Study notes · 15% of the exam

Types and Values

Know the primitive types, how equality and truthiness work, and the handful of quirks every interviewer asks about.

Key points

1
There are seven primitive types (string, number, bigint, boolean, undefined, symbol, null); everything else is an object, including arrays and functions.
2
=== compares without type conversion; == converts first. Use === by default, except x == null as shorthand for null-or-undefined.
3
The falsy values are false, 0, -0, 0n, "", null, undefined and NaN. Every object is truthy, even [] and {}.
4
typeof null is "object" (a historical bug) and NaN !== NaN. Test with value === null and Number.isNaN(value).
5
Numbers are IEEE 754 doubles, so 0.1 + 0.2 !== 0.3. Compare with a tolerance, or work in integers (such as cents).

The global isNaN('abc') is true because it coerces; Number.isNaN('abc') is false.
Object.is(0, -0) is false even though 0 === -0 is true.

Ten questions, with the answer and explanation after each one.