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TypeScriptType Narrowing & Type Guards

36 items

1

Truthiness Narrowing

Flashcard
2

Equality Narrowing

Flashcard
3

User-Defined Type Guards (is)

Flashcard
4

Assertion Functions (asserts)

Flashcard
5

Control Flow Analysis Narrowing

Flashcard
6

instanceof Type Guards

Flashcard
7

Discriminated Unions

Flashcard
8

Narrowing the unknown Type

Flashcard
9

Exhaustiveness Checking with never

Flashcard
10

Narrowing Out null and undefined

Flashcard
11

Type Assertions vs Type Guards

Flashcard
12

Array.isArray Narrowing

Flashcard
13

typeof Type Guards

Flashcard
14

Non-Null Assertion Operator (!)

Flashcard
15

in Operator Narrowing

Flashcard
16

Filter Callback Missing Type Predicate

Code Quiz
17

Helper Function Loses Narrowing

Code Quiz
18

Using unknown Without Narrowing

Code Quiz
19

Wrong Null Check Misses Undefined

Code Quiz
20

Equality Narrowing Dead Branch

Code Quiz
21

Truthiness Narrowing Drops Zero

Code Quiz
22

Optional Chaining Yields Possibly Undefined

Code Quiz
23

typeof Cannot Detect Arrays

Code Quiz
24

Reassignment Resets Narrowing

Code Quiz
25

Non-null Assertion Hides Real Bug

Code Quiz
26

Narrowing via Assignment

Quiz
27

Narrowing Loss in Closures

Quiz
28

typeof Guards for Primitives

Quiz
29

never in Exhaustiveness Checking

Quiz
30

Non-null Assertion Operator

Quiz
31

Equality Narrowing

Quiz
32

Narrowing Literal and Union Types

Quiz
33

Type Predicate Return Effect

Quiz
34

Control Flow Analysis Basics

Quiz
35

Truthiness Narrowing

Quiz
36

Array.isArray as Type Guard

Quiz
Code QuizIntermediate

typeof Cannot Detect Arrays

A typeof check misidentifies arrays instead of using Array.isArray.

Codetypescript
function process(x: string | string[]) {
  if (typeof x === "array") {
    return x.join(", ");
  } else {
    return x.toUpperCase();
  }
}

What is the bug in this code?