The typeof operator and its surprises — interview question

What does typeof return for each kind of value? Which results surprise people, and how do you reliably check for an array or null?

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Model answer. typeof returns a string:

'string', 'number', 'bigint', 'boolean', 'symbol', 'undefined' for those primitives
'function' for functions (including classes)
'object' for everything else

The surprises:

typeof null === 'object' (a bug from the first version of the language)
typeof [] === 'object', because arrays are objects
typeof NaN === 'number'

Reliable checks:

JavaScript

Array.isArray(value);          // arrays
value === null;                // null
Number.isNaN(value);           // NaN (unlike the global isNaN, no coercion)
value instanceof Date;         // a specific class

typeof is also the only safe way to test a name that may not be declared at all: typeof someGlobal === 'undefined' does not throw.

Original UniCircle question. Last reviewed 22 Oct 2026.