TypeScript compiler API basics - Time & Space Complexity

Time Complexity: TypeScript compiler API basics

O(n)

Understanding Time Complexity

When using the TypeScript compiler API, it is important to know how the time to compile grows as your code gets bigger.

We want to understand how the compiler's work increases when processing more code.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import ts from "typescript";

function compileCode(source: string) {
  const sourceFile = ts.createSourceFile("file.ts", source, ts.ScriptTarget.Latest);
  const program = ts.createProgram(["file.ts"], {});
  const checker = program.getTypeChecker();
  // Walk nodes and get types
  ts.forEachChild(sourceFile, node => {
    checker.getTypeAtLocation(node);
  });
}
    

This code creates a source file, a program, and uses the type checker to get types for each node in the source.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Walking through each node in the source file using ts.forEachChild.
• How many times: Once for each node in the source code's syntax tree.

How Execution Grows With Input

As the source code size grows, the number of syntax nodes grows roughly in proportion.

Input Size (n nodes)	Approx. Operations
10	About 10 type checks
100	About 100 type checks
1000	About 1000 type checks

Pattern observation: The work grows roughly in a straight line with the number of nodes.

Final Time Complexity

Time Complexity: O(n)

This means the compiler's work grows linearly as the source code gets bigger.

Common Mistake

[X] Wrong: "The compiler checks all types instantly, so size doesn't matter much."

[OK] Correct: Each node must be visited and checked, so more code means more work.

Interview Connect

Understanding how compiler APIs scale helps you write tools that stay fast as projects grow.

Self-Check

"What if we added nested loops to walk child nodes recursively? How would the time complexity change?"

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