design-by-contract

A TS/JavaScript library to put runtime checks using 'design by contract' approach.

``
npm i design-by-contract
`

Usage

`ts
import { functionByContract } from "design-by-contract";

try {
const thisMayThrowError =
functionByContract({
fn: () => { / your function mutating outer scope / },
preconditions: Precondition[];
postconditions: Postcondition[];
invariants: Invariant[];
async: boolean, // NOTE: must be set along with asyncFnArgs if your fn is async
asyncFnArgs: any[],
})
} catch(e) {
// Clearly handle error
console.error(e);
}
`

* Precondition | Postcondition | Invariant has type: [ConditionFunction, string?].
* preconditions are checked before the function executes
* postconditions are checked after the function executes
* invariants are checked both before and after the function executes

Basic Example

`js
let n1 = 1;
let n2 = 2;
let sum = 0;

let addTwoNumbersWithChecks = functionByContract({
fn: () => {
sum = n1 + n2;
return sum;
},
preconditions: [
[
() => typeof n1 === "number" && typeof n2 === "number",
"n1 and n2 should be numbers",
],
],
postconditions: [[() => sum - n2 === n1 && sum - n1 === n2]],
invariants: [[() => n1 === 1 && n2 === 2, "n1 & n2 must not change"]],
});

try {
// this is okay
sum = addTwoNumbersWithChecks();
} catch (error) {
console.error(error);
}

// Let's ruin it
addTwoNumbersWithChecks = functionByContract({
fn: () => {
n1 = 0; // somehow corrupted memory
sum = n1 + n2;
return sum;
},
preconditions: [
[
() => typeof n1 === "number" && typeof n2 === "number",
"n1 and n2 should be numbers",
],
],
postconditions: [[() => sum - n2 === n1 && sum - n1 === n2]],
invariants: [[() => n1 === 1 && n2 === 2, "n1 & n2 must not change"]],
});

try {
// Invariant violated so it'll throw
sum = addTwoNumbersWithChecks();
} catch (error) {
// Error: n1 & n2 must not change
console.error(error);
}
``