Union type syntax and behavior in Typescript

Introduction

Union types let you say a value can be one of several types. This helps your program accept different kinds of inputs safely.

When a function can accept either a string or a number as input.

When a variable might hold a value that can be a boolean or a string.

When you want to allow multiple types for a parameter but still keep type safety.

When reading data that might come in different formats, like a string or an array.

Syntax

Typescript

let variableName: Type1 | Type2 | Type3;

The vertical bar | means "or" between types.

You can combine as many types as you want using |.

Examples

This means value can be a string or a number.

Typescript

let value: string | number;

This function accepts an id that can be a number or a string.

Typescript

function printId(id: number | string) {
  console.log(`ID: ${id}`);
}

This variable can only be one of these three string values.

Typescript

let status: 'success' | 'error' | 'loading';

Sample Program

This program shows how to handle a value that can be a string or a number. It checks the type and uses the right method.

Typescript

function formatInput(input: string | number) {
  if (typeof input === 'string') {
    return `String: ${input.toUpperCase()}`;
  } else {
    return `Number: ${input.toFixed(2)}`;
  }
}

console.log(formatInput('hello'));
console.log(formatInput(42));

Important Notes

When using union types, you often need to check the type before using type-specific methods.

TypeScript helps catch errors if you try to use a method that doesn't exist on all types in the union.

Summary

Union types let variables hold values of different types safely.

Use the | symbol to separate types in a union.

Always check the type before using type-specific features.

Practice

1. What does a union type in TypeScript allow a variable to do?

easy

A. Hold values of multiple specified types safely

B. Hold only one fixed type value

C. Automatically convert types at runtime

D. Restrict values to a single primitive type

Solution

1. Step 1: Understand union type purpose

   A union type lets a variable accept values from multiple specified types, not just one.

2. Step 2: Compare options with definition

   Only Hold values of multiple specified types safely correctly describes this behavior; others describe fixed or automatic conversions which union types do not do.

3. Final Answer:

   Hold values of multiple specified types safely -> Option A

4. Quick Check:

   Union type = multiple types allowed [OK]

Hint: Union types use | to allow multiple types [OK]

Common Mistakes:

• Thinking union types convert values automatically
• Believing union types restrict to one type only
• Confusing union with intersection types

2. Which of the following is the correct syntax to declare a variable value that can be either a string or a number?

easy

A. let value: string | number;

B. let value: string & number;

C. let value: string, number;

D. let value: string | number[];

Solution

1. Step 1: Recall union type syntax

   Union types use the pipe symbol | between types without parentheses needed.

2. Step 2: Evaluate each option

   let value: string | number; uses correct syntax: string | number. let value: string & number; uses & which means intersection, not union. let value: string, number; uses comma which is invalid. let value: string | number[]; uses number[] which is an array of numbers, not a single number.

3. Final Answer:

   let value: string | number; -> Option A

4. Quick Check:

   Union syntax uses | without commas [OK]

Hint: Use | between types, no commas or & for union [OK]

Common Mistakes:

• Using & instead of | for union types
• Separating types with commas
• Using number[] instead of number

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

function printId(id: string | number) {
  if (typeof id === 'string') {
    console.log('ID is a string: ' + id.toUpperCase());
  } else {
    console.log('ID is a number: ' + id.toFixed(2));
  }
}
printId(42);

medium

A. ID is a string: 42

B. ID is a number: 42

C. ID is a number: 42.00

D. Runtime error due to wrong method

Solution

1. Step 1: Analyze type check and method calls

   The function checks if id is a string or number. For number, it calls toFixed(2) which formats number with 2 decimals.

2. Step 2: Apply input and output

   Input is 42 (number), so else branch runs and prints 'ID is a number: 42.00'.

3. Final Answer:

   ID is a number: 42.00 -> Option C

4. Quick Check:

   Number uses toFixed(2) = 42.00 [OK]

Hint: Check typeof before calling type-specific methods [OK]

Common Mistakes:

• Calling string methods on numbers without check
• Expecting no decimal places in toFixed output
• Ignoring type narrowing with typeof

4. Identify the error in this TypeScript code using union types:

function formatInput(input: string | number) {
  return input.toUpperCase();
}

medium

A. No error, code works fine

B. Error: toUpperCase() not available on number

C. Error: Missing return type annotation

D. Error: Union types cannot be used in functions

Solution

1. Step 1: Check method usage on union type

   Method toUpperCase() exists on string but not on number.

2. Step 2: Understand type safety with union

   Without checking type, calling toUpperCase() on input causes error because input might be number.

3. Final Answer:

   Error: toUpperCase() not available on number -> Option B

4. Quick Check:

   Call methods only after type check [OK]

Hint: Always check type before calling type-specific methods [OK]

Common Mistakes:

• Calling string methods on union without type check
• Assuming union types allow all methods
• Ignoring compiler errors about method availability

5. Given this TypeScript code, what will be the output?

type Result = string | null;
function process(value: Result) {
  return value?.toUpperCase() ?? 'No value';
}
console.log(process(null));
console.log(process('hello'));

hard

A. No value\nhello

B. null\nhello

C. Error: Cannot call toUpperCase on null

D. No value\nHELLO

Solution

1. Step 1: Understand optional chaining and nullish coalescing

   The expression value?.toUpperCase() calls toUpperCase() only if value is not null or undefined. Otherwise, it returns undefined.

2. Step 2: Apply nullish coalescing fallback

   If the left side is undefined, ?? 'No value' returns 'No value'. So for null, output is 'No value'. For 'hello', it returns 'HELLO'.

3. Final Answer:

   No value HELLO -> Option D

4. Quick Check:

   Optional chaining + ?? handles null safely [OK]

Hint: Use ?. and ?? to safely handle null in unions [OK]

Common Mistakes:

• Expecting error when calling method on null
• Ignoring optional chaining usage
• Confusing null and undefined handling