In a previous post, I explained property based testing. In this post we’ll see a simple example using fast-check

Let assume we’re building the same bank transfer code as described in the dotnet FsCheck earlier post. Here’s the TypeScript version of the test:

import BuggyBank from "../buggy-bank"
import * as fc from 'fast-check';

function transferProperties(startBalanceA: bigint
        , startBalanceB: bigint
        , amount: bigint) : void {

    const bank = new BuggyBank()
    bank.balanceOfA = startBalanceA
    bank.balanceOfB = startBalanceB
    try {
        bank.transfer(amount)
    } catch {
        //Transfer failed
        const balanceAUnchanged = bank.balanceOfA == startBalanceA
        const balanceBUnchanged = bank.balanceOfB == startBalanceB
        expect(balanceAUnchanged).toBeTruthy()
        expect(balanceBUnchanged).toBeTruthy()
        return
    }
    //Transfer succeeded
    const balanceAIsNonNegative = bank.balanceOfA >= 0
    const balanceAChanged = bank.balanceOfA != startBalanceA
    const balanceBChanged = bank.balanceOfB != startBalanceB
    expect(balanceAIsNonNegative).toBeTruthy()
    expect(balanceAChanged).toBeTruthy()
    expect(balanceBChanged).toBeTruthy()
}

test('properties of a bank transfer must be correct', () => {
    const config = { numRuns : 10000 }
    //use this if you need to control the seed for the random number generator
    //const config = { numRuns : 10000, seed:123 }
    const property = fc.property(fc.bigIntN(32)
            , fc.bigIntN(32)
            , fc.bigIntN(32)
            , transferProperties)
    fc.assert(property,config)
})

I created the quick-n-dirty example like this:

mkdir fast-check-example
cd fast-check-example
npm init --yes
npm install typescript ts-node
echo "{}" > tsconfig.json
npm install --save-dev jest ts-jest @types/jest
npm install --save-dev fast-check
mkdir src
#Start vscode
code


The first time I ran the test, it detected a defect: The code allowed transferring zero amounts:

Property failed after 1 tests
{ seed: -1444529403, path: "0:2:0:0:1:0:5:1:3:0:0:1:1:0:1:2:2:0:0:0:0:0", endOnFailure: true }
Counterexample: [0n,0n,0n]
Shrunk 21 time(s)
...stack trace to relevant expect() line in code ....

After fixing that bug it detected another defect: Transfers succeed even when the source account’s balance is insufficient:

Property failed after 4 tests
{ seed: 1922422813, path: "3:0:0:1:0", endOnFailure: true }
Counterexample: [0n,0n,1n]
Shrunk 4 time(s)
...