Adding functions dynamically to function.prototype

Question

I am a newbie to nodejs. I am writing a helper function to build JSON using a schema, I am trying to add functions (mostly setters) to set values. The following is a simple version of it.

function Task() {
  this.action = {};
  this.schedule = {};
}

function capitalize(str) {
  return `${str[0].toUpperCase()}${str.slice(1)}`;
}

const scheduleProps = [
'startAt',
'repeatEvery',
'endAt',
'count',
'interval'
];

Add methods to it dynamically

for(var i=0; i<scheduleProps.length; i++) {
  Object.defineProperty(Task.prototype, `set${capitalize(scheduleProps[i])}`, {
    enumerable: true,
    configurable: false,
    writable: true,
    value: (value) => {
      this.schedule[scheduleProps[i]] = value;
    }
  });
}

When I call the following way I expect obj.schedule.repeatEvery to contain value 10.

obj = new Task();
obj.setRepeatEvery(10);

Instead I get

TypeError: Cannot set property 'repeatEvery' of undefined

I even tried to set the functions like so

  Task.prototype[`set${capitalize(scheduleProps[i])}`] = (val) => {
    this.schedule[scheduleProps[i]] = val;
  }

In this case I get

TypeError: Cannot set property 'interval' of undefined
at Task.(anonymous function) [as setRepeatEvery]

How can I set methods to a function.prototype dynamically? Thanks a lot for your help

Where is the loop run? Lambda (arrow) functions won't have the correct this scope if that loop doesn't run inside a member method. try using a regular function — Balázs Édes
– Balázs Édes, Commented Mar 24, 2017 at 22:42
Awesome Balázs!! I had to move it into the constructor and it worked. I need to have them as lambda function in order to set the functions to the main function prototype — Gatothgaj
– Gatothgaj, Commented Mar 24, 2017 at 22:54
JSON is a data exchange format, like XML or YAML. I don't see any JSON in your example. JavaScript objects are not JSON. — Felix Kling
– Felix Kling, Commented Mar 24, 2017 at 22:58
Another question I had was which of the methods is better? Is defining by Object.defineProperty() prefered over Task.prototype[methodName] ? — Gatothgaj
– Gatothgaj, Commented Mar 24, 2017 at 22:59
What are you trying to accomplish? Can you show what you want the prototype to look like after you add these dynamic things to it? And, then how you want to access things. For starters, your for loop has already run to completion so your i variable is wrong when your value method gets called. — jfriend00
– jfriend00, Commented Mar 24, 2017 at 23:04

Jitendra Khatri · Accepted Answer · 2017-03-24 23:18:36Z

1

You were facing that issue mainly because of function assignment for "value" key.

You can find some changes there which is creating a closure and also changed the syntax of assigning a function at key of an object.

Closure is created to maintain the value of i for that particular iteration.

function Task() {
  this.action = {};
  this.schedule = {};
}

function capitalize(str) {
  return `${str[0].toUpperCase()}${str.slice(1)}`;
}

const scheduleProps = [
    'startAt',
    'repeatEvery',
    'endAt',
    'count',
    'interval'
];

for(var i=0; i<scheduleProps.length; i++) {
  Object.defineProperty(Task.prototype, `set${capitalize(scheduleProps[i])}`, {
    enumerable: true,
    configurable: false,
    writable: true,
    value: (function(i) { return function(value) { this.schedule[scheduleProps[i]] = value;} })(i)
  });
}

obj = new Task();
obj.setRepeatEvery(10);

edited Mar 24, 2017 at 23:18

answered Mar 24, 2017 at 22:53

Jitendra Khatri

7743 silver badges16 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Gatothgaj Over a year ago

Thanks Jitendra, I had to move the for loop into the Task() constructor. By adding anonymous function the this is the scope of the anonymous funciton. I wanted the methods to be added to the Task.prototype

Jitendra Khatri Over a year ago

By above code those functions are already on prototype of Task Class. You can run this code in Browser console and after that type "Task.prototype." this will shows you all there.

RobG Over a year ago

An answer should explain why the OP has their issue and how to fix it.

Jitendra Khatri Over a year ago

@RobG Thanks. Added Explanation for the same.

Gatothgaj Over a year ago

Thanks a lot Jitendra. As you had mentioned I was missing the array scope when the function was actually executing. Awesome.. !

Collectives™ on Stack Overflow

Adding functions dynamically to function.prototype

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related