Codebase Services TextChanges

Text Changes

The majority of this file is devoted to a class called the ChangeTracker. This class is nearly always created via ChangeTracker.with where you would give it a context object.

Here is an example context object:

{
  cancellationToken:CancellationTokenObject {cancellationToken: TestCancellationToken}
  errorCode:2304
  formatContext:Object {options: Object, getRule: }
  host:NativeLanguageServiceHost {cancellationToken: TestCancellationToken, settings: Object, sys: System, …}
  preferences:Object {}
  program:Object {getRootFileNames: , getSourceFile: , getSourceFileByPath: , …}
  sourceFile:SourceFileObject {pos: 0, end: 7, flags: 65536, …}
  span:Object {start: 0, length: 6}
}

You only really see ChangeTrack in use within the codefixes and refactors given that the other case where TypeScript emits files is a single operation of emission.

The change tracker keeps track of individual changes to be applied to a file. There are currently four main APIs that it works with: type Change = ReplaceWithSingleNode | ReplaceWithMultipleNodes | RemoveNode | ChangeText;

The ChangeTrack class is then used to provide high level API to describe the sort of changes you might want to make, which eventually fall into one of the four categories above.

Making Changes

The end result of using a ChangeTrack object is an array of FileTextChanges objects. The ChangeTrack.with function lets you work with a tracker instance elsewhere and passes back the ChangeTrack objects.

The core work in generating changes occurs in:

• getTextChangesFromChanges
• computeNewText
• getFormattedTextOfNode

Going from an AST node to text is done by creating a printer in getNonformattedText. The printer returns an unformatted node, which is then ran through [a formatter][./formatting.md] and the raw string substitution is done in [applyChanges][9].

Changes look like this:

[{ fileName: "/b.js", textChanges: [{ span: { start: 0, length: 0 }, newText: "// @ts-ignore\n" }] }];

Writing

newFileChanges handles passing the set of