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')
flatstr(someHeavilyConcatenatedString)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%
In the v8 C++ layer, JavaScript strings can be represented in two ways.
- As an array
- 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.
Here's the code:
module.exports = function flatstr(s) {
Number(s)
return s
}Obviously, you could just user Number in your own code, and not use
this module at all. However, this module serves the purpose of preventing
misunderstandings in your code base (and potential removal of code that
appears to be superfluous at first glance). Tests show that the additional
function wrapper adds negligible overhead.
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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