Tuple with fixed length and types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a tuple in TypeScript?

A tuple is a fixed-length array where each element has a specific type. It allows you to store multiple values of different types in a single variable.

beginner

How do you declare a tuple with a fixed length of 3 elements: a string, a number, and a boolean?

You declare it like this: <pre>let myTuple: [string, number, boolean];</pre> This means the tuple must have exactly three elements in that order and type.

intermediate

Can you change the length of a tuple after it is declared?

No, tuples have a fixed length defined by their type. You cannot add or remove elements beyond the declared length without causing a type error.

beginner

What happens if you try to assign a wrong type to a tuple element?

TypeScript will show a type error because each position in the tuple expects a specific type. For example, assigning a string to a number position is not allowed.

beginner

Example: How to access the second element of a tuple [string, number, boolean]?

You access it by index like an array:

myTuple[1]

which will give you the number value because indexes start at 0.

What does the tuple type [string, number] mean?

A A function returning string and number

B An array with exactly two elements: first a string, second a number

C An object with string and number properties

D An array with any number of strings and numbers

A tuple [string, number] means exactly two elements: first a string, second a number.

Can you assign ["hello", 42, true] to a variable typed as [string, number, boolean]?

A No, because booleans are not allowed

B No, because the length is wrong

C Yes, but only if you use type assertion

D Yes, because the types and order match

The tuple [string, number, boolean] matches the value ["hello", 42, true] exactly in type and order.

What happens if you try to push an extra element to a tuple?

A TypeScript will give a type error

B The tuple will accept it without error

C The tuple will convert to an array

D The extra element will be ignored

Tuples have fixed length, so pushing an extra element causes a type error.

How do you declare a tuple with two elements: a boolean and a string?

A let t: [boolean, string];

B let t: (boolean | string)[];

C let t: boolean & string;

D let t: {boolean, string};

The correct tuple syntax is [boolean, string] for fixed length and types.

Which index accesses the first element of a tuple?

A 1

B -1

C 0

D It depends on the tuple

Tuple elements are accessed by zero-based index, so the first element is at index 0.

Explain what a tuple with fixed length and types is in TypeScript and why it is useful.

Think about a small box with fixed slots for different items.

How do you declare and use a tuple with three elements: string, number, and boolean? Give an example.

Remember tuples are like arrays but with fixed types and length.

Practice

1. What is a tuple in TypeScript?

easy

A. A fixed-length array with specific types for each element

B. A variable-length array with any types

C. A function that returns multiple values

D. A type of object with dynamic keys

Solution

1. Step 1: Understand tuple definition

   A tuple is an array with a fixed number of elements where each element has a specific type.

2. Step 2: Compare with other options

   Options A, B, and C describe other concepts like objects, variable arrays, or functions, not tuples.

3. Final Answer:

   A fixed-length array with specific types for each element -> Option A

4. Quick Check:

   Tuple = fixed length + specific types [OK]

Hint: Remember: tuple = fixed length + fixed types [OK]

Common Mistakes:

• Thinking tuples can have variable length
• Confusing tuples with regular arrays
• Assuming tuples are objects

2. Which of the following is the correct syntax to declare a tuple with a string and a number in TypeScript?

easy

A. let tuple: [string; number] = ['hello', 5];

B. let tuple: (string, number) = ['hello', 5];

C. let tuple: {string, number} = ['hello', 5];

D. let tuple: [string, number] = ['hello', 5];

Solution

1. Step 1: Recall tuple syntax

   Tuples use square brackets with types separated by commas: [type1, type2].

2. Step 2: Check each option

   let tuple: [string, number] = ['hello', 5]; uses correct syntax. Options B, C, and D use invalid tuple syntax.

3. Final Answer:

   let tuple: [string, number] = ['hello', 5]; -> Option D

4. Quick Check:

   Tuple syntax = square brackets with comma-separated types [OK]

Hint: Use square brackets with commas for tuple types [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using curly braces instead of brackets
• Using semicolons instead of commas

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

let data: [string, number] = ['age', 30];
console.log(data[0]);
console.log(data[1]);

medium

A. 30\nage

B. undefined\nundefined

C. age\n30

D. Error: Type mismatch

Solution

1. Step 1: Understand tuple assignment

   The tuple 'data' is assigned ['age', 30], so data[0] is 'age' and data[1] is 30.

2. Step 2: Check console.log outputs

   console.log(data[0]) prints 'age', console.log(data[1]) prints 30, so output is two lines: 'age' then '30'.

3. Final Answer:

   age 30 -> Option C

4. Quick Check:

   Tuple elements print in order: string then number [OK]

Hint: Tuple elements print in order by index [OK]

Common Mistakes:

• Swapping the order of elements
• Expecting an error due to types
• Confusing tuple with object properties

4. Identify the error in this TypeScript tuple declaration:

let info: [string, number] = ['John', '25'];

medium

A. The second element should be a number, not a string

B. The tuple length is incorrect

C. The first element should be a number

D. No error, this is valid

Solution

1. Step 1: Check tuple types

   The tuple expects [string, number], so first element string 'John' is correct, second element must be a number.

2. Step 2: Identify type mismatch

   The second element is '25' (a string), which mismatches the expected number type.

3. Final Answer:

   The second element should be a number, not a string -> Option A

4. Quick Check:

   Tuple types must match exactly [OK]

Hint: Check each tuple element type carefully [OK]

Common Mistakes:

• Ignoring type mismatch in tuple elements
• Assuming string '25' is a number
• Thinking tuple length is wrong

5. Given the tuple type [string, number, boolean], which of the following assignments is valid in TypeScript?

hard

A. let t: [string, number, boolean] = ['hello', true, 42];

B. let t: [string, number, boolean] = ['hello', 42, true];

C. let t: [string, number, boolean] = ['hello', 42];

D. let t: [string, number, boolean] = ['hello', 42, 'true'];

Solution

1. Step 1: Check tuple length and types

   The tuple requires exactly three elements: string, number, boolean in that order.

2. Step 2: Evaluate each option

   let t: [string, number, boolean] = ['hello', 42, true]; matches all types and length correctly. let t: [string, number, boolean] = ['hello', true, 42]; swaps number and boolean types. let t: [string, number, boolean] = ['hello', 42]; misses the third element. let t: [string, number, boolean] = ['hello', 42, 'true']; uses a string instead of boolean.

3. Final Answer:

   let t: [string, number, boolean] = ['hello', 42, true]; -> Option B

4. Quick Check:

   Tuple must match length and types exactly [OK]

Hint: Match tuple length and types exactly in order [OK]

Common Mistakes:

• Changing order of types
• Omitting elements
• Using wrong types for elements