Building objects with attributes using constructor functions

Question

Building objects with attributes using constructor functions

My question pertains to JavaScript constructor function prototypes. Suppose I have code like the following:

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');
c.a['b'](30);

Is this code correct? And if so, how does the c object access the a['b'] function? Does it follow its proto property to reach the constructor function? Does the constructor function then set the b property on the newly created object?

javascript

Answer 1

Answer №1

Is this situation acceptable?

No, there is an error on the final line. The c object does not contain an a property:

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');
c.a['b'](30); // This line throws a TypeError

new a generates an object that inherits from the object indicated by a.prototype. However, a.prototype does not have an a property.

If you wish to access a from c, you can utilize the constructor property in a.prototype which points to a:

c.constructor['b'](30);

However, executing this would make the second function consider this as pertaining to c.constructor and consequently add an age property to the initial function:

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');
c.constructor['b'](30);
console.log(a.age); // Outputs: 30

In retrospect, the overall structure of

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');

...seems somewhat illogical. It's typically unnecessary to place b under a in such a manner.

Additional note: Your code is susceptible to The Horror of Implicit Globals (an article on my personal blog): Remember to declare your variables.

Answer 2

Is this situation acceptable?

No, there is an error on the final line. The c object does not contain an a property:

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');
c.a['b'](30); // This line throws a TypeError

new a generates an object that inherits from the object indicated by a.prototype. However, a.prototype does not have an a property.

If you wish to access a from c, you can utilize the constructor property in a.prototype which points to a:

c.constructor['b'](30);

However, executing this would make the second function consider this as pertaining to c.constructor and consequently add an age property to the initial function:

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');
c.constructor['b'](30);
console.log(a.age); // Outputs: 30

In retrospect, the overall structure of

a = function (name){this.name = name};
a['b'] = function (age){this.age = age};
c = new a('John');

...seems somewhat illogical. It's typically unnecessary to place b under a in such a manner.

Additional note: Your code is susceptible to The Horror of Implicit Globals (an article on my personal blog): Remember to declare your variables.

Building objects with attributes using constructor functions

Answer №1

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