Runtime type checking strategies in Typescript

Introduction

Sometimes, we want to check the type of data while the program is running to avoid errors. This helps make sure the program works correctly with the right kind of data.

When receiving data from users or external sources like APIs.

When you want to make sure a function gets the right type of input before using it.

When debugging to find out what type of data is causing problems.

When working with dynamic data that can change during the program.

When you want to add extra safety checks beyond TypeScript's compile-time checks.

Syntax

Typescript

if (typeof variable === 'string') {
  // code for string
} else if (typeof variable === 'number') {
  // code for number
} else {
  // code for other types
}

Use typeof to check simple types like string, number, boolean.

For objects or arrays, use Array.isArray() or check properties manually.

Examples

Check if value is a string before using it.

Typescript

const value: unknown = 'hello';

if (typeof value === 'string') {
  console.log('It is a string:', value);
} else {
  console.log('Not a string');
}

Custom type guard function to check if a value is a number.

Typescript

function isNumber(value: unknown): value is number {
  return typeof value === 'number';
}

const input: unknown = 42;
if (isNumber(input)) {
  console.log('Number:', input);
} else {
  console.log('Not a number');
}

Check if data is an array before using array methods.

Typescript

const data: unknown = [1, 2, 3];

if (Array.isArray(data)) {
  console.log('It is an array with length:', data.length);
} else {
  console.log('Not an array');
}

Sample Program

This program checks the type of different values at runtime and prints a message based on the type.

Typescript

function checkType(value: unknown) {
  if (typeof value === 'string') {
    console.log(`String with length ${value.length}`);
  } else if (typeof value === 'number') {
    console.log(`Number with value ${value}`);
  } else if (Array.isArray(value)) {
    console.log(`Array with ${value.length} items`);
  } else {
    console.log('Unknown type');
  }
}

checkType('hello');
checkType(123);
checkType([1, 2, 3]);
checkType({});

Important Notes

Runtime checks add safety but can slow down the program if overused.

TypeScript types disappear when running the program, so runtime checks are needed for real-time safety.

Use custom type guard functions to keep code clean and reusable.

Summary

Runtime type checking helps catch wrong data types while the program runs.

Use typeof, Array.isArray(), and custom functions for checks.

It is useful when working with unknown or dynamic data sources.

Practice

1. Which of the following is the best way to check if a variable value is a string at runtime in TypeScript?

easy

A. Use value === String

B. Use value instanceof String

C. Use typeof value === 'string'

D. Use Array.isArray(value)

Solution

1. Step 1: Understand the typeof operator

   The typeof operator returns a string indicating the type of the operand. For strings, it returns 'string'.

2. Step 2: Compare with 'string'

   Checking typeof value === 'string' correctly verifies if value is a string primitive.

3. Final Answer:

   Use typeof value === 'string' -> Option C

4. Quick Check:

   typeof string = 'string' [OK]

Hint: Use typeof for primitives like string or number [OK]

Common Mistakes:

• Using instanceof for string primitives
• Confusing Array.isArray with string check
• Comparing variable directly to String constructor

2. Which syntax correctly checks if a variable data is an array at runtime in TypeScript?

easy

A. Array.isArray(data)

B. data instanceof Array

C. typeof data === 'array'

D. data.constructor === 'Array'

Solution

1. Step 1: Recall the standard array check method

   JavaScript and TypeScript provide Array.isArray() to check if a value is an array.

2. Step 2: Verify the correct usage

   Calling Array.isArray(data) returns true if data is an array, false otherwise.

3. Final Answer:

   Array.isArray(data) -> Option A

4. Quick Check:

   Array.isArray([]) = true [OK]

Hint: Use Array.isArray() to check arrays safely [OK]

Common Mistakes:

• Using typeof with 'array' (invalid)
• Using instanceof which can fail across frames
• Comparing constructor to string instead of function

3. What will be the output of this TypeScript code?

function isNumber(value: unknown): boolean {
  return typeof value === 'number';
}

console.log(isNumber('123'));
console.log(isNumber(123));

medium

A. false true

B. true true

C. true false

D. false false

Solution

1. Step 1: Check typeof for '123'

   For the string '123', typeof '123' returns 'string', so isNumber('123') returns false.

2. Step 2: Check typeof for 123

   For the number 123, typeof 123 returns 'number', so isNumber(123) returns true.

3. Final Answer:

   false true -> Option A

4. Quick Check:

   typeof '123' = string, typeof 123 = number [OK]

Hint: typeof returns 'number' only for number primitives [OK]

Common Mistakes:

• Assuming string '123' is a number
• Confusing typeof results
• Ignoring return boolean values

4. Identify the error in this runtime type check function and fix it:

function isStringArray(value: unknown): boolean {
  return typeof value === 'string' && Array.isArray(value);
}

medium

A. No error, function is correct

B. Use Array.isArray(value) only, remove typeof check

C. Check should be typeof value === 'object' instead of 'string'

D. Check should be typeof value[0] === 'string' inside array

Solution

1. Step 1: Analyze the current condition

   The function checks if value is both a string and an array, which is impossible.

2. Step 2: Correct the logic for string arrays

   We should check if value is an array, then verify its elements are strings, e.g., typeof value[0] === 'string'.

3. Final Answer:

   Check should be typeof value[0] === 'string' inside array -> Option D

4. Quick Check:

   Array with string elements needs element type check [OK]

Hint: Check array type first, then element types inside [OK]

Common Mistakes:

• Checking typeof array itself as 'string'
• Ignoring element type checks
• Assuming array is string without Array.isArray

5. You receive data of unknown type and want to safely check if it is an object with a numeric property id. Which runtime type checking strategy is best?

hard

A. Use Array.isArray(data) and check data.id

B. Check typeof data === 'object' and typeof data.id === 'number'

C. Check typeof data === 'string' and parse data.id

D. Use data instanceof Number to check id

Solution

1. Step 1: Confirm data is an object

   Use typeof data === 'object' to ensure data is an object (and not null).

2. Step 2: Check the property type

   Check typeof data.id === 'number' to confirm the id property is numeric.

3. Final Answer:

   Check typeof data === 'object' and typeof data.id === 'number' -> Option B

4. Quick Check:

   Object with numeric id needs both checks [OK]

Hint: Check object type then property type stepwise [OK]

Common Mistakes:

• Using Array.isArray for objects
• Checking string type for object
• Using instanceof Number for primitives