Invoke a method (not a function) using a string

March 22, 2022

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

>Solution :

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);