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

JavaScript offers multiple ways to manipulate objects. Two common methods are Object.assign() and the spread operator (...). Both allow you to copy properties from one or more source objects to a target object. However, they differ in syntax, performance, and underlying mechanisms. This article provides a comprehensive comparison to help you choose the right tool for your specific use case.

Understanding Object.assign()

Object.assign() is a method that copies all enumerable own properties from one or more source objects to a target object. It modifies the target object directly and returns it. The basic syntax is:

            Object.assign(target, ...sources)

            

              
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• target: The target object to which properties will be copied. This object will be modified.
• sources: One or more source objects from which properties will be copied.

Example:

            const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

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

console.log(target); // Output: { a: 1, b: 4, c: 5 }
console.log(returnedTarget === target); // Output: true

            

              
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In this example, the properties of source are copied to target. Note that the b property is overwritten, and the returnedTarget is the same object as target.

Understanding the Spread Operator

The spread operator (...) allows you to expand an iterable (like an array or an object) into individual elements. When used with objects, it creates a shallow copy of the object's properties into a new object. The syntax is simple:

            const newObject = { ...sourceObject };

            

              
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Example:

            const source = { a: 1, b: 2 };
const newObject = { ...source };

console.log(newObject); // Output: { a: 1, b: 2 }
console.log(newObject === source); // Output: false

            

              
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Here, newObject contains a copy of the properties of source. Importantly, newObject is a *new* object, distinct from source.

Key Differences

While both Object.assign() and the spread operator achieve similar results (copying object properties), they have crucial differences:

• Immutability: The spread operator creates a new object, leaving the original object unchanged (immutable). Object.assign() modifies the target object directly (mutable).
• Target Object Handling: Object.assign() allows you to specify a target object, while the spread operator always creates a new one.
• Property Enumerability: Both methods copy enumerable properties. Non-enumerable properties are not copied.
• Inherited Properties: Neither method copies inherited properties from the prototype chain.
• Setters: Object.assign() invokes setters on the target object. The spread operator doesn't; it directly assigns values.
• Undefined or Null Sources: Object.assign() skips null and undefined source objects. Spreading null or undefined will throw an error.

Performance Comparison

Performance is a critical consideration, especially when dealing with large objects or frequent operations. Microbenchmarks consistently show that the spread operator is generally faster than Object.assign(). The difference stems from their underlying implementations.

Why is the Spread Operator Faster?

The spread operator often benefits from optimized internal implementations within JavaScript engines. It's specifically designed for object and array creation, allowing engines to perform optimizations that aren't possible with the more general-purpose Object.assign(). Object.assign() needs to handle various cases, including setters and different property descriptors, making it inherently more complex.

Benchmark Example (Illustrative):

            // Simplified example (actual benchmarks require more iterations and robust testing)
const object1 = { a: 1, b: 2, c: 3, d: 4, e: 5 };
const object2 = { f: 6, g: 7, h: 8, i: 9, j: 10 };

// Using Object.assign()
console.time('Object.assign');
for (let i = 0; i < 1000000; i++) {
  Object.assign({}, object1, object2);
}
console.timeEnd('Object.assign');

// Using Spread Operator
console.time('Spread Operator');
for (let i = 0; i < 1000000; i++) {
  ({...object1, ...object2 });
}
console.timeEnd('Spread Operator');

            

              
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Note: This is a basic example for illustration purposes. Real-world benchmarks should utilize dedicated benchmarking libraries (like Benchmark.js) for accurate and reliable results. The magnitude of the performance difference can vary based on the JavaScript engine, object size, and the specific operations being performed.

Use Cases: Object.assign()

Despite the performance advantage of the spread operator, Object.assign() remains valuable in specific scenarios:

• Modifying an Existing Object: When you need to update an object in place (mutation) rather than creating a new one. This is common when working with mutable state management libraries or when performance is absolutely critical, and you've profiled your code to confirm that avoiding a new object allocation is worthwhile.
• Merging Multiple Objects into a Single Target: Object.assign() can efficiently merge properties from several source objects into a single target.
• Working with Older JavaScript Environments: The spread operator is an ES6 feature. If you need to support older browsers or environments that don't support ES6, Object.assign() provides a compatible alternative (although you may need to polyfill it).
• Invoking Setters: If you need to trigger setters defined on the target object during property assignment, Object.assign() is the correct choice.

Example: Updating State in a Mutable Fashion

            let state = { name: 'Alice', age: 30 };

function updateName(newName) {
  Object.assign(state, { name: newName }); // Mutates the 'state' object
}

updateName('Bob');
console.log(state); // Output: { name: 'Bob', age: 30 }

            

              
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Use Cases: Spread Operator

The spread operator is generally preferred for its immutability and performance advantages in most modern JavaScript development:

• Creating New Objects with Existing Properties: When you want to create a new object with a copy of properties from another object, often with some modifications.
• Functional Programming: The spread operator aligns well with functional programming principles, which emphasize immutability and avoiding side effects.
• React State Updates: In React, the spread operator is commonly used to create new state objects when updating component state immutably.
• Redux Reducers: Redux reducers often use the spread operator to return new state objects based on actions.

Example: Updating React State Immutably

            import React, { useState } from 'react';

function MyComponent() {
  const [state, setState] = useState({ name: 'Charlie', age: 35 });

  const updateAge = (newAge) => {
    setState({ ...state, age: newAge }); // Creates a new state object
  };

  return (
    <div>
      <p>Name: {state.name}</p>
      <p>Age: {state.age}</p>
      <button onClick={() => updateAge(36)}>Increment Age</button>
    </div>
  );
}

export default MyComponent;

            

              
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Shallow Copy vs. Deep Copy

It's crucial to understand that both Object.assign() and the spread operator perform a *shallow* copy. This means that only the top-level properties are copied. If an object contains nested objects or arrays, only the references to those nested structures are copied, not the nested structures themselves.

Example of Shallow Copy:

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

copy.a = 3; // Modifies 'copy.a', but 'original.a' remains unchanged
copy.b.c = 4; // Modifies 'original.b.c' because 'copy.b' and 'original.b' point to the same object

console.log(original); // Output: { a: 1, b: { c: 4 } }
console.log(copy); // Output: { a: 3, b: { c: 4 } }

            

              
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To create a *deep* copy (where nested objects are also copied), you need to use techniques like:

• JSON.parse(JSON.stringify(object)): This is a simple but potentially slow method. It doesn't work with functions, dates, or circular references.
• Lodash's _.cloneDeep(): A utility function provided by the Lodash library.
• A custom recursive function: More complex but provides the most control over the deep copy process.
• Structured Clone: Uses window.structuredClone() for browsers or the structuredClone package for Node.js to deep copy objects.

Best Practices

• Prefer the Spread Operator for Immutability: In most modern JavaScript applications, favor the spread operator to create new objects immutably, especially when working with state management or functional programming.
• Use Object.assign() for Mutating Existing Objects: Choose Object.assign() when you specifically need to modify an existing object in place.
• Understand Shallow vs. Deep Copy: Be aware that both methods perform shallow copies. Use appropriate techniques for deep copying when necessary.
• Benchmark When Performance is Critical: If performance is paramount, conduct thorough benchmarks to compare the performance of Object.assign() and the spread operator in your specific use case.
• Consider Code Readability: Choose the method that results in the most readable and maintainable code for your team.

International Considerations

The behavior of Object.assign() and the spread operator is generally consistent across different JavaScript environments worldwide. However, it's worth noting the following potential considerations:

• Character Encoding: Ensure that your code handles character encoding correctly, especially when dealing with strings containing characters from different languages. Both methods copy string properties correctly, but encoding issues can arise when processing or displaying these strings.
• Date and Time Formats: When copying objects containing dates, be mindful of time zones and date formats. If you need to serialize or deserialize dates, use appropriate methods to ensure consistency across different regions.
• Number Formatting: Different regions use different conventions for number formatting (e.g., decimal separators, thousands separators). Be aware of these differences when copying or manipulating objects containing numerical data that might be displayed to users in different locales.

Conclusion

Object.assign() and the spread operator are valuable tools for object manipulation in JavaScript. The spread operator generally offers better performance and promotes immutability, making it a preferred choice in many modern JavaScript applications. However, Object.assign() remains useful for mutating existing objects and supporting older environments. Understanding their differences and use cases will help you write more efficient, maintainable, and robust JavaScript code.