---
title: Diagnostics
---

## Diagnostics

Diagnostics (compile errors) can be retrieved on the AST or on source files:

```typescript
const diagnostics = ast.getDiagnostics();
const preEmitDiagnostics = ast.getPreEmitDiagnostics();

// or on a source file
const sourceFileDiagnostics = sourceFile.getDiagnostics();
const sourceFilePreEmitDiagnostics = sourceFile.getPreEmitDiagnostics();
```

### Diagnostic

#### Message text

Returned message text could be a `string` or a `DiagnosticMessageChain`:

```typescript
const message = diagnostic.getMessageText();
```

#### Source file

Source file the diagnostic occurs in:

```typescript
const sourceFile = diagnostic.getSourceFile(); // returns: SourceFile | undefined
```

#### Start & length

Position in the file and length of the diagnostic:

```typescript
const start = diagnostic.getStart();   // returns: number
const length = diagnostic.getLength(); // returns: number
```

#### Category

Categories can be warnings, errors, or just messages.

```typescript
const category = diagnostic.getCategory(); // returns: ts.DiagnosticCategory
```

#### Code

This is the error code number:

```typescript
const code = diagnostic.getCode(); // returns: number
```

#### Source

todo: I don't know what this is, but it's available to get from the diagnostic.

```typescript
const source = diagnostic.getSource(); // returns: string | undefined
```

### DiagnosticMessageChain

A diagnostic message chain (DMC) will be returned by `diagnostic.getMessageText()` in certain scenarios.

According to the typescript compiler:

```
/**
* A linked list of formatted diagnostic messages to be used as part of a multiline message.
* It is built from the bottom up, leaving the head to be the "main" diagnostic.
* While it seems that DiagnosticMessageChain is structurally similar to DiagnosticMessage,
* the difference is that messages are all preformatted in DMC.
*/
```

The properties of a DMC are similar to a Diagnostic:

```typescript
const messageText = dmc.getMessageText(); // returns: string
const category = dmc.getCategory();       // returns: ts.DiagnosticCategory
const code = dmc.getCode();               // returns: number
```

#### Next DMC in linked list

Call `.getNext()`:

```typescript
const next = dmc.getNext(); // returns: DiagnosticMessageChain | undefined
```