Script SpringsISSUE #7 of 29

data types · primitives vs objects

SparkyVSNaN the Trickster

Sparky saysJS has primitives (string, number, boolean, null, undefined, bigint, symbol) and objects (everything else).

Seven primitive types hold single values and are copied by value. Everything else — arrays, functions, dates, plain objects — is an object, copied and compared by reference. typeof reveals a value's type (with the famous quirk that typeof null is "object").

Power-ups you unlock

Primitives: string, number, boolean, null, undefined, bigint, symbol
Objects (arrays, functions, …) are reference types
Primitives copy by value; objects by reference
typeof null === "object" (a historic bug)

NaN the Trickster attacks — common mistakes

Expecting object equality by value (it’s by reference)
Confusing null and undefined
Forgetting typeof null quirk

Boss battleLog typeof for a string, number, array, function, and null — note the surprises.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre>
<script>const t=[typeof 'a',typeof 1,typeof [],typeof (()=>{}),typeof null];
document.getElementById('o').textContent = t.join('\n');</script></body></html>

▶ Open the interactive comic issue

‹ Hoisting · What Runs Before What Numbers · Int · Float · NaN · Infinity ›