Are JavaScript Strings Immutable, and Is a String Builder Necessary for Efficient Concatenation?

Are JavaScript Strings Immutable? Do I Need a "String Builder" in JavaScript?

JavaScript strings are immutable, meaning they cannot be changed after they have been created. For instance, attempting to modify a character within a string using the syntax myString[2] = 'c' will not alter the original string. Instead, string manipulation methods like trim and slice create new strings with the desired modifications.

Even when multiple references exist for the same string, changing one does not affect the others. Consider the following example:

let a = b = "hello";
a = a + " world";
// b remains unchanged

Debunking the Myth: String Concatenation is Not Inefficient

While it has been widely believed that string concatenation is slow in JavaScript, benchmarks have revealed that this is not true. In fact, using Array.join for concatenation is not significantly faster than simply concatenating strings directly.

Sample Benchmarks:

To demonstrate this, the following tests were conducted:

• Array Indexing and Array.join: Using an array to store strings, avoiding Array.push, and then joining the strings with Array.join.
• Straight String Concatenation: Appending strings directly without using any intermediate storage or methods.

In both cases, constant values and random strings were appended in a loop.

The results indicate that both approaches perform nearly identically, regardless of the type of strings being concatenated.

Conclusion

JavaScript strings are immutable, and using a "string builder" is unnecessary for performance optimization. Concatenating strings directly in JavaScript is not slow, making it the preferred method for most situations.

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