Slice vs. For Loop: Which is Faster for Shallow Cloning Arrays in JavaScript?

Shallow Cloning in JavaScript: Comparing the Performance of Array Duplication Techniques

When dealing with arrays in JavaScript, it becomes necessary to create copies or duplicates. Two commonly used methods for this task are the slice method and the traditional for loop. But which one is faster?

Challengers: Slice vs. For Loop

The slice method is part of the standard Array object and creates a shallow copy of the array, preserving the structure and elements, but not deep-copying object references.

On the other hand, the for loop uses a straightforward approach to iterate through the array and manually copy each element into a new array.

Benchmarks and Optimization

According to extensive benchmarks, the slice method proves to be the faster option in browsers such as Chrome and Firefox. This is due to browser optimizations that have been implemented for built-in methods like slice.

However, for other browsers that don't have these optimizations, the for loop method may be faster. This is because the for loop's predictable structure is easier for the browser's compiler to optimize.

Implementation Considerations

Here are the sample scripts for testing these methods in the browser's console:

While Loop

n = 1000*1000;
start = + new Date();
a = Array(n); 
b = Array(n); 
i = a.length;
while(i--) b[i] = a[i];
console.log(new Date() - start);

Slice

n = 1000*1000;
start = + new Date();
a = Array(n); 
b = a.slice();
console.log(new Date() - start);

Conclusion

The choice of which method to use for duplicating arrays in JavaScript depends on the targeted browser. For browsers with optimization for slice method, it's the faster option. In other cases, the for loop may provide better performance.

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