhawksey/4876f901591fb7fd304a41923c2b4fcf)) is an example of a custom function that generates a SHA-256 hash of text input:
```
/**
* Generates a SHA-256 hash of a text input.
*
* @param {String||Array} value to SHA-256 hash.
* @param {String} key to use to generate the hash with.
* @return the SHA-256 hash of the value
* @customfunction
*/
function HASH(value, key) {
if (typeof key !== 'string'){ // test the key is a string
throw "Key needs to be a text value";
}
if (value.map) { // Test whether input is an array.
return value.map(function(v){return HASH(v, key)}); // Recurse over array if so.
} else { // The bit that actual creates the hash
// based on https://stackoverflow.com/a/27933459
return Utilities.computeHmacSha256Signature(value, key).reduce(function(str, chr) {
chr = (chr < 0 ? chr + 256 : chr).toString(16);
return str + (chr.length == 1 ? '0' : '') + chr;
}, '');
}
}
```
In the above example our function has two variables `value` and `key`. The `key` is used to hash the `value`. In this scenario let's assume that the key can only be a string input while value can either be a string or an array. In the above code we detect if `key` is not a string a throw an error for the user. If the `value` is mappable we recurse over the array sending the array value v to our function `HASH` also passing the `key`. When a string value is sent to the `HASH` function we compute and return the SHA-256 hash of a text input.
The result of using this formula with different values inputs is shown below:
If you have other custom function tips and tricks feel free to share them in the comments :)
**Update:** Eric Koleda has a helper function you might also want to look at:
> Nice article! A while back I created a general purpose helper function called "multimap" that allows you to do this automatically and across multiple parameters:
>
> — Eric Koleda (@erickoleda) [April 26, 2019](https://twitter.com/erickoleda/status/1121834325698334721?ref_src=twsrc%5Etfw)