1.0.12 • Public • Published


Flattens the underlying C structures of a concatenated JavaScript string


If you're doing lots of string concatenation and then writing that string somewhere, you may find that passing your string through flatstr vastly improves performance.


var flatstr = require('flatstr')


Benchmarks test flat vs non-flat strings being written to an fs.WriteStream.

unflattenedManySmallConcats*10000: 147.540ms
flattenedManySmallConcats*10000: 105.994ms
unflattenedSeveralLargeConcats*10000: 287.901ms
flattenedSeveralLargeConcats*10000: 226.121ms
unflattenedExponentialSmallConcats*10000: 410.533ms
flattenedExponentialSmallConcats*10000: 219.973ms
unflattenedExponentialLargeConcats*10000: 2774.230ms
flattenedExponentialLargeConcats*10000: 1862.815ms

In each case, flattened strings win, here's the performance gains from using flatstr

ManySmallConcats: 28%
SeveralLargeConcats: 21% 
ExponentialSmallConcats: 46%
ExponentialLargeConcats: 33%

How does it work

In the v8 C++ layer, JavaScript strings can be represented in two ways.

  1. As an array
  2. As a tree

When JavaScript strings are concatenated, tree structures are used to represent them. For the concat operation, this is cheaper than reallocating a larger array. However, performing other operations on the tree structures can become costly (particularly where lots of concatenation has occurred).

V8 has a a method called String::Flattenwhich converts the tree into a C array. This method is typically called before operations that walk through the bytes of the string (for instance, when testing against a regular expression). It may also be called if a string is accessed many times over, as an optimization on the string. However, strings aren't always flattened. One example is when we pass a string into a WriteStream, at some point the string will be converted to a buffer, and this may be expensive if the underlying representation is a tree.

String::Flatten is not exposed as a JavaScript function, but it can be triggered as a side effect.

There are several ways to indirectly call String::Flatten (see alt-benchmark.js), but coercion to a number appears to be (one of) the cheapest.

However since Node 10 the V8 version has stopped using Flatten in all places identified. Thus the code has been updated to seamlessly use the native runtime function %FlattenString without having to use the --allow-natives-syntax flag directly.

One final note: calling flatstr too much can in fact negatively effect performance. For instance, don't call it every time you concat (if that was performant, v8 wouldn't be using trees in the first place). The best place to use flatstr is just prior to passing it to an API that eventually runs non-v8 code (such as fs.WriteStream, or perhaps xhr or DOM apis in the browser).


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  • davidmarkclements