TypeScript compiler API basics - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript compiler API code?

Consider the following TypeScript code that uses the compiler API to create a source file and print its text.

import ts from 'typescript';

const source = ts.createSourceFile('test.ts', 'const x = 42;', ts.ScriptTarget.Latest, true);
console.log(source.text);

What will be printed to the console?

Typescript

import ts from 'typescript';

const source = ts.createSourceFile('test.ts', 'const x = 42;', ts.ScriptTarget.Latest, true);
console.log(source.text);

A An empty string

B SyntaxError

C const x = 42;

D undefined

💡 Hint

Think about what source.text represents in a source file object.

The createSourceFile function creates a source file object that contains the original source code text. Accessing source.text returns the exact string passed in, which is const x = 42;.

🧠 Conceptual

intermediate

Which TypeScript compiler API method creates a program from source files?

You want to analyze multiple TypeScript files together. Which method from the compiler API should you use to create a program that represents all these files?

A ts.transpileModule

B ts.createSourceFile

C ts.createPrinter

D ts.createProgram

💡 Hint

Think about which method handles multiple files and type checking.

ts.createProgram creates a program that includes multiple source files and provides type checking and semantic analysis. createSourceFile is for single files, transpileModule is for quick transpilation, and createPrinter is for printing AST nodes.

🔧 Debug

advanced

What error does this TypeScript compiler API code produce?

Examine this code snippet:

import ts from 'typescript';

const source = ts.createSourceFile('test.ts', 'let a = 1;', ts.ScriptTarget.Latest, true);
const printer = ts.createPrinter();

const result = printer.printNode(ts.EmitHint.Unspecified, source, source);
console.log(result);

What error or output will this code produce?

Typescript

import ts from 'typescript';

const source = ts.createSourceFile('test.ts', 'let a = 1;', ts.ScriptTarget.Latest, true);
const printer = ts.createPrinter();

const result = printer.printNode(ts.EmitHint.Unspecified, source, source);
console.log(result);

A TypeError: printNode expects a Statement node, not SourceFile

B Prints the entire source code: 'let a = 1;'

C SyntaxError: Argument of type 'SourceFile' is not assignable to parameter of type 'Node'

D Prints an empty string

💡 Hint

Check what printNode accepts as arguments and what SourceFile is.

printNode can print any node, including the root SourceFile. So printing the source file node outputs the original source code text. No error occurs.

📝 Syntax

advanced

Which option correctly creates a TypeScript source file with strict mode enabled?

You want to create a source file with the text "let y = 10;" and specify the script target as ES2020 with strict mode enabled. Which code snippet is correct?

A ts.createSourceFile('file.ts', 'let y = 10;', ts.ScriptTarget.ES2020, true);

B ts.createSourceFile('file.ts', 'let y = 10;', ts.ScriptTarget.ES2020, strict=true);

C ts.createSourceFile('file.ts', 'let y = 10;', ts.ScriptTarget.ES2020, {strict: true});

D ts.createSourceFile('file.ts', 'let y = 10;', ts.ScriptTarget.ES2020);

💡 Hint

Check the parameters of createSourceFile in the TypeScript compiler API.

The fourth parameter of createSourceFile is a boolean indicating whether to set the source file as 'strict'. Passing true enables strict mode. Options objects or named parameters are not accepted here.

🚀 Application

expert

How many diagnostics are reported by this TypeScript program?

Given these two TypeScript files:

// file1.ts
const a: number = 'hello';

// file2.ts
const b: string = 123;

And this code to create a program and get diagnostics:

import ts from 'typescript';

const program = ts.createProgram(['file1.ts', 'file2.ts'], {});
const diagnostics = ts.getPreEmitDiagnostics(program);
console.log(diagnostics.length);

How many diagnostics will be printed?

A 2

B 0

C 1

D 4

💡 Hint

Each file has one type error. Think about how many errors the compiler reports.

Both files have one type mismatch error each. The compiler reports one diagnostic per error, so total diagnostics are 2.