CODEWITHSUNDEEP
javascript
N Sharma
N Sharma

Reputation: 34497

TypeError: Cannot read property 'hello' of undefined in JavaScript

Hi I am trying to run below program. I have one function hello and I am calling it inside b. It gives me an error

TypeError: Cannot read property 'hello' of undefined

class ChildClass {
  // constructor
  constructor(param, ...args) {

    this.hello = function (a) {
      console.log(a);
    }

    this.obj = {
      a: {
        b() {
          this.hello(2)
        }
      }
    }
  }
}

Does anyone know what I am doing wrong here ?

Upvotes: 0

Views: 266

Answers (1)

Andrei CACIO
Andrei CACIO

Reputation: 2119

The problem is that this isn't pointing to the ChildClass instance but to the inner a object. One solution might be:

class ChildClass {
  // constructor
  constructor(param, ...args) {

    const hello = this.hello = function (a) {
      console.log(a);
    }

    this.obj = {
      a: {
        b() {
         hello(2)
        }
      }
    }
  }
}

However if you don't intend to use hello() as an instance method, you can simply declare it in a private const and don't put it on this:

class ChildClass {
  // constructor
  constructor(param, ...args) {

    const hello = function (a) {
      console.log(a);
    }

    this.obj = {
      a: {
        b() {
         hello(2)
        }
      }
    }
  }
}

Depends on what you want to do.

Or, you can save the "real" this in a const:

class ChildClass {
  // constructor
  constructor(param, ...args) {
    const instance = this;

    this.hello = function (a) {
      console.log(a);
    }

    this.obj = {
      a: {
        b() {
         instance.hello(2)
        }
      }
    }
  }
}

Upvotes: 1

Related Questions