JavaScript Object.assign vs Spread: Performance Comparison and Use Cases

In JavaScript, manipulating objects is a common task. Two popular methods for achieving this are Object.assign() and the spread syntax (...). While both can be used to copy properties from one or more objects into a target object, they differ in performance characteristics, use cases, and overall behavior. This article provides a comprehensive comparison to help you choose the right tool for the job.

Understanding Object.assign()

Object.assign() is a method that copies the values of all enumerable own properties from one or more source objects to a target object. It returns the modified target object.

Syntax:

            Object.assign(target, ...sources)
            

              
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Example:

            const target = { a: 1 };
const source = { b: 2, c: 3 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 2, c: 3 }
console.log(returnedTarget === target); // true

            

              
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In this example, properties b and c from the source object are copied to the target object. Object.assign() modifies the original target object and returns it.

Understanding Spread Syntax

The spread syntax (...) allows an iterable such as an array or object to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) or key-value pairs (for object literals) are expected.

Syntax (Object Literal):

            const newObject = { ...object1, ...object2 };
            

              
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Example:

            const obj1 = { a: 1 };
const obj2 = { b: 2, c: 3 };

const mergedObj = { ...obj1, ...obj2 };

console.log(mergedObj); // { a: 1, b: 2, c: 3 }

            

              
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Here, the spread syntax creates a new object mergedObj by combining the properties of obj1 and obj2.

Performance Comparison

The performance difference between Object.assign() and the spread syntax can vary depending on the JavaScript engine and the complexity of the objects being manipulated. Generally, for simple object cloning and merging, the spread syntax tends to be slightly faster. However, the difference is often negligible for small objects. For larger objects, more complex scenarios, and repeated operations, micro-benchmarking is recommended to determine the fastest approach for your specific use case. Let's consider different scenarios:

Scenario 1: Simple Object Cloning

When cloning a single object, the spread syntax generally exhibits better performance due to its more streamlined operation.

            const original = { a: 1, b: 2, c: 3 };

// Spread Syntax
const cloneSpread = { ...original };

// Object.assign()
const cloneAssign = Object.assign({}, original);

            

              
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Scenario 2: Merging Multiple Objects

When merging multiple objects, the performance difference between the two methods is often minimal, but spread syntax often maintains a slight edge, primarily because it is implemented natively in modern JavaScript engines.

            const obj1 = { a: 1 };
const obj2 = { b: 2 };
const obj3 = { c: 3 };

// Spread Syntax
const mergedSpread = { ...obj1, ...obj2, ...obj3 };

// Object.assign()
const mergedAssign = Object.assign({}, obj1, obj2, obj3);

            

              
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Scenario 3: Large Objects with Many Properties

When dealing with large objects containing hundreds or thousands of properties, the performance differences can become more noticeable. In these cases, spread syntax often retains its advantage due to more efficient memory allocation and property copying within the engine.

Benchmarking

To obtain precise performance measurements, consider using benchmarking tools like Benchmark.js. These tools allow you to run repeated tests and collect statistics to determine which method performs best under specific conditions.

Example using Benchmark.js:

            const Benchmark = require('benchmark');

const suite = new Benchmark.Suite;

const obj1 = { a: 1 };
const obj2 = { b: 2 };
const obj3 = { c: 3 };

// add tests
suite.add('Spread Syntax', function() {
  const mergedSpread = { ...obj1, ...obj2, ...obj3 };
})
.add('Object.assign()', function() {
  const mergedAssign = Object.assign({}, obj1, obj2, obj3);
})
// add listeners
.on('cycle', function(event) {
  console.log(String(event.target));
})
.on('complete', function() {
  console.log('Fastest is ' + this.filter('fastest').map('name'));
})
// run async
.run({ 'async': true });

            

              
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This code snippet demonstrates how to set up a performance benchmark using Benchmark.js to compare the performance of spread syntax and Object.assign() when merging multiple objects. Remember to install the library using npm install benchmark before running the script.

Use Cases

While performance is a factor, the choice between Object.assign() and the spread syntax often depends on the specific use case and coding style preferences.

Object.assign() Use Cases

• Modifying the Target Object: Object.assign() modifies the target object directly, which can be useful when you want to update an existing object in place.
• Compatibility with Older Browsers: Object.assign() has broader browser support compared to the spread syntax, making it suitable for projects targeting older environments. You might need a polyfill for older browsers.
• Integrating with Existing Codebases: If you're working with an existing codebase that extensively uses Object.assign(), sticking with it can maintain consistency and reduce the risk of introducing bugs.
• Setting Default Values: It can be used to apply default values to an object, ensuring that certain properties are always defined.

              const defaults = { a: 1, b: 2, c: 3 };
  const options = { a: 10, d: 4 };
  
  const config = Object.assign({}, defaults, options);
  console.log(config); // { a: 10, b: 2, c: 3, d: 4 }
  
              
  
                
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Spread Syntax Use Cases

• Creating New Objects: The spread syntax excels at creating new objects without modifying the original objects, promoting immutability.
• Concise Syntax: The spread syntax often results in more readable and concise code, especially when merging multiple objects.
• React and Redux: In React and Redux, where immutability is crucial for performance and state management, the spread syntax is widely used for creating updated versions of state objects.
• Functional Programming: It aligns well with functional programming principles, where avoiding side effects and working with immutable data are encouraged.

Shallow Copy vs. Deep Copy

It's crucial to understand that both Object.assign() and the spread syntax perform a shallow copy. This means that if the object contains nested objects, only the references to those nested objects are copied, not the nested objects themselves. Modifying a nested object in the copied object will also affect the original object, and vice versa.

Example:

            const original = {
  a: 1,
  b: { c: 2 }
};

const copied = { ...original };

copied.b.c = 3;

console.log(original.b.c); // 3 - The original object is modified!

            

              
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If you need to create a deep copy, where nested objects are also copied, you can use techniques like:

• JSON.parse(JSON.stringify(object)): This is a simple but potentially inefficient approach, especially for large or complex objects. It also doesn't handle functions or circular references correctly.
• Using a library like Lodash's _.cloneDeep(): Libraries like Lodash provide optimized deep cloning functions that handle various edge cases.
• Writing a custom recursive deep copy function: This allows you to control the cloning process and handle specific data types or structures.

Immutability

Immutability is a programming concept that emphasizes creating new data structures instead of modifying existing ones. This approach can lead to more predictable code, easier debugging, and improved performance in certain scenarios. Both Object.assign() and the spread syntax can be used to promote immutability, but the spread syntax is generally preferred due to its ability to create new objects more directly.

Using Object.assign() to achieve immutability requires creating a new target object first:

            const original = { a: 1, b: 2 };
const updated = Object.assign({}, original, { a: 10 });

console.log(original); // { a: 1, b: 2 }
console.log(updated);  // { a: 10, b: 2 }

            

              
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            const original = { a: 1, b: 2 };
const updated = { ...original, a: 10 };

console.log(original); // { a: 1, b: 2 }
console.log(updated);  // { a: 10, b: 2 }

            

              
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Practical Examples

Example 1: Updating User Profile Data

Imagine you have a user profile object and you want to update it with new information from a form. Using the spread syntax, you can easily create a new object with the updated data:

            const userProfile = {
  id: 123,
  name: 'Alice',
  email: 'alice@example.com',
  location: 'New York'
};

const updatedData = {
  email: 'alice.new@example.com',
  location: 'London'
};

const updatedProfile = { ...userProfile, ...updatedData };

console.log(updatedProfile);
// {
//   id: 123,
//   name: 'Alice',
//   email: 'alice.new@example.com',
//   location: 'London'
// }

            

              
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Example 2: Managing Shopping Cart Items

In an e-commerce application, you might need to update the quantity of an item in the shopping cart. Using the spread syntax, you can create a new cart array with the updated item:

            const cart = [
  { id: 1, name: 'Product A', quantity: 2 },
  { id: 2, name: 'Product B', quantity: 1 }
];

const productIdToUpdate = 1;
const newQuantity = 3;

const updatedCart = cart.map(item =>
  item.id === productIdToUpdate ? { ...item, quantity: newQuantity } : item
);

console.log(updatedCart);
// [
//   { id: 1, name: 'Product A', quantity: 3 },
//   { id: 2, name: 'Product B', quantity: 1 }
// ]

            

              
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Example 3: Configuring Application Settings

When configuring application settings, you might want to merge default settings with user-provided settings. Object.assign() can be useful for this purpose, especially if you need to modify the default settings object directly:

            const defaultSettings = {
  theme: 'light',
  fontSize: 'medium',
  language: 'en'
};

const userSettings = {
  theme: 'dark',
  fontSize: 'large'
};

Object.assign(defaultSettings, userSettings);

console.log(defaultSettings);
// {
//   theme: 'dark',
//   fontSize: 'large',
//   language: 'en'
// }

            

              
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In this case, the defaultSettings are modified in place, which may or may not be desirable depending on your application's requirements.

Best Practices

• Understand Shallow Copying: Be aware that both methods perform shallow copies. For deep copying, use appropriate techniques or libraries.
• Consider Immutability: When possible, favor the spread syntax for creating new objects and promoting immutability.
• Benchmark When Necessary: For performance-critical code, benchmark both methods to determine the fastest option for your specific use case.
• Choose Based on Context: Select the method that best aligns with your coding style, project requirements, and compatibility needs.
• Use Linters and Code Style Guides: Enforce consistent usage of Object.assign() and the spread syntax through linters and code style guides.
• Document Your Choices: Clearly document your reasoning for choosing one method over the other, especially in complex codebases.

Conclusion

Object.assign() and the spread syntax are valuable tools for object manipulation in JavaScript. While the spread syntax often offers slightly better performance and promotes immutability, Object.assign() remains relevant for modifying existing objects and maintaining compatibility with older environments. Understanding the nuances of each method allows you to make informed decisions and write more efficient and maintainable code.

By considering the performance characteristics, use cases, and best practices outlined in this article, you can effectively leverage both Object.assign() and the spread syntax to enhance your JavaScript development workflow and build robust and scalable applications for a global audience. Remember to always prioritize code clarity and maintainability while optimizing for performance when necessary. Micro-benchmarking and profiling your code can also help you identify performance bottlenecks and make data-driven decisions about which method to use in specific scenarios.