Comparing for loops vs spread and concat for merging arrays in JavaScript — performance and best practices? [closed]

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I'm learning JavaScript and practicing merging arrays. Here's one of the methods I wrote using two for loops:

const arr1 = [4, 5, 7, 9, 8];
const arr2 = [-1, -2, 0, 12];
const arr3 = [];

for (let i = 0; i < arr1.length; i++) {
  arr3.push(arr1[i]);
}

for (let i = 0; i < arr2.length; i++) {
  arr3.push(arr2[i]);
}

console.log(arr3); // [4, 5, 7, 9, 8, -1, -2, 0, 12]

My questions:

Are there any performance or readability trade-offs between the manual for loop approach and using concat() or spread?
Especially in large-scale applications or performance-sensitive cases — is there a real difference?

Would love to learn the best practice here.

Thank you so much for improving the formatting, Nina! 🙏 That looks much cleaner now — I appreciate your help as I'm still learning how to write good questions on Stack Overflow. 😊 — user31029120
– user31029120, Commented Jul 13 at 7:16
This question is similar to: What is the most efficient way to concatenate N arrays?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — jonrsharpe
– jonrsharpe, Commented Jul 13 at 7:30

Aqil Contractor · Accepted Answer · 2025-07-13 07:29:06Z

I would suggest going with popular, JavaScript native methods. concat is going to be the best option here! For loops add lines of code and can obscure much simpler logic. Most competent/experienced developers can understand words like concat and know syntax much more easily than having to parse what is going on in for loops or other control flow.

When caring about performance, I would never worry about it until it is explicitly needed. JavaScript parsers and engines constantly optimize methods that are commonly used in their parser and code generators, one good example of this was that [...arr] spreading/cloning syntax used to be incredibly slow in (as far as I can recall) ALL engines when it was added by ECMA, but it is currently one of the fastest methods for cloning an array. Engines will keep evolving, and simpler constructs are always much more optimizable than obscure ones, like for looping to clone/concat an array.

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