Null and undefined types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between null and undefined in TypeScript?

null means a variable explicitly has no value. undefined means a variable has been declared but not assigned a value yet.

beginner

How do you declare a variable that can hold either a string or null in TypeScript?

<p>Use a union type like <code>let name: string | null;</code> to allow <code>name</code> to be a string or <code>null</code>.</p>

intermediate

What happens if you try to access a property on undefined in TypeScript?

You get a runtime error because undefined has no properties. TypeScript helps catch this with strict null checks.

intermediate

What is the effect of enabling strictNullChecks in TypeScript?

It forces you to explicitly handle null and undefined values, preventing accidental errors by requiring checks or unions.

beginner

How can you safely access a property of an object that might be null or undefined?

Use the optional chaining operator ?., for example: obj?.property. This returns undefined if obj is null or undefined without error.

In TypeScript, what does undefined mean?

A A variable explicitly has no value

B A variable is a number zero

C A variable has been declared but not assigned a value

D A variable is a boolean false

undefined means a variable exists but has not been given a value yet.

Which TypeScript setting helps catch errors with null and undefined?

A strictNullChecks

B noImplicitAny

C esModuleInterop

D allowJs

strictNullChecks forces explicit handling of null and undefined.

What does the optional chaining operator ?. do?

A Converts null to undefined

B Accesses a property only if the object is not null or undefined

C Throws an error if the object is null

D Assigns a default value if the property is missing

?. safely accesses properties without errors if the object is null or undefined.

How do you declare a variable that can be a string or null?

A let x: string | null;

B let x: string & null;

C let x: string?

D let x: null | undefined;

Use a union type with | to allow multiple types.

What error occurs if you try to access a property on undefined?

A No error

B Syntax error

C Compile error

D Runtime error

Accessing properties on undefined causes a runtime error.

Explain the difference between null and undefined in TypeScript and how to handle them safely.

Think about how variables get these values and how TypeScript helps avoid errors.

Describe how enabling strictNullChecks changes the way you write TypeScript code.

Consider what happens if you don't check for null or undefined.

Practice

1. What does null represent in TypeScript?

easy

A. A variable that has not been declared

B. A variable that holds a number zero

C. A variable that is still loading

D. A variable that has no value assigned

Solution

1. Step 1: Understand the meaning of null

   null means a variable explicitly has no value.

2. Step 2: Compare with other options

   The incorrect options describe different states or values, not null.

3. Final Answer:

   A variable that has no value assigned -> Option D

4. Quick Check:

   null means no value [OK]

Hint: Remember: null means no value assigned [OK]

Common Mistakes:

• Confusing null with undefined
• Thinking null means variable is undeclared
• Assuming null means zero or empty string

2. Which of the following is the correct way to declare a variable that can hold a string or null in TypeScript?

easy

A. let name: string | null;

B. let name: string & null;

C. let name: string or null;

D. let name: string? null;

Solution

1. Step 1: Recall union type syntax

   In TypeScript, the pipe | is used to allow multiple types.

2. Step 2: Check each option

   let name: string | null; uses | correctly. The other options use invalid syntax.

3. Final Answer:

   let name: string | null; -> Option A

4. Quick Check:

   Union types use | symbol [OK]

Hint: Use | to combine types like string | null [OK]

Common Mistakes:

• Using & instead of | for union types
• Writing 'or' instead of |
• Using invalid syntax like ? null

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

let value: number | undefined;
console.log(value);

medium

A. undefined

B. null

C. 0

D. Error: Variable not initialized

Solution

1. Step 1: Understand variable initialization

   The variable value is declared but not assigned, so it defaults to undefined.

2. Step 2: Check console.log output

   Logging value prints undefined because it has no assigned value.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   Unassigned variable = undefined [OK]

Hint: Uninitialized variables print undefined [OK]

Common Mistakes:

• Expecting null instead of undefined
• Assuming default is zero
• Thinking it causes an error

4. Identify the error in this TypeScript code:

let age: number;
age = null;

medium

A. No error, code is correct

B. Variable age is not declared

C. Cannot assign null to a variable of type number

D. Missing semicolon after declaration

Solution

1. Step 1: Check variable type and assignment

   age is declared as number only, so it cannot hold null.

2. Step 2: Understand TypeScript strict null checks

   By default, null is not assignable to number unless union type includes null.

3. Final Answer:

   Cannot assign null to a variable of type number -> Option C

4. Quick Check:

   Assigning null to number causes error [OK]

Hint: Add | null to allow null assignment [OK]

Common Mistakes:

• Ignoring TypeScript strict null checks
• Thinking null is allowed by default
• Confusing missing semicolon as error

5. How can you declare a function parameter in TypeScript that accepts either a string, null, or undefined?

hard

A. function greet(name?: string & null) { console.log(name); }

B. function greet(name: string | null | undefined) { console.log(name); }

C. function greet(name: string & null & undefined) { console.log(name); }

D. function greet(name: string | null?) { console.log(name); }

Solution

1. Step 1: Understand union types for parameters

   To accept multiple types, use | to combine them.

2. Step 2: Analyze each option

   function greet(name: string | null | undefined) { console.log(name); } correctly uses string | null | undefined. function greet(name: string & null & undefined) { console.log(name); } uses & which means intersection (not correct). function greet(name?: string & null) { console.log(name); } uses & which means intersection (not correct). function greet(name: string | null?) { console.log(name); } has invalid syntax.

3. Final Answer:

   function greet(name: string | null | undefined) { console.log(name); } -> Option B

4. Quick Check:

   Use | to allow multiple types in parameters [OK]

Hint: Use | to combine string, null, and undefined types [OK]

Common Mistakes:

• Using & instead of | for union types
• Confusing optional parameter with undefined union
• Writing invalid syntax like null?