Diagnostic production
- TypeScript course
- Expert
- Lesson 11/35
- 4 min
Learn to produce your own diagnostics.
Position information
When a rule violation is found, you report it with position info. node.getStart(sourceFile) gives the node's start offset; ts.getLineAndCharacterOfPosition(sf, offset) returns a { line, character } object. line is 0-indexed.
const start = node.getStart(sf);
const { line } = ts.getLineAndCharacterOfPosition(sf, start);
console.log(`line ${line + 1}: error`);Collecting errors
To collect errors at once you use an array. On each violation you push text with errors.push(...), and at the end you print with errors.join("\n"). This pattern mirrors TypeScript's own diagnostics mechanism.
const errors: string[] = [];
// on violation:
errors.push(`line ${line + 1}: message`);
console.log(errors.join("\n"));Practice Report any types
Create a source with three variables, each on its own line: let a: any = 1;, let b: number = 2; and let c: any = 3;. Write a visit function that finds VariableDeclaration nodes whose type annotation is any and collects an error with its line number into a list. Walk with ts.forEachChild and print one error per line.
Starter code
import * as ts from "typescript";
// Üç satırlık kaynak: let a: any = 1; / let b: number = 2; / let c: any = 3;
// SourceFile oluştur
// visit fonksiyonu yaz: any türü olan VariableDeclaration bul
// Satır numarasıyla hatayı topla
// forEachChild ile gez
// Hataları her satırda bir tane olarak yazdır
// kodunu buraya yazSample solution
import ts from "typescript";
const sourceText = 'let a: any = 1;\nlet b: number = 2;\nlet c: any = 3;';
const sf = ts.createSourceFile("app.ts", sourceText, ts.ScriptTarget.Latest, true);
const errors: string[] = [];
function visit(node: ts.Node) {
if (ts.isVariableDeclaration(node) && node.type?.kind === ts.SyntaxKind.AnyKeyword) {
const { line } = ts.getLineAndCharacterOfPosition(sf, node.getStart(sf));
errors.push(`line ${line + 1}: any`);
}
ts.forEachChild(node, visit);
}
visit(sf);
console.log(errors.join("\n"));Expected output
line 1: any line 3: any
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does ts.getLineAndCharacterOfPosition(sf, offset) return?
- A { line, character } object Correct answer
- Only the line number
- An offset number
- A Type object
Explanation: getLineAndCharacterOfPosition returns a { line, character } object where line is 0-indexed. It does not return a Type, a single line number or an offset.
2 Get the line number
const start = node.getStart(sf);
const pos = ts.____(sf, start);
console.log(pos.line + 1);const start = node.getStart(sf);
const pos = ts.getLineAndCharacterOfPosition(sf, start);
console.log(pos.line + 1);Explanation: ts.getLineAndCharacterOfPosition returns a { line, character } object; getLineOfPosition, getPosition and getLineNumber do not exist.
3 Error reporting parts
| Term | Meaning |
|---|---|
node.getStart(sf) |
Gives the start offset |
getLineAndCharacterOfPosition |
Gives line and character |
errors.push |
Adds the error to the list |
errors.join |
Turns the list into text |