TypeScript compiler API basics - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the TypeScript compiler API.

Typescript

import * as ts from '[1]';

Drag options to blanks, or click blank then click option'

Atypescript

Btsc

Ctypescript-api

Dts

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'tsc' or 'ts' as the package name instead of 'typescript'.

Trying to import from a non-existent package.

The TypeScript compiler API is imported from the 'typescript' package.

2fill in blank

medium

Complete the code to create a TypeScript program from source files.

Typescript

const program = ts.createProgram({ rootNames: ['file.ts'], [1]: {} });

Drag options to blanks, or click blank then click option'

Aconfig

Boptions

Csettings

DcompilerOptions

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'options' or 'config' instead of 'compilerOptions'.

Omitting compiler options entirely.

The property to specify compiler options is 'compilerOptions'.

3fill in blank

hard

Fix the error in the code to get the source file from the program.

Typescript

const sourceFile = program.getSourceFile([1]);

Drag options to blanks, or click blank then click option'

A0

Bundefined

C'file.ts'

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a number or null instead of the file name string.

Passing undefined which returns undefined.

The method getSourceFile expects the file name as a string.

4fill in blank

hard

Fill both blanks to create a printer and print the source file.

Typescript

const printer = ts.createPrinter([1]);
const result = printer.printFile([2]);

Drag options to blanks, or click blank then click option'

A{}

BsourceFile

Cnull

D{ removeComments: true }

Attempts:

3 left

💡 Hint

Common Mistakes

Passing null instead of an options object.

Passing a file name string instead of the source file object to printFile.

The printer can be created with options like removing comments, and printFile needs the source file object.

5fill in blank

hard

Fill all three blanks to create a diagnostic reporter and report errors.

Typescript

const diagnostics = program.getSemanticDiagnostics();
diagnostics.forEach(diagnostic => {
  const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, [1]);
  console.[2](`Error in file: ${diagnostic.file?.fileName} - ${message}`);
  if (diagnostic.start !== undefined) {
    const { line, character } = diagnostic.file!.getLineAndCharacterOfPosition([3]);
    console.log(`At line ${line + 1}, character ${character + 1}`);
  }
});

Drag options to blanks, or click blank then click option'

A2

Berror

Cdiagnostic.start

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.log instead of console.error for errors.

Passing wrong arguments to flattenDiagnosticMessageText.

Using diagnostic.file instead of diagnostic.start for position.

The flattenDiagnosticMessageText needs a newline character, console.error is used for errors, and getLineAndCharacterOfPosition needs the diagnostic start position.

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