TypeScript compiler API basics

Introduction

The TypeScript compiler API lets you work with TypeScript code inside your programs. It helps you read, check, and change TypeScript code automatically.

You want to build a tool that checks your TypeScript code for errors.

You want to create a program that changes TypeScript code automatically.

You want to analyze TypeScript code to understand how it works.

You want to make a custom code formatter or linter for TypeScript.

You want to convert TypeScript code into another format or language.

Syntax

Typescript

import * as ts from 'typescript';

const sourceCode = `let x: number = 5;`;
const sourceFile = ts.createSourceFile('example.ts', sourceCode, ts.ScriptTarget.Latest);

// Use the sourceFile object to read or change the code

Use ts.createSourceFile to turn code text into a structure you can work with.

The sourceFile object holds the code in a way the API understands.

Examples

This example creates a source file and prints how many statements it has.

Typescript

import * as ts from 'typescript';

const code = `const a = 10;`;
const file = ts.createSourceFile('file.ts', code, ts.ScriptTarget.Latest);
console.log(file.statements.length);

This example prints the type of each statement in the code.

Typescript

import * as ts from 'typescript';

const code = `function greet() { console.log('Hi'); }`;
const file = ts.createSourceFile('greet.ts', code, ts.ScriptTarget.Latest);

file.statements.forEach(stmt => {
  console.log(ts.SyntaxKind[stmt.kind]);
});

Sample Program

This program reads a simple TypeScript code string, counts how many statements it has, and prints the type of each statement.

Typescript

import * as ts from 'typescript';

const code = `let message: string = 'Hello, world!';`;
const sourceFile = ts.createSourceFile('hello.ts', code, ts.ScriptTarget.Latest);

console.log('Number of statements:', sourceFile.statements.length);

sourceFile.statements.forEach(statement => {
  console.log('Statement kind:', ts.SyntaxKind[statement.kind]);
});

Important Notes

The TypeScript compiler API uses SyntaxKind to name parts of the code.

Working with the API can be tricky at first, but it lets you do powerful things with code.

Summary

The TypeScript compiler API helps you read and change TypeScript code inside programs.

You start by creating a SourceFile from code text.

You can then explore and work with the code using the API's tools.

Practice

1.

What is the primary purpose of the TypeScript compiler API?

SourceFile is a key part of it. What does it help you do?

easy

A. Run TypeScript code directly in the browser

B. Read and modify TypeScript code programmatically

C. Convert TypeScript to JavaScript manually

D. Create user interfaces with TypeScript

Solution

1. Step 1: Understand the role of the compiler API

   The TypeScript compiler API is designed to let programs read and change TypeScript code, not to run it or build UI.

2. Step 2: Identify what SourceFile represents

   SourceFile is an object representing the code text, allowing exploration and modification.

3. Final Answer:

   Read and modify TypeScript code programmatically -> Option B

4. Quick Check:

   Compiler API = Code reading and modification [OK]

Hint: Remember: compiler API reads and changes code, not runs it [OK]

Common Mistakes:

• Thinking it runs TypeScript code directly
• Confusing it with UI libraries
• Assuming it manually converts code without automation

2.

Which of the following is the correct way to create a SourceFile using the TypeScript compiler API?

const sourceFile = ts.createSourceFile(____);

easy

A. (codeText, ts.ScriptTarget.Latest)

B. ('file.ts', codeText)

C. (codeText, 'file.ts', ts.ScriptTarget.Latest)

D. ('file.ts', codeText, ts.ScriptTarget.Latest)

Solution

1. Step 1: Recall the createSourceFile signature

   The method requires filename, source text, and script target in that order.

2. Step 2: Match the correct argument order

   ('file.ts', codeText, ts.ScriptTarget.Latest) correctly uses ('file.ts', codeText, ts.ScriptTarget.Latest).

3. Final Answer:

   ('file.ts', codeText, ts.ScriptTarget.Latest) -> Option D

4. Quick Check:

   Filename, text, target order = A [OK]

Hint: Filename first, then code text, then script target [OK]

Common Mistakes:

• Swapping filename and code text
• Omitting the script target argument
• Passing arguments in wrong order

3.

Given this code snippet using the TypeScript compiler API:

const sourceFile = ts.createSourceFile('test.ts', 'let x = 5;', ts.ScriptTarget.Latest);
const firstStatement = sourceFile.statements[0];
console.log(firstStatement.kind);

What will be printed?

medium

A. SyntaxKind.VariableStatement

B. 5

C. SyntaxKind.Identifier

D. undefined

Solution

1. Step 1: Understand what sourceFile.statements[0] is

   The first statement is let x = 5;, which is a variable declaration statement.

2. Step 2: Identify the kind of this statement

   Its kind is SyntaxKind.VariableStatement, a numeric enum value representing variable statements.

3. Final Answer:

   SyntaxKind.VariableStatement -> Option A

4. Quick Check:

   First statement kind = VariableStatement [OK]

Hint: Statements array holds top-level code pieces; kind shows their type [OK]

Common Mistakes:

• Expecting the value 5 to be printed
• Confusing statement kind with identifier kind
• Assuming undefined because of missing imports

4.

Consider this code snippet:

const sourceFile = ts.createSourceFile('file.ts', 'const a = 10;', ts.ScriptTarget.Latest);
const firstStatement = sourceFile.statements[0];
console.log(firstStatement.name.text);

What is the error in this code?

medium

A. firstStatement has no property name

B. sourceFile is not created correctly

C. text is not a valid property

D. No error, code runs fine

Solution

1. Step 1: Check the type of firstStatement

   firstStatement is a VariableStatement node, which does not have a direct name property.

2. Step 2: Understand how to access variable name

   The variable name is inside firstStatement.declarationList.declarations[0].name, not directly on firstStatement.

3. Final Answer:

   firstStatement has no property name -> Option A

4. Quick Check:

   VariableStatement lacks direct name property [OK]

Hint: Variable names are nested inside declarations, not on statement directly [OK]

Common Mistakes:

• Accessing name directly on VariableStatement
• Assuming sourceFile creation is wrong
• Confusing property names on nodes

5.

You want to write a function using the TypeScript compiler API that extracts all variable names from a given source code string. Which approach is best?

1. Create a SourceFile from the code string.
2. Traverse its statements and declarations.
3. Collect names from variable declarations.

Which code snippet correctly starts this process?

function getVariableNames(code: string): string[] {
  const sourceFile = ts.createSourceFile('temp.ts', code, ts.ScriptTarget.Latest);
  const names: string[] = [];
  sourceFile.statements.forEach(statement => {
    if (ts.isVariableStatement(statement)) {
      statement.declarationList.declarations.forEach(decl => {
        if (ts.isIdentifier(decl.name)) {
          names.push(decl.name.text);
        }
      });
    }
  });
  return names;
}

hard

A. Incorrect because it uses forEach on a non-iterable

B. Fails because it does not parse the code into a SourceFile

C. Correctly traverses variable declarations and extracts names

D. Wrong because it pushes declaration nodes instead of names

Solution

1. Step 1: Verify SourceFile creation

   The code correctly creates a SourceFile from the input string.

2. Step 2: Check traversal and extraction logic

   It loops over statements, filters variable statements, then loops declarations to extract identifier names properly.

3. Final Answer:

   Correctly traverses variable declarations and extracts names -> Option C

4. Quick Check:

   Traversal and extraction logic is correct [OK]

Hint: Traverse statements, check variable statements, then extract names [OK]

Common Mistakes:

• Not creating SourceFile before traversal
• Trying to loop on non-iterable objects
• Pushing nodes instead of text names