Exhaustive checking with never in Typescript - Step-by-Step Execution

Concept Flow - Exhaustive checking with never

Start with union type

↓

Switch on union cases

↓

Handle each case explicitly

↓

Use 'never' for unhandled cases

↓

Compiler error if any case missed

↓

End

This flow shows how to check all cases of a union type using a switch, and use 'never' to catch missing cases at compile time.

Execution Sample

Typescript

type Shape = 'circle' | 'square';
function area(shape: Shape) {
  switch(shape) {
    case 'circle': return 3.14 * 1 * 1;
    case 'square': return 1 * 1;
    default: const _exhaustiveCheck: never = shape;
  }
}

This code calculates area for shapes and uses 'never' in default to ensure all cases are handled.

Execution Table

Step	Input shape	Switch case matched	Action	Result / Error
1	'circle'	case 'circle'	Calculate area as 3.14 * 1 * 1	3.14
2	'square'	case 'square'	Calculate area as 1 * 1	1
3	'triangle'	default	Assign shape to never variable	Type error: 'triangle' not assignable to never

💡 Execution stops because 'triangle' is not part of Shape union, causing compile-time error at default case.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
shape	undefined	'circle'	'square'	'triangle'
_exhaustiveCheck	undefined	N/A	N/A	Error: type mismatch

Key Moments - 2 Insights

Why does assigning 'shape' to a variable of type 'never' cause an error in the default case?

Because 'never' means no value is allowed. If the default case runs, it means some case was missed. The compiler uses this to catch unhandled cases, as shown in execution_table row 3.

What happens if we add a new shape to the union but forget to handle it in the switch?

The default case will catch it by assigning to 'never', causing a compile-time error. This forces you to handle the new shape explicitly, as seen in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the result when input shape is 'circle'?

A Type error

B 1

C 3.14

D undefined

💡 Hint

Check execution_table row 1 under 'Result / Error'

At which step does the compiler error occur due to an unhandled case?

A Step 3

B Step 1

C Step 2

D No error

💡 Hint

See execution_table row 3 where default case triggers error

If we remove the default case, what happens when a new shape is added but not handled?

A Compiler error still occurs

B No error, but runtime may be wrong

C Code runs correctly

D TypeScript warns but compiles

💡 Hint

Default case with 'never' enforces compile-time check; without it, missing cases are not caught (see key_moments)

Concept Snapshot

Exhaustive checking with 'never':
- Use a switch on union types
- Handle all cases explicitly
- Add a default case assigning to a 'never' variable
- Compiler errors if any case is missed
- Helps catch bugs early by ensuring all cases handled

Full Transcript

This example shows how to use TypeScript's 'never' type to ensure all cases of a union type are handled in a switch statement. The code defines a union type 'Shape' with 'circle' and 'square'. The function 'area' switches on the shape and returns the area for each case. The default case assigns the shape to a variable of type 'never'. Since 'never' means no value is possible, if the default case runs, the compiler throws an error. This helps catch missing cases at compile time. The execution table shows the steps for inputs 'circle', 'square', and an invalid 'triangle'. The variable tracker shows how 'shape' changes and how '_exhaustiveCheck' causes an error when assigned an unexpected value. Key moments clarify why 'never' causes errors and how it helps maintain code safety. The quiz tests understanding of results, error steps, and the role of the default case. The snapshot summarizes the pattern for quick reference.