Runtime type checking strategies in Typescript - Step-by-Step Execution

Concept Flow - Runtime type checking strategies

Start: Receive input

↓

Check type at runtime

↓

Primitive

↓

Validate

↓

If valid: proceed

↓

If invalid: throw error or handle

↓

End

The program receives input, checks its type at runtime using different strategies for primitives, arrays, and objects, then proceeds if valid or handles errors if invalid.

Execution Sample

Typescript

function isString(value: any): boolean {
  return typeof value === 'string';
}

const input = "hello";
if (isString(input)) {
  console.log("Valid string");
}

This code checks if the input is a string at runtime and prints a message if it is.

Execution Table

Step	Action	Input Value	Type Check Result	Next Step
1	Receive input	"hello"	N/A	Proceed to type check
2	Check if input is string	"hello"	true	Print 'Valid string'
3	Print output	N/A	N/A	End execution

💡 Input passes the string type check, so execution ends after printing.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
input	undefined	"hello"	"hello"	"hello"
isString(input)	N/A	N/A	true	true

Key Moments - 2 Insights

Why do we need to check types at runtime if TypeScript already has types?

TypeScript types are erased at compile time, so runtime checks ensure the actual data matches expected types during execution, as shown in Step 2 of the execution_table.

What happens if the type check fails?

If the check returns false, the program should handle the error or reject the input. This is implied after Step 2 if the result was false, preventing further execution.

Visual Quiz - 3 Questions

Look at the execution_table, what is the type check result at Step 2?

A true

B false

C undefined

D null

💡 Hint

Check the 'Type Check Result' column in Step 2 of the execution_table.

At which step does the program print 'Valid string'?

A Step 1

B Step 2

C Step 3

D No printing occurs

💡 Hint

Look at the 'Action' column for printing in the execution_table.

If the input was a number instead of a string, what would change in the execution_table?

A Type Check Result at Step 2 would be false

B Input value at Step 1 would be 'true'

C Program would print 'Valid string' anyway

D No change in execution_table

💡 Hint

Refer to the 'Type Check Result' column and consider how type checking works.

Concept Snapshot

Runtime type checking in TypeScript:
- Use functions to check types at runtime (e.g., typeof, Array.isArray)
- Validate primitives, arrays, and objects differently
- Proceed if check passes, else handle errors
- Necessary because TypeScript types are erased at runtime
- Helps ensure data safety during execution

Full Transcript

This visual execution shows how runtime type checking works in TypeScript. The program starts by receiving input, then checks its type using a function like isString. If the input is a string, it prints a confirmation message. Variables like input and the result of isString are tracked step-by-step. Key moments include understanding why runtime checks are needed despite TypeScript's static typing and what happens if a check fails. The quizzes test understanding of the type check result, when output occurs, and how the table changes with different inputs.