Run results for: JavaScript spread operator vs Object.assign vs for-in loop safe performance (lots of properties)

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:139.0) Gecko/20100101 Firefox/139.0

Browser: Firefox 139

Operating system: Windows

Device Platform: Desktop

Date tested: one year ago

Test name	Executions per second
Using the spread operator	373262.5 Ops/sec
Using Object.assign	958826.2 Ops/sec
Using For-In loop	585062.9 Ops/sec
Using For-In loop with hasOwnProperty	360429.7 Ops/sec

Script Preparation code:

const firstObject = {
    sampleData: 'Hello world',
    id: 1,
    name: 'Test Object',
    isActive: true,
    count: 42,
    tags: ['test', 'demo', 'sample'],
    createdAt: new Date(),
    updatedAt: new Date(),
    description: 'This is a test object',
    category: 'general',
    priority: 'high',
    value: 99.99,
    units: 'kg',
    isVerified: false,
    owner: 'Aaron',
    permissions: ['read', 'write'],
    metadata: { source: 'system', version: '1.0' },
    location: { lat: 35.6895, lon: 139.6917 },
    dimensions: { width: 10, height: 20, depth: 5 },
    status: 'pending',
    isArchived: false,
    color: 'blue',
    weight: 15.5,
    isEditable: true,
    notes: ['First note', 'Second note'],
    history: [{ timestamp: new Date(), action: 'created' }],
    settings: { theme: 'dark', notifications: true },
    lastLogin: new Date(),
    preferences: { language: 'English', currency: 'USD' },
    securityLevel: 'medium',
    groups: ['admin', 'user'],
    rating: 4.5,
    isPremium: false,
    attributes: ['responsive', 'scalable'],
    dependencies: ['moduleA', 'moduleB'],
    transactions: [{ id: 1001, amount: 50.75, date: new Date() }],
    schedule: { startTime: '08:00', endTime: '17:00' },
    version: '1.2.3',
    compatibility: ['v1', 'v2'],
    relatedIds: [100, 200, 300],
    extraInfo: 'Additional details here',
    warnings: ['Low battery', 'High temperature'],
    validation: { isValid: true, errors: [] },
    approvals: [{ user: 'manager', status: 'approved' }],
    associatedFiles: ['doc1.pdf', 'image.png'],
    lastAccessed: new Date(),
    trends: { growthRate: 2.5, declineRate: 0.8 }
};

const secondObject = {
    moreData: 'foo bar',
    id: 2,
    name: 'Another Test Object',
    isActive: false,
    count: 24,
    tags: ['development', 'test'],
    createdAt: new Date(),
    updatedAt: new Date(),
    description: 'Second test object',
    category: 'specific',
    priority: 'medium',
    value: 75.50,
    units: 'lbs',
    isVerified: true,
    owner: 'Developer',
    permissions: ['read'],
    metadata: { source: 'user', version: '2.0' },
    location: { lat: 51.5074, lon: -0.1278 },
    dimensions: { width: 5, height: 15, depth: 8 },
    status: 'completed',
    isArchived: true,
    color: 'red',
    weight: 20.1,
    isEditable: false,
    notes: ['Important note'],
    history: [{ timestamp: new Date(), action: 'updated' }],
    settings: { theme: 'light', notifications: false },
    lastLogin: new Date(),
    preferences: { language: 'French', currency: 'EUR' },
    securityLevel: 'high',
    groups: ['user'],
    rating: 3.9,
    isPremium: true,
    attributes: ['secure', 'efficient'],
    dependencies: ['moduleX', 'moduleY'],
    transactions: [{ id: 2002, amount: 120.99, date: new Date() }],
    schedule: { startTime: '09:00', endTime: '18:00' },
    version: '2.4.7',
    compatibility: ['v2', 'v3'],
    relatedIds: [400, 500, 600],
    extraInfo: 'Extended details',
    warnings: ['Slow network'],
    validation: { isValid: false, errors: ['Timeout'] },
    approvals: [{ user: 'admin', status: 'rejected' }],
    associatedFiles: ['spreadsheet.xlsx', 'diagram.svg'],
    lastAccessed: new Date(),
    trends: { growthRate: 1.9, declineRate: 0.5 }
};

Tests:

Using the spread operator
const finalObject = { ...firstObject, ...secondObject };
Using Object.assign
const finalObject = Object.assign({}, firstObject, secondObject);
Using For-In loop
const finalObject = {} for (let key in firstObject) { finalObject[key] = firstObject[key]; } for (let key in secondObject) { finalObject[key] = secondObject[key]; }
Using For-In loop with hasOwnProperty
const finalObject = {} for (let key in firstObject) { if (Object.prototype.hasOwnProperty.call(firstObject, key)) { finalObject[key] = firstObject[key]; } } for (let key in secondObject) { if (Object.prototype.hasOwnProperty.call(secondObject, key)) { finalObject[key] = secondObject[key]; } }

Run results for: JavaScript spread operator vs Object.assign vs for-in loop safe performance (lots of properties)

Measure the fastest way to merge objects, without mutation.

Using the spread operator

Using Object.assign

Using For-In loop

Using For-In loop with hasOwnProperty