Index signatures for dynamic keys in Typescript - Mini Project: Build & Apply

Index Signatures for Dynamic Keys

📖 Scenario: You are building a simple app to store the number of items sold for different products in a store. The product names are not fixed and can change dynamically.

🎯 Goal: Create a TypeScript object that can hold product names as keys and their sold quantities as values using index signatures. Then, add some products and their quantities, and finally print the total quantity sold.

📋 What You'll Learn

Create an interface called SalesRecord with an index signature for string keys and number values.

Create a variable called sales of type SalesRecord.

Add three products with exact names and quantities: 'apple': 10, 'banana': 5, 'orange': 8.

Create a variable called totalSold to hold the sum of all quantities.

Use a for...in loop to iterate over sales and sum the quantities into totalSold.

Print the total quantity sold using console.log.

💡 Why This Matters

🌍 Real World

Stores and inventory systems often need to track quantities of products where product names can change or be added dynamically.

💼 Career

Understanding index signatures helps in building flexible TypeScript applications that handle dynamic data structures, common in web development and backend services.

1

Create the SalesRecord interface

Create an interface called SalesRecord with an index signature that allows string keys and number values.

Typescript

// Your code here

Hint

Use [key: string]: number; inside the interface to allow dynamic keys.

2

Create the sales object with products

Create a variable called sales of type SalesRecord and assign it an object with these exact entries: 'apple': 10, 'banana': 5, 'orange': 8.

Typescript

interface SalesRecord {
  [productName: string]: number;
}

// Your code here

Hint

Use const sales: SalesRecord = { ... } to create the object with the exact keys and values.

3

Calculate total quantity sold

Create a variable called totalSold and set it to 0. Then use a for...in loop with variable product to iterate over sales and add each quantity to totalSold.

Typescript

interface SalesRecord {
  [productName: string]: number;
}

const sales: SalesRecord = {
  apple: 10,
  banana: 5,
  orange: 8
};

// Your code here

Hint

Initialize totalSold to 0. Use for (const product in sales) to loop through keys and add sales[product] to totalSold.

4

Print the total quantity sold

Use console.log to print the value of totalSold.

Typescript

interface SalesRecord {
  [productName: string]: number;
}

const sales: SalesRecord = {
  apple: 10,
  banana: 5,
  orange: 8
};

let totalSold = 0;
for (const product in sales) {
  totalSold += sales[product];
}

// Your code here

Hint

Use console.log(totalSold); to display the total quantity sold.

Practice

1. What is the main purpose of an index signature in TypeScript when working with objects?

easy

A. To create arrays with fixed length

B. To define fixed keys with different types for each key

C. To restrict objects to only one key-value pair

D. To allow objects to have dynamic keys with values of a specific type

Solution

1. Step 1: Understand index signatures

   Index signatures let you define objects that can have any number of keys, but all keys must have values of the same type.

2. Step 2: Compare options

   To allow objects to have dynamic keys with values of a specific type correctly describes this purpose. Options B, C, and D describe other unrelated concepts.

3. Final Answer:

   To allow objects to have dynamic keys with values of a specific type -> Option D

4. Quick Check:

   Index signature = dynamic keys with uniform value type [OK]

Hint: Index signatures mean dynamic keys with same value type [OK]

Common Mistakes:

• Thinking index signatures fix keys instead of allowing dynamic keys
• Confusing index signatures with tuple or array types
• Assuming index signatures restrict to one key-value pair

2. Which of the following is the correct syntax for an index signature allowing string keys and number values in TypeScript?

easy

A. { [key: string]: number }

B. { key: string; value: number }

C. { [key: number]: string }

D. { key: number; value: string }

Solution

1. Step 1: Recall index signature syntax

   The syntax is { [key: KeyType]: ValueType }. For string keys and number values, it is { [key: string]: number }.

2. Step 2: Check options

   { [key: string]: number } matches the correct syntax. { [key: number]: string } reverses key and value types. Options B and D are object properties, not index signatures.

3. Final Answer:

   { [key: string]: number } -> Option A

4. Quick Check:

   Index signature syntax = {[key: string]: number} [OK]

Hint: Index signature syntax: {[key: KeyType]: ValueType} [OK]

Common Mistakes:

• Swapping key and value types in the signature
• Using object property syntax instead of index signature
• Using number keys when string keys are intended

3. Given the following TypeScript code, what will be the output of console.log(userScores['alice'])?

interface Scores {
  [username: string]: number;
}
const userScores: Scores = {
  alice: 10,
  bob: 15
};
console.log(userScores['alice']);

medium

A. 10

B. alice

C. undefined

D. 15

Solution

1. Step 1: Understand the object and index signature

   The interface allows any string key with number values. The object has keys 'alice' and 'bob' with values 10 and 15.

2. Step 2: Access the value for key 'alice'

   Accessing userScores['alice'] returns 10, the value assigned to 'alice'.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   userScores['alice'] = 10 [OK]

Hint: Access dynamic key value directly with bracket notation [OK]

Common Mistakes:

• Confusing key with value and expecting 'alice' as output
• Assuming undefined for existing keys
• Mixing up values of different keys

4. Identify the error in the following TypeScript code using an index signature:

interface Data {
  [key: string]: number;
}
const data: Data = {
  id: 1,
  name: "John"
};

medium

A. The interface should use [key: number] instead

B. The value of 'name' is a string, but should be a number

C. Index signatures cannot use string keys

D. The key 'id' should be a number, not a string

Solution

1. Step 1: Check index signature type

   The interface says all keys (strings) must have number values.

2. Step 2: Check object values

   The 'id' key has value 1 (number), which is correct. The 'name' key has value "John" (string), which violates the number value rule.

3. Final Answer:

   The value of 'name' is a string, but should be a number -> Option B

4. Quick Check:

   Value types must match index signature [OK]

Hint: All values must match index signature type [OK]

Common Mistakes:

• Thinking keys must be numbers instead of strings
• Ignoring value type mismatch errors
• Assuming index signatures allow mixed value types

5. You want to create a TypeScript object to store user preferences where keys are user IDs (numbers) and values are objects with dynamic string keys and boolean values. Which of the following interface definitions correctly models this?

hard

A. interface Preferences { [userId: string]: { [prefKey: number]: boolean } }

B. interface Preferences { userId: number; prefKey: string; value: boolean }

C. interface Preferences { [userId: number]: { [prefKey: string]: boolean } }

D. interface Preferences { [userId: number]: { prefKey: string; value: boolean } }

Solution

1. Step 1: Understand the requirement

   We need an object with number keys (user IDs) and values that are objects with string keys and boolean values.

2. Step 2: Match interface to requirement

   interface Preferences { [userId: number]: { [prefKey: string]: boolean } } uses an index signature for userId as number keys, and nested index signature for preference keys as strings with boolean values, matching the requirement exactly.

3. Final Answer:

   interface Preferences { [userId: number]: { [prefKey: string]: boolean } } -> Option C

4. Quick Check:

   Nested index signatures for dynamic keys [OK]

Hint: Use nested index signatures for nested dynamic keys [OK]

Common Mistakes:

• Using string keys for userId instead of number
• Confusing property syntax with index signatures
• Not nesting index signatures for inner dynamic keys