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.