closure-builder/third_party/closure-templates/documentation/concepts/functions-plugins.md

Simple Closure, Soy and JavaScript Build system

# Functions

[TOC]

## What is a `Closure Templates` function?

Similar to other programing languages, `Closure Templates` functions are a way to
perform particular tasks in your `Closure Templates` templates. Examples include
checking the length of a list, basic number operations such as rounding and
flooring, etc.

Since `Closure Templates` is a template language, functions are *not*
defined/declared in templates. Built-in functions are included in the compiler,
and custom functions (aka, plugins) can be implemented by developers in multiple
backends.

## What does a function look like?

Functions are called from within expressions, which perform operations, and are
evaluated for their results or side-effects (or both). Expressions can be used
pretty much anywhere within a template. (`Closure Templates`'s rules for parsing
and evaluating expressions are actually quite complicated, but the majority of
places where you'd want them to work, they just do.)

For example:

```soy
{param myStringLength: (strLen($myString)) /}
```

The above param finds the length of `$myString`, and assigns it to a new
parameter `$myStringLength`.

## What types of functions are there?

There are two main types of functions in `Closure Templates`, ones included in the
compiler, simply known as functions, and custom functions known as plugins.

See [reference](../reference/functions) for full list of built-in functions.

### Plugins

A plugin is a custom `Closure Templates` function that is hard-coded in the
compiler. Developers can write plugins for any additional functions, print
directives, or other message file formats. (Print directives are post-processing
on the output of a print command.)

Developers implement the plugin outside of the `Closure Templates` codebase, and
`Closure Templates` provides an API to connect the implementations so that the
custom functions can be used in templates. To learn how to setup a plugin, see
[the dev guide](../dev/plugins) section for details.

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