TypeScript contract-based decorators

tscontract.ts

type MethodDecorator = (target: any, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor) =>
    TypedPropertyDescriptor | void;

type AssertFn = (...args: Array<any>) => void;

class TSContract {
    private static isCustomContractInterruptionCb: boolean = false;

    private static contractInterruptionCb(message: string): void {
        console.warn(message);
    }

    static setupContractInterruptionCb(cb: (message: string) => void): void {
        if (TSContract.isCustomContractInterruptionCb) {
            console.warn('Custom contract interruption callback already setted, break');
            return;
        }
        TSContract.contractInterruptionCb = cb;
        TSContract.isCustomContractInterruptionCb = true;
    }

    static assert(expression: boolean, errorMessage: string): void {
        if (!expression) {
            TSContract.contractInterruptionCb(errorMessage);
        }
    }

    static In(assertFn: AssertFn): MethodDecorator {
        return function(target: any, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor): TypedPropertyDescriptor {
            const originalValue = descriptor.value;
            function newValue(...args) {
                assertFn(...args);
                return originalValue.apply(this, args);
            }
            descriptor.value = newValue;
            return descriptor;
        };
    }

    static Out(assertFn: AssertFn): MethodDecorator {
        return function(target: any, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor): TypedPropertyDescriptor {
            const originalValue = descriptor.value;
            function newValue(...args) {
                result = originalValue.apply(this, args);
                assertFn(result);
                return result;
            }
            descriptor.value = newValue;
            return descriptor;
        };
    }
}

class ContractTest {
    @TSContract.In(a => {
        TSContract.assert(typeof a === 'number', 'Not a number!');
    })
    @TSContract.Out(retVal => {
        TSContract.assert(!retVal, 'Wat?! Return?!');
    })
    doSome(a: number, withReturn: boolean = false): void {
        console.log(a);
        if (withReturn) {
            return a;
        }
    }
}

TSContract.setupContractInterruptionCb((message: string) => {
    console.error(message);
});

// cant setup another callback
TSContract.setupContractInterruptionCb((message: string) => {
    console.error(message);
});

const ct = new ContractTest();
ct.doSome(12);
ct.doSome('12');
ct.doSome(12, true);