> For the complete documentation index, see [llms.txt](https://radlohead.gitbook.io/typescript-deep-dive/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://radlohead.gitbook.io/typescript-deep-dive/overview/program.md).

# Program

Defined in `program.ts`. The compilation context ([a concept we covered previously](/typescript-deep-dive/project/compilation-context.md)) is represented within the TypeScript compiler as a `Program`. It consists of `SourceFile`s and compiler options.

## Usage of `CompilerHost`

Its interaction mechanism with the OE:

`Program` *-uses->* `CompilerHost` *-uses->* `System`

The reason for having a `CompilerHost` as a point of indirection is that it allows its interface to be more finely tuned for `Program` needs and not bother with OE needs (e.g. the `Program` doesn't care about `fileExists` a function provided by `System`).

There are other users of `System` as well (e.g. tests).

## SourceFile

The program provides an API to get the Source Files `getSourceFiles(): SourceFile[];`. Each is represented as a root-level node for an AST (called `SourceFile`).