I was confusing myself a little with a thought experiment and now I'm looking for some advice. Its about ECMAscript references and the Array.prototype.indexOf()
method.
Lets start easy:
var container = [ ];
// more code
container.push( 5 );
container.push( 7 );
container.push( 10 );
So now we pushed some " primitive values " into our ECMAscript array (whether or not that statement is true I'll come back for), at least I imagined it like this so far. A call to
container.indexOf( 7 );
will return 1
as expected. The big question I'm having is, if .indexOf()
really compares the primitive value or if in reality a Number()
object is created + stored and its reference is getting compared. This becomes a little more obvious if we re-write that like so:
var a = 5,
b = 7,
c = 10;
var container = [ ];
container.push( a );
container.push( b );
container.push( c );
container.indexOf( b );
Until this point, one could still easily argue that all .indexOf()
needs to do is to compare values, but now lets look at something like this:
var a = { name: 'a', value: 5 },
b = { name: 'b', value: 10 },
c = { name: 'c', value: 15 };
var container = [ ];
// more code
container.push( a );
container.push( b );
container.push( c );
Here, we filled that container array with object-references and still, .indexOf()
works as expected
container.indexOf( b ) // === 1
while a call like this
container.indexOf({ name: 'b', value: 10 });
obviously returns -1
since we are creating a new object and get a new reference. So here it must internally compare references with each other, right?
Can some ECMAscript spec genius confirm that or even better link me some material about that ?
A side question on this would be if there is any possibly way to access an internally stored object-reference within a lexicalEnvironment respectively Activation Object .
It boils down to indexOf()
comparing against each array property in turn using the same algorithm as the ===
operator.
The relevant section of the ECMAScript 5 spec is section 15.4.4.14, step 9, section b (highlighting mine):
If kPresent is true, then
i. Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
ii. Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement and elementK.
iii. If same is true, return k.
References:
I'm not sure if this is guaranteed across all ECMAScript implementations or not, but the Mozilla documentation states that it uses strict equality to make the comparison (===). As such this would exhibit the behaviour you describe, comparing by values on primitives, but by reference on objects (see strict equality ).
@Tim Down is right. indexOf
does strict comparison. I am giving a demonstration of this by overriding valueOf
function
var MyObject = function(n, v){
this.name = n;
this.value = v;
}
MyObject.prototype.valueOf = function(){
return this.value;
}
var a = new MyObject("a", 5);
var b = new MyObject("b", 10);
var c = new MyObject("c", 15);
var container = [ ];
container.push( a );
container.push( b );
container.push( c );
console.log(b == 10); // true
console.log(container[1] == 10); // true
console.log(b === 10); // false
container.indexOf(10); // -1
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