02 — Values and types

One line: JavaScript has eight types — seven immutable primitives and the object — and most surprises about values come from the line between them: typeof null, wrapper objects, and assignment that copies a primitive but shares an object.

The chapter starts with the eight types and the operator that names them, typeof, including the answer it gets wrong. Then the two empty values, null and undefined. Next come the wrapper objects that let a primitive have methods, and the two newer primitives, symbols and BigInt, each with rules of its own. It ends with what assignment copies, because that is where primitives and objects part ways.

Every lesson ends with a kata whose solution is a program CI runs. The last three pages are the chapter's companions: the errors a learner meets with these values, each with its fix; the lints that catch the chapter's mistakes, and the ones no lint catches; and resources — the books by chapter, the documentation, and the claims in them that fail when run.

The pages

Page	Level	In one line	Status
Eight types	101	Every value has one of eight types — seven primitives (undefined, null, boolean, number, bigint, string, symbol) and Object — and Object(v) === v is true only for an object; a primitive takes no property and never changes, while arrays, functions, dates, regular expressions and the instances of your own classes are kinds of Object, not types of their own.	checked
typeof	101	typeof answers with one of eight strings, and two of its answers do not name the value's type: typeof null is "object" though no other test calls null an object, and a function or a class, which is an object, answers "function"; for a name nothing declares it answers "undefined", yet it throws a ReferenceError for a let read before its line.	checked
null and undefined	101	undefined is what the language fills in when nothing was assigned — a missing property, argument or return value — while null appears only where some code returns it; the two are == but not ===, and null + 1 is 1 while undefined + 1 is NaN.	checked
Primitives and wrappers	201	"abc".length works because the lookup wraps the string in a temporary String object — a sloppy-mode method is even handed a new wrapper as this on every call — while new String("abc") keeps its wrapper, so typeof says "object", two equal ones are not ===, and new Boolean(false) is truthy.	checked
Symbols	201	Symbol("id") === Symbol("id") is false, so two modules' symbol keys never collide, while Symbol.for("id") hands every caller one shared symbol and brings the collision back; Object.keys, for...in, JSON.stringify and structuredClone skip a symbol key, but spread and Object.assign copy it and Reflect.ownKeys lists it — hidden from accidents, not from anyone looking.	checked
BigInt	201	2n ** 64n is exact and -7n / 2n is -3n, because division truncates toward zero, but 1n + 1 throws a TypeError: arithmetic never mixes BigInt with Number, while comparison does and compares exact values, so 1n == 1 is true and 2n ** 53n + 1n == 2 ** 53 + 1 is false; a BigInt grows as needed up to V8's ceiling of about 2^30 bits.	checked
Values and references	101	Assignment copies what a variable holds, and for an object that is a reference: after const b = a, b.count = 2 changes a.count too; a function can change an object you pass it, but assigning to its parameter never changes which object your variable holds; and { n: 1 } === { n: 1 } is false, because === on two objects asks whether they are one object.	checked
Errors around values and types	201	Seventeen messages, each printed by a run on this page — thirteen by Node while a program ran, four by tsc before one could — and under each, the program behind it, what that program assumed, and a fix that runs.	checked
Lints around values and types	201	Twelve core ESLint rules each flag a mistake from this chapter — nine of the bad programs print a wrong answer, two throw, one is only untidy — and pass the fixed program with no report; tsc --strict refuses seven of those mistakes on its own and accepts five, and six more mistakes pass both tools and still print a wrong answer.	checked
Resources for values and types	101	Thirteen books and the ES5 standard cover this chapter in the sections named here, beside three free online books, the specification, MDN, the TypeScript handbook, six articles and four videos; the older sources count five or six types, one number type and a writable undefined, and eight programs on this page show what Node 24 and TypeScript 7 do instead.	checked

Boundaries

Numbers and strings get chapters of their own (10 and 9), and converting one type into another is chapter 3.