Performance Analysis of Array Shallow Copying in JavaScript: slice vs. Loops vs. Spread Operator

Introduction

Array copying represents a fundamental operation in JavaScript development. While all shallow copying methods only duplicate references without cloning objects themselves, significant performance differences exist among various approaches. This article systematically analyzes performance characteristics of array copying methods based on high-quality StackOverflow discussions and independent benchmarking.

Overview of Six Array Shallow Copying Methods

JavaScript provides multiple approaches for array shallow copying, primarily including:

• Traditional for loop: Copying each element through index iteration
• Slice method: Utilizing built-in array slice functionality
• Array.from(): ES6 introduced array construction method
• Concat method: Achieving copy through empty array concatenation
• Spread operator: ES6 syntax sugar using three dots (...) operator
• Map method: Returning original array elements through mapping function

Performance Benchmarking Data

According to large-scale benchmarking results, different JavaScript engines demonstrate varying optimization levels for copying methods:

Blink Engine Browsers (Chrome, Edge, etc.)

In Blink-based browsers, slice() method shows optimal performance, concat() performs slightly slower, while while loop is approximately 2.4 times slower than slice. This performance gap stems from engine-specific internal optimizations for built-in methods.

Other Browser Engines

In browsers lacking specialized optimizations, while loop emerges as the fastest option, as these environments lack internal optimization mechanisms for slice and concat methods.

Detailed Performance Analysis

Slice Method Optimization Techniques

Testing reveals that explicitly specifying start index with slice(0) provides slight performance advantages over parameterless slice(). Though minimal, this difference warrants consideration in high-frequency operation scenarios.

Limitations of Spread Operator

Despite syntactic conciseness, spread operator carries stack overflow risks with large arrays. As documented in reference article 1, push(...largeArray) pattern may cause call stack overflow when array element count reaches million-scale, even after adjusting stack size parameters.

Performance Variations Across Array Sizes

Benchmarking covers diverse scales from 256 to 1,048,576 elements:

• Small arrays (256 elements): Array.slice() demonstrates overall best performance
• Medium arrays (2,048 elements): Spread operator and JSON cloning become worst performers
• Large arrays (16,384+ elements): Engine-specific anomalies emerge, with concat outperforming slice in V8 at certain scales

Engine-Specific Analysis

V8 Engine (Node.js, Chrome)

In Node.js environments, slice method maintains leadership in most cases, but concat method may outperform at specific array sizes. This nonlinear performance variation reflects complexity in V8's internal optimization strategies.

SpiderMonkey Engine (Firefox)

Firefox's SpiderMonkey engine exhibits distinct optimization patterns, with multiple methods showing similar performance characteristics, suggesting potential unified underlying implementation optimizations.

Practical Development Recommendations

Based on performance testing results, the following recommendations are provided for different scenarios:

• General scenarios: Prioritize slice() method, balancing performance and browser compatibility
• Performance-sensitive scenarios: Select optimal method based on target runtime environment - slice for Blink engines, consider while loops for other environments
• Large arrays: Avoid spread operator to prevent stack overflow issues
• Code readability: Choose semantically clearer methods within acceptable performance margins

Code Examples and Implementation Details

Below are typical implementations of various copying methods:

// Slice method
var dup_array = original_array.slice();

// Optimized slice
var dup_array = original_array.slice(0);

// For loop
var dup_array = [];
for(var i = 0, len = original_array.length; i < len; ++i) {
    dup_array[i] = original_array[i];
}

// While loop (faster in some environments)
var dup_array = [];
var i = original_array.length;
while(i--) dup_array[i] = original_array[i];

// Spread operator
var dup_array = [...original_array];

// Array.from()
var dup_array = Array.from(original_array);

// Concat method
var dup_array = [].concat(original_array);

// Map method
var dup_array = original_array.map(function(e){ return e; });

Conclusion

Performance optimization for JavaScript array shallow copying represents a complex multi-factor problem. While slice method demonstrates optimal performance in most modern browsers, practical selection should comprehensively consider target runtime environment, array size, and code maintainability. Developers should conduct performance testing based on specific scenarios rather than blindly following generic advice. As JavaScript engines continue evolving, these performance characteristics may change over time, making ongoing attention to latest benchmarking crucial.