Chapter 1Lesson 7

Quiz - The JavaScript value model

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Which of these lines throws a TypeError at runtime?

const config = { feature: true, env: { tier: 'pro' } };

config.feature = false;          // A
config.env.tier = 'free';        // B
config = { feature: false };     // C

Only line A.

Only line C.

Lines A and C.

All three.

const locks the binding (the arrow), not the value (the box). Lines A and B are property mutations through the existing reference — const has nothing to say about those. Line C is the one true reassignment: it tries to repoint config at a different object, which is exactly what const forbids.

You spread a nested object to make a “copy” before mutating it:

const original = { name: 'Ada', address: { city: 'London' } };
const copy = { ...original };
copy.address.city = 'Paris';

What does original.address.city hold after this runs?

'London' — the spread copied the whole structure, so the mutation stays on copy.

'Paris' — spread is shallow; copy.address and original.address are the same object.

{ ...original } copies the top level only. Nested objects keep their references — both copy.address and original.address point at the same allocation, so mutating through one is visible through the other. When the data shape is genuinely nested, structuredClone is the deep-copy default.

For which of these values would a senior reach for BigInt rather than a regular number?

A user’s shopping cart total in USD.

A 64-bit Snowflake ID coming back from an external API.

The number of unread notifications on a user’s account.

A nanosecond-resolution counter accumulated across a multi-year telemetry pipeline.

BigInt earns its weight only when values genuinely exceed Number.MAX_SAFE_INTEGER (2^53 − 1) — external 64-bit IDs and rare large counters are the canonical cases. Money is stored as integer cents in a regular number (covers amounts up to ~$90 trillion), and ordinary counts comfortably fit. BigInt carries real friction (no mixing with number, no Math.*, JSON serialization breaks) — pay it only when overflow is real.

A bio field has a “280-character” limit. The user types 280 emoji, each of which they perceive as one character, and gets rejected as “too long.” Which check belongs in the validator?

input.length <= 280

[...input].length <= 280

[...new Intl.Segmenter('en', { granularity: 'grapheme' }).segment(input)].length <= 280

.length counts UTF-16 code units (a flag emoji is 4, a family emoji is 11). Spreading counts Unicode code points — better, but a family emoji still comes out as 7. Only Intl.Segmenter with grapheme granularity counts what the user actually perceives, and it’s the 2026 default for any length check on a user-facing field.

Two database rows come back with identical contents:

const a = { id: 1, name: 'Ada' };
const b = { id: 1, name: 'Ada' };
console.log(a === b);

What prints, and why?

true — === walks the object and compares each property.

false — === on objects compares the reference (the allocation), not the shape; a and b are two separate allocations.

false — === only works on primitives; comparing objects always returns false.

For objects, === asks “are these the same allocation?” not “do they have the same shape?” Two object literals are two separate allocations, so the answer is false. JavaScript ships no structural-equality operator — the patterns the course uses instead are comparing by primary key, deriving a string key from the relevant fields, or discriminated unions.

Why does Drizzle’s sql`SELECT * FROM users WHERE id = ${userId}` resist SQL injection, while `SELECT * FROM users WHERE id = '${userId}'` does not?

The sql tag escapes the value as a string before splicing it into the query text.

The sql tag receives the static segments and the dynamic values separately, and sends the values to the database as bound parameters — they never become part of the SQL text.

Plain backticks evaluate ${userId} at runtime; sql-tagged backticks evaluate it lazily, so injection can’t happen.

A tagged template is a function call: the tag receives the array of static string segments and the interpolated values as separate arguments. Drizzle’s sql tag uses that to bind each value to a numbered placeholder ($1, $2, …) — the database driver handles the dynamic value as data, not as SQL text. The plain template literal concatenates the value straight into the query string, leaving the injection door open.

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