This directory gathers multiple examples of properties you might come with when using fast-check.

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Teach yourself property based through examples

Following examples show how you could think about properties given algorithms ranging from basic pure functions returning easy to assess outputs to complex state machines.

Simple data structures

101 for property based:

• decompPrime - Returns the list of prime factors corresponding to the input value
• fibonacci - Returns the item at a given position in the sequence of Fibonacci
• indexOf - Returns the position of the first occurrence of pattern in text
• sort - Returns a sorted copy of the input array

Recursive structures

Let's see how to generate recursive inputs using letrec, memo or even none of them:

• isSearchTree - Returns true if the tree is a binary search tree, false otherwise

Misc

Various algorithms to have more random examples:

• knight - Multi dimensional dichotomy given as a coding exercise
• mazeGenerator - Maze generator
• roman - Convert from and to roman notation for numbers

State machines to user interfaces

Property based testing applied to state machines or user interfaces:

• MusicPlayer - Simple music player with play, pause, addTrack and next

Race conditions

Property based testing used to detect race conditions in various kind of JavaScript snippets:

• AutocompleteField - An autocomplete field written in React providing suggestions as soon as possible
• Counter - Increment a counter stored in a DB - non atomic and atomic versions
• DebouncedAutocomplete - An autocomplete field written in React providing suggestions in a debounced way (uses timers)
• dependencyTree - Fetch recursively dependencies for a npm package
• TodoList - Simple todolist React app
• UserProfilePage - A simple React component loading user profile on mount

1. Properties do not replace examples, they are just an extra layer of tests
2. Properties can be used at any level: unit, integration, end-to-end

1. Characteristics independent of the inputs

Examples:

for any floating point number d, Math.floor(d) is an integer

for any integer n, Math.abs(n) ≥ 0

2. Characteristics derived from the inputs

Examples:

for any a and b integers the average of a and b is between a and b

for any array — data, sorted(data) and data contain the same elements

3. Restricted set of inputs with useful characteristics

Examples:

for any prime number p, its decomposition into prime factors is itself

for any a, b and c strings the concatenation of a, b and c always contains b

4. Characteristics on combination of functions

Examples:

for any file f, unzip(zip(f)) is the original file

for any a, b numbers lcm(a, b) * gcd(a, b) equals a * b

5. Comparison with a simpler implementation