Invoke a method (not a function) using a string

Question

Note to dup finders: please feel free to mark this as a dup if the dup shows how to solve with classes and methods, not just functions.

I'm building a command line tool that solicits string input from the user then tries to invoke a class with matching methods and params. How do I call so that this is defined?

I have a class:

class MyClass {
  constructor() {
    this.foo = 'bar';
  }
  myMethod(param) {
    console.log(param, this.foo);  // this is undefined, based on how I invoke it
  }
}

I'd like to do this, once I get the user input...

let userInputMethod = 'myMethod';
let userInputParam = 'param';

const myInstance = new MyClass();
const method = myInstance[userInputMethod];
method(userInputParam);  // error, because I need somehow to set the context of this

Eliaz Bobadilla · Accepted Answer · 2022-03-22 17:45:23Z

You could bind it 😎

let userInputMethod = 'myMethod';
let userInputParam = 'param';

const myInstance = new MyClass();
const method = myInstance[userInputMethod].bind(myInstance)
method(userInputParam);

But at that point why not use plain objects?

// foo.js
const foo = "bar";

const hey = {
  myMethod(param) {
    console.log(param, foo);
  },
};

//main.js
let userInputMethod = 'myMethod';
let userInputParam = 'param';

const method = hey[userInputMethod];
method(userInputParam);

If you need your class to receive data you can use closures like this 👇

function makeInstance({ foo }) {
  return {
    myMethod(param) {
      console.log(param, foo);
    },
  };
}

axtck · Accepted Answer · 2022-03-22 17:47:52Z

The this context is lost, you need to bind it.

class MyClass {
  constructor() {
    this.foo = 'bar';
  }
  myMethod(param) {
    console.log(param, this.foo);
  }
}

let userInputMethod = 'myMethod';
let userInputParam = 'param';

const myInstance = new MyClass();
const method = myInstance[userInputMethod].bind(myInstance); // bind
method(userInputParam);

Or you could use arrow functions.

class MyClass {
  constructor() {
    this.foo = 'bar';
  }
  myMethod = (param) => {
    console.log(param, this.foo);
  }
}

let userInputMethod = 'myMethod';
let userInputParam = 'param';

const myInstance = new MyClass();
const method = myInstance[userInputMethod]
method(userInputParam);

Very similar to my answer, posted some minutes later after mine :) btw you shouldn't use arrow functions like that but in the constructor.
Actually, @EliazBobadilla, I saw both answers, and preferred this one because of the working snippet. (I hope you didn't downvote on this basis).

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