Runtime type checking strategies in Typescript - Mini Project: Build & Apply

Runtime type checking strategies

📋 What You'll Learn

💡 Why This Matters

🌍 Real World

When working with data from APIs or user input, runtime type checking helps prevent errors by verifying data shapes before using them.

💼 Career

Many jobs require validating external data safely. Knowing runtime type checking strategies in TypeScript is useful for frontend and backend development.

1

Create the user data object

Create a constant object called userData with these exact properties and values: name as "Alice" (string), age as 30 (number), and isMember as true (boolean).

Typescript

// Create the userData object with name, age, and isMember properties
// Your code here

Hint

Use const userData = { name: "Alice", age: 30, isMember: true };

2

Define expected property names

Create a constant array called expectedProps containing the strings "name", "age", and "isMember".

Typescript

const userData = { name: "Alice", age: 30, isMember: true };
// Create expectedProps array with "name", "age", "isMember"
// Your code here

Hint

Use const expectedProps = ["name", "age", "isMember"];

3

Write a runtime type check function

Write a function called isValidUser that takes a parameter obj of type any and returns true if obj has all properties in expectedProps with types string for name, number for age, and boolean for isMember. Otherwise, return false. Use typeof checks inside the function.

Typescript

const userData = { name: "Alice", age: 30, isMember: true };
const expectedProps = ["name", "age", "isMember"];
// Write isValidUser function to check property existence and types
// Your code here

Hint

Check each property with typeof obj.property === "type" and combine with &&.

4

Print the runtime type check result

Write a console.log statement that prints the result of calling isValidUser(userData).

Typescript

const userData = { name: "Alice", age: 30, isMember: true };
const expectedProps = ["name", "age", "isMember"];

function isValidUser(obj: any): boolean {
    return typeof obj.name === "string" &&
           typeof obj.age === "number" &&
           typeof obj.isMember === "boolean";
}
// Print the result of isValidUser(userData)
// Your code here

Hint

Use console.log(isValidUser(userData)); to show the check result.

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