Is JavaScript String Concatenation Slow, and Do I Need a String Builder?

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

JavaScript strings are immutable, meaning you can't modify a character within a string. The following code will not change the value of myString:

var myString = "abbdef";
myString[2] = 'c';

String manipulation methods like trim and slice return new strings, leaving the original string unchanged.

Myth Debunking: String Concatenation is NOT Slow

Contrary to popular belief, string concatenation in JavaScript is not slow. Tests have shown that it is on par with other concatenation methods, including those using arrays.

Benchmark Results

Here are benchmark results for different string concatenation approaches:

Appending Constant String

Approach	Time (ms)
Array Indexing (StringBuilderArrayIndex)	19.28
String Concatenation (StringBuilderStringAppend)	19.32

Appending Random Strings

Approach	Time (ms)
Array Indexing (StringBuilderArrayIndex)	58.79
String Concatenation (StringBuilderStringAppend)	57.92

As you can see, the performance difference is negligible.

Conclusion

• JavaScript strings are immutable, and you don't need a string builder.
• String concatenation in JavaScript is not slow. It's comparable to other methods, even for large strings.

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