What types exist only at compile time in Typescript - Step-by-Step Execution

Concept Flow - What types exist only at compile time

Write TypeScript code with types

↓

Compiler checks types

↓

Types removed during compilation

↓

Output JavaScript without types

↓

Run JavaScript code at runtime

TypeScript types exist only during compilation to check code correctness and are removed before running JavaScript.

Execution Sample

Typescript

type User = { name: string; age: number };

function greet(user: User) {
  return `Hello, ${user.name}`;
}

const person = { name: "Alice", age: 30 };
greet(person);

Defines a type for a user object and uses it to check the function argument at compile time.

Execution Table

Step	Action	Type Check	Result
1	Define type User	User type created	No runtime code generated
2	Define function greet with User parameter	Parameter checked against User type	Function code generated (types erased)
3	Create object person with name and age	Object matches User type	Object created at runtime
4	Call greet(person)	Argument matches User type	Function runs, returns greeting string
5	Compile to JavaScript	Types removed	Output JS has no types
6	Run JavaScript code	No type checks	Outputs: Hello, Alice

💡 Compilation ends with types removed; runtime runs plain JavaScript without types

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
User	Type defined	Type exists only at compile time	Removed after compilation	No runtime presence
person	Undefined	{ name: 'Alice', age: 30 }	{ name: 'Alice', age: 30 }	Same object at runtime
greet	Function defined with type	Function exists	Returns 'Hello, Alice'	Function runs at runtime

Key Moments - 3 Insights

Why don't types appear in the final JavaScript code?

Types are only for compile-time checks and are removed during compilation, as shown in execution_table step 5.

Can types affect runtime behavior?

No, types do not exist at runtime and cannot change how the JavaScript runs, confirmed by step 6 in execution_table.

What happens if an object doesn't match the type?

The compiler shows an error before generating JavaScript, preventing runtime errors, as implied in step 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what happens to the types?

A They are converted to runtime checks

B They are removed from the output JavaScript

C They become variables in JavaScript

D They are ignored but stay in the code

💡 Hint

Check the 'Result' column in step 5 of execution_table

According to variable_tracker, what is the state of 'User' after compilation?

A It is removed and does not exist at runtime

B It becomes a JavaScript function

C It exists as a runtime object

D It is converted to a string

💡 Hint

Look at the 'User' row in variable_tracker under 'Final'

If the object 'person' did not match the 'User' type, what would happen during compilation?

A The types would be ignored and code runs

B The code would run but with warnings

C The compiler would show an error and stop

D The runtime would throw an error

💡 Hint

Refer to key_moments about type mismatch and execution_table steps 2 and 3

Concept Snapshot

TypeScript types exist only at compile time.
They help check code correctness before running.
Types are removed when compiling to JavaScript.
No types or checks remain at runtime.
Errors happen during compilation if types don't match.

Full Transcript

In TypeScript, types like 'User' exist only during compilation to check code correctness. When you write a function with typed parameters, the compiler verifies the arguments match the types. These types do not produce any JavaScript code and are removed during compilation. The final JavaScript runs without any type information or checks. If an object does not match the expected type, the compiler shows an error and stops before generating JavaScript. This ensures safer code without affecting runtime performance.