1.24 — Object Methods: Quick Revision

Compact cheat sheet. Print-friendly.

How to use this material (instructions)

1. Skim before labs or interviews.
2. Drill gaps — reopen README.md -> 1.24.a...1.24.h.
3. Practice — 1.24-Exercise-Questions.md.
4. Polish answers — 1.24-Interview-Questions.md.

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All 8 methods at a glance

Method	Returns	Purpose
Object.keys(obj)	string[]	Own enumerable property names
Object.values(obj)	any[]	Own enumerable property values
Object.entries(obj)	[string, any][]	Own enumerable [key, value] pairs
Object.fromEntries(iter)	object	Build object from [key, value] pairs
Object.assign(target, ...src)	target	Copy props into target (mutates)
Object.freeze(obj)	obj	Make object immutable (shallow)
Object.seal(obj)	obj	Fix shape, allow value changes (shallow)
Object.is(a, b)	boolean	Same-value equality (fixes NaN, +/-0)

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Iteration trio

const obj = { name: "Alice", age: 30 };

Object.keys(obj);     // ["name", "age"]
Object.values(obj);   // ["Alice", 30]
Object.entries(obj);   // [["name", "Alice"], ["age", 30]]

Common rules for all three:

• Own properties only (not inherited)
• Enumerable only
• String keys only (no Symbols)
• Order: integer keys sorted, then string keys by insertion order

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entries/fromEntries pipeline

// Filter
Object.fromEntries(Object.entries(obj).filter(([k, v]) => v > 0));

// Transform values
Object.fromEntries(Object.entries(obj).map(([k, v]) => [k, v * 2]));

// Pick keys
Object.fromEntries(Object.entries(obj).filter(([k]) => allowed.includes(k)));

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Object.assign quick reference

// Merge (safe — empty target)
const merged = Object.assign({}, defaults, overrides);

// Clone with extras
const clone = Object.assign({}, original, { extra: true });

// Mutate existing
Object.assign(target, source);  // target is modified and returned

Remember:

• Shallow copy (nested objects shared by reference)
• Later sources overwrite earlier (last wins)
• Copies Symbol keys (unlike entries)
• Triggers setters on target

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assign vs spread

	Object.assign(target, src)	{ ...src }
Mutates target?	Yes	No
Triggers setters?	Yes	No
Creates new object?	Only if target is {}	Always

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Immutability hierarchy

preventExtensions  <  seal  <  freeze
     (weakest)                (strongest)

Operation	preventExtensions	seal	freeze
Add properties	No	No	No
Delete properties	Yes	No	No
Modify values	Yes	Yes	No
Reconfigure descriptors	Yes	No	No

All three are SHALLOW. Nested objects unaffected.

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freeze vs const

	const	Object.freeze()
Protects	Variable binding	Object contents
Prevents reassignment?	Yes	No
Prevents property mutation?	No	Yes

Use both: const CONFIG = Object.freeze({ ... })

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Object.is vs === vs ==

	==	===	Object.is()
Type coercion?	Yes	No	No
NaN === NaN	false	false	true
+0 === -0	true	true	false

Rule of thumb: Use === everywhere. Use Object.is for NaN-safe comparisons (React does this).

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Checking object state

Object.isFrozen(obj);      // true if frozen
Object.isSealed(obj);      // true if sealed (or frozen)
Object.isExtensible(obj);  // true if can add properties

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Common patterns

Empty check

Object.keys(obj).length === 0

Property count

Object.keys(obj).length

Value existence

Object.values(obj).includes(searchValue)

Sum values

Object.values(obj).reduce((sum, v) => sum + v, 0)

Object to Map

new Map(Object.entries(obj))

Map to Object

Object.fromEntries(map)

Object to query string

Object.entries(obj).map(([k, v]) => `${k}=${v}`).join("&")

Deep freeze

function deepFreeze(obj) {
  Object.freeze(obj);
  Object.values(obj).forEach(v => {
    if (v && typeof v === "object" && !Object.isFrozen(v)) deepFreeze(v);
  });
  return obj;
}

Shallow clone

{ ...obj }                    // spread (preferred)
Object.assign({}, obj)        // assign

Deep clone

structuredClone(obj)           // modern (recommended)
JSON.parse(JSON.stringify(obj)) // JSON-safe data only

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What each method copies/skips

	Own	Enumerable only	String keys	Symbol keys
Object.keys()	Yes	Yes	Yes	No
Object.values()	Yes	Yes	Yes	No
Object.entries()	Yes	Yes	Yes	No
Object.assign()	Yes	Yes	Yes	Yes
{ ...obj }	Yes	Yes	Yes	Yes
for...in	+inherited	Yes	Yes	No

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Master workflow

1. Inspect — Object.keys / values / entries to examine an object
2. Transform — entries -> map/filter -> fromEntries pipeline
3. Copy — { ...obj } for shallow; structuredClone for deep
4. Merge — { ...defaults, ...overrides } or Object.assign({}, defaults, overrides)
5. Lock — Object.seal for fixed shape; Object.freeze for full immutability
6. Compare — === normally; Object.is for NaN-safe checks

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End of 1.24 quick revision.