Runtime type checking strategies in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a variable is a string at runtime.

Typescript

function isString(value: unknown): boolean {
  return typeof value === [1];
}

Drag options to blanks, or click blank then click option'

A"string"

B"number"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' or 'boolean' instead of 'string' in the typeof check.

Forgetting to use quotes around the type string.

Use typeof value === "string" to check if the value is a string at runtime.

2fill in blank

medium

Complete the code to check if a value is an array at runtime.

Typescript

function isArray(value: unknown): boolean {
  return Array.[1](value);
}

Drag options to blanks, or click blank then click option'

AisObject

Bfrom

Cof

DisArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array.of or Array.from which create arrays but don't check types.

Using a non-existent method like isObject.

Use Array.isArray(value) to check if the value is an array at runtime.

3fill in blank

hard

Fix the error in the code to check if a value is a number at runtime.

Typescript

function isNumber(value: unknown): boolean {
  return typeof value === [1];
}

Drag options to blanks, or click blank then click option'

Anumber

B'number'

C"number"

DNumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using the bare word number without quotes causes a syntax error.

Using the Number object instead of the string type name.

The typeof operator returns a string, so the type must be in quotes like "number".

4fill in blank

hard

Fill both blanks to create a runtime check for an object that is not null.

Typescript

function isObject(value: unknown): boolean {
  return typeof value === [1] && value !== [2];
}

Drag options to blanks, or click blank then click option'

A"object"

Bnull

C"null"

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing value !== "null" instead of null.

Not checking for null and getting false positives.

Check that typeof value === "object" and that value !== null because null is also of type "object" in JavaScript.

5fill in blank

hard

Fill all three blanks to create a runtime check for a string array.

Typescript

function isStringArray(value: unknown): boolean {
  return Array.[1](value) && value.every(item => typeof item === [2] && item !== [3]);
}

Drag options to blanks, or click blank then click option'

AisArray

B"string"

Cnull

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using typeof item === 'object' instead of 'string'.

Not checking for null values inside the array.

Use Array.isArray(value) to check if it's an array, then check every item is a string and not null.