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What's new in fast-check 3.12.0?

· 7 min read
Nicolas Dubien
fast-check maintainer

This release introduces performance optimizations for float, double and ulid, along with the ability to define ranges with excluded boundaries for float and double.

Continue reading to explore the detailed updates it brings.

Performance optimizations


When it comes to optimizing JavaScript code, developers have a variety of tricks to choose from. These range from minimizing the number of operations to adopting memory-efficient algorithms and utilizing caches. However, achieving the best performance often involves navigating trade-offs, considering factors such as garbage collection costs, V8 optimization of monomorphic operations, and V8's internal representation of certain data types.

To determine the most effective option, we conducted benchmarks using tinybench. All the following figures are based on measurements from running tinybench with 100k iterations on GitHub Actions workers.

Faster decomposition of floating point values

When creating arbitraries for float and double, fast-check relies on an algorithm able to compute the position of any floating-point number within the entire range of existing values. For instance, the value 2-52 is the 629,145,600th 32-bits float and the 1,018,167,296th 64-bits float.

This numbering system enables the enumeration of all possible floating-point numbers. For instance, immediately following 1, the value is 1.0000001192092896 for 32-bit floats and 1.0000009536743164 for 64-bit floats.

The algorithm responsible for finding the index of any floating-point number relies on a code snippet capable of decomposing a float into its significand and exponent. Initially, our implementation involved a loop to accomplish this task.

export function decomposeFloat(f: number): { exponent: number; significand: number } {
// 1 => significand 0b1 - exponent 1 (will be preferred)
// => significand 0b0.1 - exponent 2
const maxSignificand = 1 + (2 ** 23 - 1) / 2 ** 23;
for (let exponent = -126; exponent !== 128; ++exponent) {
const powExponent = 2 ** exponent;
const maxForExponent = maxSignificand * powExponent;
if (Math.abs(f) <= maxForExponent) {
return { exponent, significand: f / powExponent };
return { exponent: Number.NaN, significand: Number.NaN };

This implementation relied on iterating over up to 253 values to extract significand and exponent to later find the index of a floating-point number. However, this approach proved to be inefficient. Thanks to the contributions of @zbjornson, an optimized version has been introduced in the latest release (see #4059).

const f32 = new Float32Array(1);
const u32 = new Uint32Array(f32.buffer, f32.byteOffset);

function bitCastFloatToUInt32(f: number): number {
f32[0] = f;
return u32[0];

export function decomposeFloat(f: number): { exponent: number; significand: number } {
const bits = bitCastFloatToUInt32(f);
const signBit = bits >>> 31;
const exponentBits = (bits >>> 23) & 0xff;
const significandBits = bits & 0x7fffff;

const exponent = exponentBits === 0 ? -126 : exponentBits - 127;
let significand = exponentBits === 0 ? 0 : 1;
significand += significandBits / 2 ** 23;
significand *= signBit === 0 ? 1 : -1;

return { exponent, significand };

This optimization will speed-up the instantiation of any instance of float or double or any arbitrary derived from them.

Higher throughput thanks to less allocations

The initial implementation of ulid in fast-check encountered several performance issues. However, in version 3.12.0, significant improvements have been made, resulting in a twofold increase in performance. In version 3.11.0, the code below performed at 1,774 ops/sec (±0.06%), but after all the optimizations, it now runs at 3,447 ops/sec (±0.10%):

fc.assert(, (_unused) => true));

The first set of optimizations primarily focused on reducing the number of allocations, aiming for a more memory-efficient solution. These optimizations were mainly addressed in #4088 and #4091.

One of the optimizations involved transforming our internal pad function to avoid unnecessary array allocations during filling and joining. The original implementation looked like this:

function pad(value: string, constLength: number) {
return (
Array(constLength - value.length)
.join('') + value

To achieve a more efficient version, we transformed it as follows:

function pad(value: string, constLength: number) {
let extraPadding = '';
while (value.length + extraPadding.length < constLength) {
extraPadding += '0';
return extraPadding + value;

With this optimization, the execution time of the function pad has been reduced by a factor of 2. The performance improvements can be observed in the following results: pad('', 10) improved from 1,411,410 ops/sec (±2.31%) to 2,385,090 ops/sec (±14.86%), pad('01234', 10) increased from 1,627,234 ops/s (±11.54%) to 3,817,072 ops/sec (±4.11%) and pad('0123456789', 10) saw a rise from 3,895,766 ops/sec (±2.72%) to 4,848,174 ops/sec (±3.15%).

Similar improvements were made in other parts of the code, such as replacing array joins with simple concatenations, replacing the code below:

return [compute(a), compute(b), compute(c)].join('');

By a more efficient version of it:

return compute(a) + compute(b) + compute(c);

This optimization led to significantly faster execution times, approaching a factor of 2 speed-up with: 2,507,281 ops/sec (±17.68%) for [compute('a'), compute('b'), compute('c')].join('') compared to 4,884,371 ops/sec (±0.56%) for compute('a') + compute('b') + compute('c').

Not only we addressed memory footprint, but also performed algorithmic optimizations. The code below was responsible for both excessive memory allocations and redundant recomputation of powers of 32:

const symbols = normalizedBase32str.split('').map((char) => decodeSymbolLookupTable[char]);
return symbols.reduce((prev, curr, i) => prev + curr * Math.pow(32, symbols.length - 1 - i), 0);

We first started to drop the unwanted memory allocations and moved from 677,844 ops/sec (±4.28%) to 809,730 ops/sec (±0.94%) with simple for-loop based version:

let sum = 0;
for (let index = 0; index !== normalizedBase32str.length; ++index) {
const char = normalizedBase32str[index];
const symbol = decodeSymbolLookupTable[char];
sum += symbol * Math.pow(32, normalizedBase32str.length - 1 - i);
return sum;

We continued by addressing the power of 32 part:

let sum = 0;
for (let index = 0, base = 1; index !== normalizedBase32str.length; ++index, base *= 32) {
const char = normalizedBase32str[normalizedBase32str.length - index - 1];
const symbol = decodeSymbolLookupTable[char];
sum += symbol * base;
return sum;

With this last optimization, the throughput reached an 2,198,923 ops/sec (±1.46%).

For further details on these and other optimizations, you can refer to the pull requests: #4088, #4091,#4092 and #4098. Some of these PRs also address low-level issues by keeping produced values within the int32 range, leveraging slight performance optimizations of V8 on integer values.

Excluded min and max

Until now, when utilizing arbitraries for float and double, the min and max values were always included in the set of generated values. This means that requesting double({min: 0, max: 1}) would produce values such that 0 ≤ value ≤ 1. If you wanted to exclude the value 1 from the range, you had to manually specify a max value slightly below 1.

With version 3.12.0, fast-check now offers two new options

  • minExcluded: This option excludes the minimum value from the set of generated values.
  • maxExcluded: This option excludes the maximum value from the set of generated values.

For example, if you want to exclude the value 1 from the range, you can now write: double({min: 0, max: 1, maxExcluded: true}), and this will produce values where 0 ≤ value < 1, effectively excluding the value 1 from the generated set.

Changelog since 3.11.0

The version 3.12.0 is based on version 3.11.0.


  • (PR#4100) Support excluded min/max in double
  • (PR#4105) Support excluded min/max in float


  • (PR#4094) Bug: Stop unwrapping ulid we cannot build
  • (PR#4095) Bug: Be resilient to poisoning with ulid
  • (PR#4041) CI: Ensure we use latest node in range
  • (PR#4062) CI: Update devcontainer configuration
  • (PR#4065) CI: Better configuration for renovate
  • (PR#4068) CI: Refine configuration of renovate
  • (PR#4073) CI: New attempt to configure renovate
  • (PR#4075) CI: Configure renovate to bump non-package
  • (PR#4078) CI: Disable nodenv bumps on renovate
  • (PR#4080) CI: Stop bumping node via renovate
  • (PR#4040) Doc: Prepare release note for 3.11.0
  • (PR#4087) Doc: Add new contributor zbjornson
  • (PR#4059) Performance: Faster decomposeFloat/Double
  • (PR#4088) Performance: Drop some unneeded allocs in ulid
  • (PR#4091) Performance: Faster unmap for ulid
  • (PR#4092) Performance: Faster generation of ulid
  • (PR#4098) Performance: Faster ulid mapper function
  • (PR#4039) Script: Add support for more gitmojis