# Extension Quick Start

## Function

This page starts with a `calc` compute extension example, demonstrating how to create the extension development directory, build the `.bts` package, install it into the project and call it in `main.bt`. Users with no basic knowledge are recommended to run through the process according to this page first, and then read [Operating Mechanism ](/en/docs/extensions/mechanism).

## Syntax

 quick start will use these commands:

```text
bt ext new calc
bt ext build calc -o calc.bts
bt ext check calc.bts
bt ext info calc.bts
bt ext install calc.bts project
bt main.bt
```

## Parameters

| Parameter | Description |
| ------ | ------ |
| `calc` | Extended development directory name, will also be used as the default extension. |
| `calc.bts` | Build output expansion pack file. |
| `project` | The target BT project directory to install the extension. |
| `main.bt` | Project entry script. |

## Prepare the project

 First prepare a common BT project directory:

```text
project/
└── main.bt
```

`main.bt` to call the extension:

```bt
result = calc(1).add(2).value()

// Output: 3
print result
```

 The `calc` extension has not been installed at this time. If you run it directly, you will not find `calc`. The next step is to create the extension.

## Create an extension

 and execute:

```text
bt ext new calc
```

 This will generate a pure BT extension development directory:

```text
calc/
├── manifest.json
├── bindings.json
└── src/
    └── lib.bt
```

 The default scaffolding already contains a chained calculation object: `calc(1).add(2).value()`.

## Build the extension package

 and execute it outside the `calc` directory: When building,

```text
bt ext build calc -o calc.bts
```

 will read `manifest.json` and `bindings.json`, check the entry source code, and package the development directory into `calc.bts`. After

 is built, you can check the package information first:

```text
bt ext check calc.bts
bt ext info calc.bts
```

## Install into the project

 Install the extension into the project directory:

```text
bt ext install calc.bts project
```

 The structure after installation is:

```text
project/
├── main.bt
└── extensions/
    └── calc.bts
```

 Then run the project in the `project` directory:

```text
bt main.bt
```

 The output should be:

```text
3
```

## Return Value

`bt ext` When the command is successful, the creation, build, check or installation results will be output in the terminal; when it fails, a Chinese error will be output. `calc(1).add(2).value()` in the example script returns the integer `3`.

## Code Examples

 extension object can continue to be called in the chain:

```bt
num = calc(10)
value = num.add(5).add(2).value()
closed = num.close()

// Output: [17,true]
print [value, closed]
```

## Notes

- `bt ext new calc` will generate an extension name based on the directory name. The directory name must use lowercase letters, numbers and underscores, and start with a lowercase letter.
- `bt ext install calc.bts project` will copy the package to `project/extensions/calc.bts`; extensions with the same name will be overwritten.
- The `.bts` package under `extensions/` will be loaded only when the project is started. The development directory `calc/` will not be automatically loaded by the project.
- If the BT build used for running does not enable the `extensions` feature, projects that exist in the `extensions/` directory will report an error indicating that the extension capability is not available.