Nullish coalescing with types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the nullish coalescing operator (??) do in TypeScript?

It returns the right-hand side value only if the left-hand side value is null or undefined; otherwise, it returns the left-hand side value.

intermediate

How does nullish coalescing differ from the logical OR (||) operator?

Nullish coalescing (??) only treats null and undefined as empty, while || treats any falsy value (like 0, '', false) as empty.

intermediate

Given: <pre>let value: string | null = null;<br>let result = value ?? 'default';</pre> What is the type of <code>result</code>?

The type of result is string because if value is null, the default string is used.

intermediate

Can nullish coalescing be chained with other operators in TypeScript? Give an example.

Yes. Example: <pre>let a: string | null = null;<br>let b: string | undefined = undefined;<br>let c = a ?? b ?? 'fallback';</pre> This returns the first defined and non-null value.

beginner

Why is nullish coalescing useful when working with optional or nullable types in TypeScript?

Because it safely provides a default value only when a variable is null or undefined, avoiding unintended fallback for falsy but valid values like 0 or empty string.

What will let x = 0 ?? 5; assign to x?

A null

B 0

C undefined

D 5

Since 0 is not null or undefined, the nullish coalescing operator returns 0.

Which values cause the right side of ?? to be used?

A null and undefined

B false and 0

C empty string and false

D all falsy values

Only null and undefined trigger the right side value with ??.

What is the type of let y: string | undefined = undefined; let z = y ?? 'hello';?

A never

B string | undefined

C undefined

D string

The result z is always a string because if y is undefined, 'hello' is used.

Which operator should you use to provide a default only when a value is null or undefined?

A ??

B ||

C &&

D !

The nullish coalescing operator ?? checks specifically for null or undefined.

What will let a = false ?? true; assign to a?

A true

B undefined

C false

D null

False is not null or undefined, so the left value is used.

Explain how the nullish coalescing operator (??) works with nullable types in TypeScript.

Think about when you want to keep 0 or false but still provide a fallback for null or undefined.

Describe the difference between using || and ?? when assigning default values in TypeScript.

Consider what happens if your value is 0 or false.

Practice

1.

What does the nullish coalescing operator ?? do in TypeScript?

easy

A. Throws an error if the left value is null or undefined.

B. Returns the right value if the left value is any falsy value like 0 or empty string.

C. Always returns the left value regardless of its content.

D. Returns the right value only if the left value is null or undefined.

Solution

1. Step 1: Understand nullish coalescing behavior

   The operator ?? checks if the left value is null or undefined.

2. Step 2: Determine when right value is returned

   If the left value is null or undefined, it returns the right value; otherwise, it returns the left value.

3. Final Answer:

   Returns the right value only if the left value is null or undefined. -> Option D

4. Quick Check:

   Null or undefined triggers right value [OK]

Hint: Only null or undefined triggers right side [OK]

Common Mistakes:

• Confusing with || operator that checks all falsy values
• Thinking 0 or empty string triggers right value
• Assuming it throws errors on null or undefined

2.

Which of the following is the correct syntax to use nullish coalescing in TypeScript?

const value = input ?? 'default';

easy

A. const value = input ?? 'default';

B. const value = input ??= 'default';

C. const value = input || 'default';

D. const value = input ? 'default' : input;

Solution

1. Step 1: Identify correct nullish coalescing syntax

   The operator ?? is used between two values to provide a fallback if the left is null or undefined.

2. Step 2: Check each option

   const value = input ?? 'default';: const value = input ?? 'default'; uses ?? correctly. const value = input ? 'default' : input; uses ternary, const value = input ??= 'default'; uses nullish assignment ??=, const value = input || 'default'; uses logical OR.

3. Final Answer:

   const value = input ?? 'default'; -> Option A

4. Quick Check:

   Correct nullish coalescing syntax uses ?? [OK]

Hint: Look for ?? between values, not ?: or || [OK]

Common Mistakes:

• Using || which treats 0 or empty string as false
• Confusing with ternary operator syntax
• Using invalid operators like ??=

3.

What is the output of this TypeScript code?

let name: string | null = "";
const displayName = name ?? "Guest";
console.log(displayName);

medium

A. "Guest"

B. null

C. "" (empty string)

D. undefined

Solution

1. Step 1: Check the value of name

   The variable name is an empty string "", which is NOT null or undefined.

2. Step 2: Apply nullish coalescing operator

   Since name is not null or undefined, name ?? "Guest" returns the left value, which is "" (empty string).

3. Final Answer:

   "" (empty string) -> Option C

4. Quick Check:

   Empty string is not nullish, so left value stays [OK]

Hint: Empty string is NOT nullish, so ?? returns left [OK]

Common Mistakes:

• Assuming empty string triggers right side
• Confusing ?? with || operator
• Expecting 'Guest' instead of empty string

4.

Identify the error in this TypeScript code using nullish coalescing:

let count: number | undefined = 0;
const result = count ?? 10;
console.log(result);

medium

A. Error: nullish coalescing cannot be used with numbers

B. No error; output is 0

C. Error: count cannot be 0

D. Error: must use || instead of ??

Solution

1. Step 1: Analyze the value of count

   The variable count is 0, which is NOT null or undefined.

2. Step 2: Apply nullish coalescing operator

   Since count is 0 (a valid number), count ?? 10 returns 0, no error occurs.

3. Final Answer:

   No error; output is 0 -> Option B

4. Quick Check:

   0 is not nullish, so left value returned [OK]

Hint: 0 is valid, ?? returns left, no error [OK]

Common Mistakes:

• Thinking 0 triggers right side fallback
• Believing ?? cannot be used with numbers
• Confusing ?? with || operator

5.

You have a function that accepts an optional string parameter input. You want to return input if it is not null or undefined, but if it is, return the string "No input". Which is the best way to write this using nullish coalescing?

function getInput(input?: string): string {
  // Fill in the return statement
}

hard

A. return input ?? "No input";

B. return input || "No input";

C. return input ? input : "No input";

D. return input === null || input === undefined ? "No input" : input;

Solution

1. Step 1: Understand the requirement

   Return input if it is NOT null or undefined; otherwise, return "No input".

2. Step 2: Choose the best nullish coalescing usage

   input ?? "No input" returns input if it is not nullish, else "No input". This is concise and clear.

3. Final Answer:

   return input ?? "No input"; -> Option A

4. Quick Check:

   ?? returns left unless null/undefined [OK]

Hint: Use ?? to check only null or undefined [OK]

Common Mistakes:

• Using || which treats empty string as false
• Using ternary unnecessarily
• Checking null and undefined manually instead of ??