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?

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?

• javascript
• inheritance
• prototype

• 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 Commented Nov 7, 2012 at 14:51
• Maybe a situation where A inherited a constructor from another object? – Ilia Choly Commented Nov 7, 2012 at 14:58
• Alright, but then the two calls are not equivalent. You're talking about two different situations. – I Hate Lazy Commented Nov 7, 2012 at 15:01
• Hence the last part of the question. – Ilia Choly Commented Nov 7, 2012 at 15:02
• 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 Commented Nov 7, 2012 at 15:03

1 Answer 1

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

1. It's faster. The interpreter doesn't need to access prototype.constructor.
2. 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;