03 — Equality and coercion

One line: Most JavaScript operators convert their operands before they work, and every famous surprise — [] == false, "1" + 1, null >= 0 — follows from a handful of fixed conversion rules you can run and read.

The chapter starts with the two equality operators, === and ==, and the four algorithms behind every equality check in the language. Then the conversion to boolean that every if performs, the + operator that chooses between adding and joining, and ToPrimitive, the step that turns an object into something an operator can use. It ends with < and >, which convert differently from ==, and with the explicit conversions to write instead of relying on the implicit ones.

The pages

Page	Level	In one line	Status
=== and ==	101	== converts its operands before comparing, which makes "" == 0 and "0" == 0 both true while "" == "0" is false, so == is not even transitive; === never converts, and only NaN breaks its rule that a value equals itself.	stub
Object.is, NaN and -0	201	The three equality checks that never convert types disagree only on NaN and -0: === says NaN !== NaN and 0 === -0, Object.is says the opposite of both, and includes, Map and Set match NaN but treat 0 and -0 as one.	stub
Truthy and falsy	101	Eight values are falsy in Node — false, 0, -0, 0n, "", null, undefined, NaN — and a browser adds document.all; so "0", "false", " ", [], {} and even new Boolean(false) all make an if run.	stub
The + operator	201	+ turns both operands into primitives, joins if either is a string and adds if not, left to right: 1 + 2 + "3" is "33", "1" + 2 + 3 is "123", [1, 2] + [3] is "1,23", and "3" - 1 is 2.	stub
ToPrimitive	301	When an operator needs a primitive from an object it passes a hint — "number", "string" or "default" — to Symbol.toPrimitive, or else tries valueOf then toString (a string hint reverses the order); Date treats "default" as string, so date + 1 concatenates.	stub
< and >	201	< compares two strings by UTF-16 code units, so "10" < "9" and "Z" < "a" are true, and anything else as numbers, where null becomes 0 — so null >= 0 is true although null > 0 and null == 0 are false.	stub
Converting on purpose	101	Unary +x is Number(x) with one exception — +1n throws where Number(1n) is 1; both read "" and " " as 0 and "12px" as NaN, and String(sym) works where "" + sym throws.	stub

Boundaries

Reading numbers out of text with parseInt and parseFloat is chapter 10, and ordering strings with localeCompare is chapter 9.