Typescript | Call a function every time a function is called

Question

I am trying to write a Typescript API service. For the service I need a way to check that the method exists when a function such as get is called.

I realise I can do like

get(endpoint: string) {
    this.handleRequest();
}

post(endpoint: string, data: any) {
    this.handleRequest();
}

But I don't particularly want to do that are the top of every method so I didn't know if there was a way to listen within the constructor of the Typescript class for a call of a child function.

It seems a little far fetched to be able to do this but it would be extremely useful in cases like mine so I don't have to keep on doing it.

export class ApiService {
    base_url: string = 'https://jsonplaceholder.typicode.com/posts';
    methods = ['GET', 'POST', 'PUT', 'PATCH', 'DELETE'];

    /**
     * Create an instance of ApiService.
     * @param {string} base_url
     */
    constructor(base_url: string = null) {
        this.base_url = base_url ? base_url : this.base_url;
    }

    get(endpoint: string): string {
        // duplicated line
        this.handleRequest();

        return 'get method';
    }

    post(endpoint: string, data: any): string {
        // duplicated line
        this.handleRequest();

        return 'post method';
    }

    protected handleRequest(): string {
        return 'handle the request';
    }
}

Which method (or function) does or does not exist? How do you want to check it? Can you please post the complete code with all the duplication of doing the checks everywhere? — Bergi, Mar 05 '18 at 21:27
How do you mean? I want to be able to call `api.get('endpoint');` which will then return the values but whenever `api.get('endpoint')` or `api.post('endpoint', ['name' => 'tallent']);` to both run the `this.handleRequest();`'but I don't want to have to duplicate `this.handleRequest();` for all of the methods. — , Mar 05 '18 at 21:28
Would decorators be a viable solution? You'd still have to attach the decorator to every method. — y2bd, Mar 05 '18 at 21:32
Does handleRequest have to be called for all methods in the class ? Most of them ? Or just a few ? — Titian Cernicova-Dragomir, Mar 05 '18 at 21:35
@TitianCernicova-Dragomir It'll be called in all of the methods — , Mar 05 '18 at 21:35
@tallent123 See https://www.typescriptlang.org/docs/handbook/decorators.html#method-decorators — y2bd, Mar 05 '18 at 21:37
@TitianCernicova-Dragomir yeah, all other than `handleRequest` itself. I didn't know if it was possible to just literally listen for a function call from the constructor. Been Googling but no luck. — , Mar 05 '18 at 21:37

score 5 · Accepted Answer · answered Mar 05 '18 at 21:46

You can do this using a decorator that will override all methods of the class with a method that calls the original implementation and your extra method:

function handleRequest() {
    return function<TFunction extends Function>(target: TFunction){
        for(let prop of Object.getOwnPropertyNames(target.prototype)){
            if (prop === 'handleRequest') continue;
            // Save the original function 
            let oldFunc: Function = target.prototype[prop];
            if(oldFunc instanceof Function) {
                target.prototype[prop] = function(){
                    this['handleRequest'](); // call the extra method
                    return oldFunc.apply(this, arguments); // call the original and return any result
                }
            }
        }
    }
}

@handleRequest()
export class ApiService {
    base_url: string = 'https://jsonplaceholder.typicode.com/posts';
    methods = ['GET', 'POST', 'PUT', 'PATCH', 'DELETE'];

    /**
     * Create an instance of ApiService.
     * @param {string} base_url
     */
    constructor(base_url: string = null) {
        this.base_url = base_url ? base_url : this.base_url;
    }

    get(endpoint: string): string {
        return 'get method';
    }

    post(endpoint: string, data: any): string {
        return 'post method';
    }

    protected handleRequest(): void {
        console.log('handle the request');
    }
}

let s = new ApiService();
s.get("");
s.post("", null);

Typescript | Call a function every time a function is called

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