TypeScript compiler API basics - Step-by-Step Execution

Concept Flow - TypeScript compiler API basics

Start: Load TypeScript API

↓

Create Program with files

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Get Source Files

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Parse and Analyze AST

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Perform Type Checking

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Emit JavaScript Output

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End

The TypeScript compiler API flow starts by loading the API, creating a program with source files, parsing and analyzing the code, checking types, and finally emitting JavaScript output.

Execution Sample

Typescript

import * as ts from 'typescript';

const program = ts.createProgram(['example.ts'], {});
const sourceFile = program.getSourceFile('example.ts');

console.log(sourceFile?.statements.length);

This code loads the TypeScript API, creates a program for 'example.ts', gets its source file, and logs the number of top-level statements.

Execution Table

Step	Action	Input/Condition	Result/Output
1	Import TypeScript API	ts module	ts object available
2	Create Program	Files: ['example.ts'], Options: {}	Program object created with files
3	Get Source File	Filename: 'example.ts'	SourceFile object or undefined
4	Access Statements	sourceFile?.statements	Array of statements in source file
5	Log statements length	sourceFile?.statements.length	Number of top-level statements printed
6	End	No more actions	Program ends

💡 All steps complete; program ends after logging statement count.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
ts	undefined	ts module imported	ts module imported	ts module imported	ts module imported
program	undefined	Program object created	Program object created	Program object created	Program object created
sourceFile	undefined	undefined	SourceFile object or undefined	SourceFile object or undefined	SourceFile object or undefined
sourceFile?.statements.length	undefined	undefined	undefined	Number of statements	Number of statements

Key Moments - 2 Insights

Why might sourceFile be undefined after calling getSourceFile?

If the filename is not included in the program's files or the file does not exist, getSourceFile returns undefined, as shown in step 3 of the execution_table.

What does sourceFile?.statements.length represent?

It represents the number of top-level statements in the TypeScript source file, as accessed in step 4 and logged in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of step 3 (Get Source File)?

A A SourceFile object or undefined

B An array of statements

C A Program object

D A number representing statement count

💡 Hint

Check the 'Result/Output' column for step 3 in the execution_table.

At which step is the number of top-level statements logged?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the action 'Log statements length' in the execution_table.

If 'example.ts' is missing, how does the variable 'sourceFile' change after step 3?

A It becomes a Program object

B It becomes undefined

C It contains an empty array

D It throws an error

💡 Hint

Refer to key_moments explanation about sourceFile being undefined if file is missing.

Concept Snapshot

TypeScript compiler API basics:
- Import 'typescript' module
- Create a Program with source files
- Get SourceFile objects
- Access AST nodes like statements
- Use API to analyze or emit code
- Handle undefined if file missing

Full Transcript

This visual execution shows how to use the TypeScript compiler API. First, the API module is imported. Then, a Program is created with a list of source files. Next, a SourceFile object is retrieved for a specific file. The code accesses the statements array from the SourceFile, which represents top-level code parts. Finally, it logs the number of statements. If the file is missing, the SourceFile is undefined. This step-by-step trace helps beginners see how the API loads and processes TypeScript code.