calculate an object property based on the value of another property of the same object

I need to find out the value of "name" inside on the obj object. How can I find it without function invocation?

I wanna use just obj.isActive not obj.isActive()

let obj = {
  name: "X Æ A-12 Musk",
  isActive: function () {
    return this.name.length > 4;
  },
};

// and after a while I need to check if is active:

console.log(obj);

// { 
//   name: 'X Æ A-12 Musk',
//   isActive: [Function: isActive]  <--------- NOT COOL ! 
// }

If use an IFEE:

let obj = {
  name: "X Æ A-12 Musk",
  isActive: (function () {
    return this.name.length > 4;
  })(),
};

I get:

return this.name.length > 4;
                ^
TypeError: Cannot read properties of undefined (reading 'length') 

If you do not want to have to call isActive as a function, you can use a getter.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const obj = {
  name: "X Æ A-12 Musk",
  get isActive () {
    return this.name.length > 4;
  },
};

console.log(obj.isActive);

<!-- end snippet -->

It's not clear from your question why you can do an IIFE but not a getter, but here are some other potential workarounds.

1. Use a Proxy to intercept the property accessor:

If you can't modify the object itself you could wrap it in a Proxy, and pass the proxy to the library.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let obj = {
  name: "X Æ A-12 Musk",
  isActive: function () {
    return this.name.length > 4;
  },
};

const proxy = new Proxy(obj, {
  get(target, prop) {
    if (prop === 'isActive') {
      return target?.isActive?.();
    }
    return target[prop]
  }
});

console.log(proxy.isActive); // true
console.log(proxy.name); // "X Æ A-12 Musk"
console.log(proxy);
/*
{
  "name": "X Æ A-12 Musk",
  "isActive": true
}
*/

<!-- end snippet -->

This could be generalized to invoke any getter function if it's present on the target:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let obj = {
  name: "X Æ A-12 Musk",
  isActive: function () {
    return this.name.length > 4;
  },
};

const proxy = new Proxy(obj, {
  get(target, prop) {
    const x = target[prop];
    return typeof x === 'function' ? target[prop]() : x;
  }
});

console.log(proxy.isActive); // true
console.log(proxy.name); // "X Æ A-12 Musk"
console.log(proxy);
/*
{
  "name": "X Æ A-12 Musk",
  "isActive": true
}
*/

<!-- end snippet -->

Note: I'm invoking target[prop]() instead of x() here because invoking the disassociated function x() has scope implications that could produce errors or unexpected results.(The same issue that makes your IIFE blow up.)

2. Use Object.defineProperty to establish the getter.

Again, it's not clear to me why you can't put a getter on the object literal directly, so I don't know if this is viable either.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let obj = {
  name: "X Æ A-12 Musk",
  isActive: function() {
    return this.name.length > 4;
  },
};

Object.defineProperty(obj, "isActive", {
  get() {
    return this.name.length > 4
  }
})

console.log(obj);
/*
{
  "name": "X Æ A-12 Musk",
  "isActive": true
}
*/

<!-- end snippet -->

Similarly to ray's answer, what about something like this:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let obj = {
    name: "test"
};

Object.defineProperty(obj, "isActive", {
    get: function() {
        return obj.name.length > 3;
    }
});

console.log(obj.isActive); // Output: true

<!-- end snippet -->

If it helps, and you were to use it constantly, you could create a helper function:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function defineIsActive(object, condition) {
    
    Object.defineProperty(obj, "isActive", {
        get: function() {
            return condition
        }
    });
}

let obj = {
  name: 'test'
}

// And simply use it like so:
defineIsActive(obj, obj.name.length > 3)

console.log(obj.isActive)

<!-- end snippet -->

> I cannot add more properties to that object

You can create wrapper object prototyped by the original object, and define getter in the former.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let obj = {
  name: "X Æ A-12 Musk",
  isActive: function () {
    return this.name.length > 4;
  },
};

function makeWrapper (obj) {
  let wrapper = Object.create(obj);
  Object.defineProperty(wrapper, 'isActive', {
    get () { return obj.isActive(); },
    enumerable: true
  });
  return wrapper;
}

let obj2 = makeWrapper(obj);
console.log(obj2);

<!-- end snippet -->