Eye Training · Objects and Arrays

Two questions catch most of these: does this method change the original, and is this a copy or the same object?

Part 2 · Surface 20. Bug counts are hidden and some snippets are clean. See Start Here for how to use these drills.

J · The basics

M · Three years in, these should jump out

S · Where seniors stand out

What your eye checks

Whenever you see an array or object method, ask:

1. Does it change the original? sort, reverse, splice, push, fill, Object.assign(target, …) mutate. slice, map, filter, toSorted, spread don't.
2. Is this a copy or the same object? Spread copies one level, fill repeats one reference, JSON round trips change types. structuredClone makes a deep copy.
3. What does it return when nothing matches? find gives undefined, indexOf gives -1 (truthy), Math.max() gives -Infinity.
4. Is equality by value or by identity? === on arrays and objects compares references.
5. Does the callback get extra arguments? map passes (value, index, array), which breaks parseInt.
6. Does reduce have a starting value?
7. Do defaults cover null? Destructuring defaults and default parameters only replace undefined.
8. Are user-controlled strings used as object keys? Inherited keys collide; __proto__ pollutes. Use Map or Object.create(null).

Tells

Go deeper

• L5 · Security: prototype pollution and mass assignment in bigger codebases
• Previous surface: Values, equality and numbers · Next: Loops and iteration