Type predicates in practice in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a type predicate function that checks if a value is a string.

Typescript

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

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

A"object"

B"number"

C"boolean"

D"string"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' without quotes causes a syntax error.

Checking for 'number' instead of 'string' returns wrong results.

The typeof operator returns a string describing the type. To check for strings, use "string".

2fill in blank

medium

Complete the code to use the type predicate function in a conditional statement.

Typescript

function printLength(value: unknown) {
  if ([1](value)) {
    console.log(value.length);
  } else {
    console.log("Not a string");
  }
}

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

AisNumber

BisString

CisBoolean

DisObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong predicate function like isNumber causes errors.

Not calling the function with parentheses causes syntax errors.

The function isString is the type predicate that checks if value is a string.

3fill in blank

hard

Fix the error in the type predicate function that checks if a value is an array of numbers.

Typescript

function isNumberArray(value: unknown): value is number[] {
  return Array.isArray(value) && value.every(item => typeof item === [1]);
}

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

A"number"

B"string"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for "string" instead of "number" causes wrong results.

Not using quotes around the type string causes syntax errors.

Each item in the array must be checked to be of type number using typeof item === "number".

4fill in blank

hard

Fill both blanks to create a type predicate that checks if a value is an object with a 'name' property of type string.

Typescript

function hasNameProperty(value: unknown): value is [1] {
  return typeof value === [2] && value !== null && 'name' in value && typeof (value as any).name === 'string';
}

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

Aobject

B"string"

C"object"

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the type in the first blank causes errors.

Using 'object' without quotes in the second blank causes wrong runtime checks.

The type predicate returns true if value is an object (TypeScript type) and the runtime typeof is "object".

5fill in blank

hard

Fill all three blanks to create a type predicate that checks if a value is a function with a 'length' property greater than 1.

Typescript

function isFunctionWithLength(value: unknown): value is [1] {
  return typeof value === [2] && value !== null && (value as Function).length [3] 1;
}

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

AFunction

B"function"

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'function' without quotes in the second blank causes errors.

Using '<' instead of '>' in the third blank causes wrong logic.

The type predicate checks if value is a Function type, the runtime typeof is "function", and the function's length property is greater than 1.