Truthiness narrowing in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is truthiness narrowing in TypeScript?

Truthiness narrowing is when TypeScript uses checks like if or ! to refine the type of a variable based on whether it is truthy or falsy.

beginner

Which values are considered falsy in JavaScript and TypeScript?

The falsy values are: false, 0, '' (empty string), null, undefined, and NaN. Everything else is truthy.

beginner

How does TypeScript narrow the type of a variable in this code?<br><pre>let value: string | null = getValue();
if (value) {
  // What is the type of value here?
}</pre>

Inside the if (value) block, TypeScript narrows value to string because null is falsy and filtered out.

intermediate

What happens to the type of a variable after a if (!variable) check?

After if (!variable), TypeScript narrows the variable's type to only the falsy types it can be, excluding truthy ones.

beginner

Why is truthiness narrowing useful in TypeScript?

It helps catch errors early by letting TypeScript know when a variable cannot be null or undefined, so you can safely use it without extra checks.

Which of these values is truthy in TypeScript?

A "hello"

B "" (empty string)

C null

D 0

Only non-empty strings like "hello" are truthy. 0, empty string, and null are falsy.

Given let x: string | null;, what is the type of x inside if (x) block?

A string | null

B string

C null

D never

Inside the if block, null is excluded because it is falsy, so x is narrowed to string.

What does if (!value) check for in TypeScript?

A value is falsy

B value is a string

C value is truthy

D value is a number

The ! operator negates truthiness, so it checks if value is falsy.

Which of these is NOT a falsy value in TypeScript?

A undefined

B NaN

C "false" (string)

D 0

The string "false" is truthy because it is a non-empty string.

Why does TypeScript use truthiness narrowing?

A To convert all variables to boolean

B To make variables immutable

C To remove all null values from code

D To help safely use variables by refining their types based on runtime checks

Truthiness narrowing helps TypeScript know when variables are definitely not null or undefined, improving safety.

Explain how TypeScript uses truthiness narrowing to refine variable types in conditional statements.

Think about how an if statement filters out falsy values.

List the common falsy values in TypeScript and explain why knowing them helps with truthiness narrowing.

Recall the basic falsy values from JavaScript.