[Typescript] Build Method decorators in Typescript

To using decorate, we can modifiy tsconfig.json:

{
  "compilerOptions": {
    ...
    "experimentalDecorators": true,
    ...
  }
}

So for example we want to build a '@LogMethod' decorator, which arroding to the system logging level to decide whether should log the action.

enum LoggingLevel {
    INFO,
    WARNING,
    DEBUG,
    TRACE
}
const loggingLevel = LoggingLevel.DEBUG;

class Database {
    name = 'ABC';

    @LogMethod(LoggingLevel.DEBUG)
    saveData(data: any) {
        console.log(`save user ${data.name} to the database ${this.name}`);
    }
}

const db = new Database();
db.saveData({name: 'zhentian'});

So for example, current is 'DEBUG' for the system, so when log level set to the 'DEBUG' or 'TRACE', action will be logged, 'INFO', 'WARNING' will be not.

Decorator:

which is simplya function return a meta function:

function LogMethod(level: LoggingLevel):Function {
    return (target: any, propertyKey: string,
            descriptor: PropertyDescriptor) => {
    }

}

If we log out each params:

'descriptor' is something we are looking for, 'descriptor.value' holding the function which is 'saveData' function, we can put into a variable:

const originalFunction:Function = descriptor.value;

Then we can create a new function to wrap this 'originalFunction' in order to provide some additional functionality:

        descriptor.value = function(...args:any[]) {
            if (level <= loggingLevel) {
                console.log(">> " + propertyKey + " " +  JSON.stringify(args));
            }

            originalFunction.apply(this,args);
        };

Full code:

enum LoggingLevel {
    INFO,
    WARNING,
    DEBUG,
    TRACE
}
const loggingLevel = LoggingLevel.DEBUG;

function LogMethod(level: LoggingLevel):Function {
    return (target: any, propertyKey: string,
            descriptor: PropertyDescriptor) => {

        const originalFunction:Function = descriptor.value;

        descriptor.value = function(...args:any[]) {
            if (level <= loggingLevel) {
                console.log(">> " + propertyKey + " " +  JSON.stringify(args));
            }

            originalFunction.apply(this,args);
        };

    }

}

class Database {
    name = 'ABC';

    @LogMethod(LoggingLevel.DEBUG)
    saveData(data: any) {
        console.log(`save user ${data.name} to the database ${this.name}`);
    }
}

const db = new Database();
db.saveData({name: 'zhentian'});