Summary - Custom type checker
- TypeScript course
- Expert
- Lesson 12/35
- 4 min
Write a custom rule checker.
Mini type checker
A mini type checker combines three pieces: walk the AST with ts.forEachChild, recognize nodes you care about via guards like ts.isVariableDeclaration, and report violations with ts.getLineAndCharacterOfPosition. Collect errors in an array and print them at the end.
function visit(node: ts.Node) {
if (ts.isVariableDeclaration(node) && !node.type) {
const { line } = ts.getLineAndCharacterOfPosition(sf, node.getStart(sf));
errors.push(`line ${line + 1}: missing type`);
}
ts.forEachChild(node, visit);
}Practice Report untyped variables
Create a source with three variables, each on its own line: let x = 1;, let y: string = "a"; and let z = true;. Write a visit function that finds VariableDeclaration nodes without a type annotation and collects an error in the form line N: missing type with its line number. Walk with ts.forEachChild and print one error per line.
Starter code
import * as ts from "typescript";
// Üç satırlık kaynak: let x = 1; / let y: string = "a"; / let z = true;
// SourceFile oluştur
// visit fonksiyonu yaz: Tür eklemesi olmayan VariableDeclaration bul
// Satır numarasıyla line N: missing type biçiminde topla
// forEachChild ile gez
// Hataları her satırda bir tane olarak yazdır
// kodunu buraya yazSample solution
import ts from "typescript";
const sourceText = 'let x = 1;\nlet y: string = "a";\nlet z = true;';
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) {
const { line } = ts.getLineAndCharacterOfPosition(sf, node.getStart(sf));
errors.push(`line ${line + 1}: missing type`);
}
ts.forEachChild(node, visit);
}
visit(sf);
console.log(errors.join("\n"));Expected output
line 1: missing type line 3: missing type
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which trio comes together for a mini type checker?
- getTypeChecker + writeFile + readFile
- forEachChild + isVariableDeclaration + getLineAndCharacterOfPosition Correct answer
- getSourceFile + getText + console.log
- createSourceFile + JSON.stringify + writeFile
Explanation: forEachChild walks the AST, isVariableDeclaration recognizes the node, and getLineAndCharacterOfPosition reports the position. Together they form a mini checker.
2 Node start
const start = node.____(sf);const start = node.getStart(sf);Explanation: node.getStart(sf) returns the node's start offset; getOffset, getPos and getStartPosition do not exist on Node.
3 Mini checker summary
| Term | Meaning |
|---|---|
ts.forEachChild |
Walks the AST |
ts.isVariableDeclaration |
Recognizes the node kind |
node.getStart |
Gives the start offset |
getLineAndCharacterOfPosition |
Gives the line number |