TypeScript dynamic runtime contracts example

wt-typescript-contracts.ts

import 'reflect-metadata';

let _contractDictMetadataKey = 'contractDict';

let ApplyContracts = () => {
    return (target, key) => {

        let originalMethod = target[key];
        let contractDict = Reflect.getOwnMetadata(_contractDictMetadataKey, target, key);

        target[key] = function (...argList) {

            if (contractDict !== undefined) {

                argList.forEach((value, index) => {

                    let contract = contractDict[index];

                    if (contract === undefined) {
                        return;
                    }

                    if (!contract(value)) {
                        throw new Error(`Value '${value}' does not respect contract for \`${target.constructor.name}.${key}(param_${index})\`.`);
                    }

                });

            }

            return originalMethod(...argList);

        }

    };
};

let Contract = (contract: (value: any) => boolean) => {
    return (target, key, index):any => {

        let contractDict = Reflect.getOwnMetadata(_contractDictMetadataKey, target, key);

        if (contractDict === undefined) {
            contractDict = {};
        }

        contractDict[index] = contract;

        Reflect.defineMetadata(_contractDictMetadataKey, contractDict, target, key);

    };
};

let isPositive = value => value > 0;

class Utils {

    @ApplyContracts()
    double(@Contract(isPositive) number: number): number {
        return number * 2;
    }

}

console.log(new Utils().double(2)); // 4
console.log(new Utils().double(0)); // Error: Value '0' does not respect contract for `Utils.double(param_0)`.