Summary - Custom type checker

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);
}

Exercise 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.

Show example 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"));

Test yourself

Which trio comes together for a mini type checker?
- getTypeChecker + writeFile + readFile
- forEachChild + isVariableDeclaration + getLineAndCharacterOfPosition
- getSourceFile + getText + console.log
- createSourceFile + JSON.stringify + writeFile
Show answer

forEachChild + isVariableDeclaration + getLineAndCharacterOfPosition — forEachChild walks the AST, isVariableDeclaration recognizes the node, and getLineAndCharacterOfPosition reports the position. Together they form a mini checker.

Node start

const start = node.____(sf);

Show answer

const start = node.getStart(sf);

Mini checker summary

ts.forEachChild

Walks the AST
ts.isVariableDeclaration

Recognizes the node kind
node.getStart

Gives the start offset
getLineAndCharacterOfPosition

Gives the line number

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