Calling base constructor - Javascript

Question

There are 2 ways to call the parent constructor in the child.

var A = function A() {
  this.x = 123;
};

var B = function B() {

  // 1. call directly
  A.call(this);

  // 2. call from prototype
  A.prototype.constructor.call(this);
};

B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;

Are there any situations when one would be safer/better than the other, or are they always equivalent?

What would compel you to use the second version? You're just taking a long path to get to the same function. — I Hate Lazy, Nov 07 '12 at 14:51
Maybe a situation where `A` inherited a constructor from another object? — Ilia Choly, Nov 07 '12 at 14:58
Alright, but then the two calls are not equivalent. You're talking about two different situations. — I Hate Lazy, Nov 07 '12 at 15:01
But if you're describing a situation where they are not equivalent, then you already know they may not be not equivalent, so why ask the question? If the situation is such that you know they *are* equivalent, then what would compel you to use the second version? — I Hate Lazy, Nov 07 '12 at 15:03
@iliacholy - From what I see you're doing in your code, perhaps you might find the following answer interesting: http://stackoverflow.com/a/12041555/783743 — Aadit M Shah, Nov 07 '12 at 15:14

Aadit M Shah · Accepted Answer · 2012-11-07T14:59:22.320

It's always better to use the base constructor directly for the following reasons:

It's faster. The interpreter doesn't need to access prototype.constructor.
It's safer. Consider the program below.

A inherits from C, but I forgot to set A.prototype.constructor back to A. So it now points to C. This causes problems in the constructor B if we use the second method:

var C = function C() {
    // some code
};

var A = function A() {
  this.x = 123;
};

A.prototype = Object.create(C.prototype);
// I forgot to uncomment the next line:
// A.prototype.constructor = A;

var B = function B() {

  // 1. call directly
  A.call(this);

  // 2. call from prototype
  A.prototype.constructor.call(this); // A.prototype.constructor is C, not A
};

B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;

Calling base constructor - Javascript

1 Answers1