JavaScript Runtime Semantics

Truthiness & Control Flow

JavaScript conditionals evaluate truthiness, not only booleans. This is convenient for presence checks, but dangerous when 0, false, or an empty string are valid values.

Falsy values are a small fixed set
Truthiness is not the same as existence
?? checks nullishness only
|| falls back for any falsy value
Control flow affects TypeScript narrowing

Defaulting operators
Expression	Falls back when	Preserves 0/false/''?
value \|\| fallback	Any falsy value	No
value ?? fallback	null or undefined	Yes
condition ? a : b	Explicit condition	Depends on condition

Defaulting bug

function pageSize(input) {
  return input || 20
}

pageSize(0) // 20, maybe wrong

function pageSizeSafe(input) {
  return input ?? 20
}

pageSizeSafe(0) // 0

Use `??` when zero, false, or empty string are valid values.