What is Time Complexity for string concatenation in JavaScript

Question

I am learning algorithms. And I didn't find any articles about how to calculate Time Complexity of string concatenation in JavaScript.

function concatString(str1, str2) {
  // return str1 + str2;
  // return str1.concat(str2)
}

We can concat strings using operator '+' or using built-in method String.prototype.concat

Which method is better? What is the Time Complexity of both methods?

Answer 1

According to the documentation, the difference between using + and concat() is that contat() will convert the argument directly to a string while the + operator will convert the operands to a primitive value and then into a string. Summarizing, it will not make a significant difference.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/concat#description

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The time complexity of concat operations in another language like Java is usually O(n^2), and is a common practice to use a StringBuilder to make it O(n). In JavaScript though, there is an optimized implementation of concatenation based on the rope data structure, making the time complexity O(log n) in most cases.

https://josephmate.github.io/java/javascript/stringbuilder/2020/07/27/javascript-does-not-need-stringbuilder.html