JavaScript - Using Objects as Keys


// JavaScript - Problem with using object as key:
var a={},

The above code will log 456 instead of 123 to the console.  When setting an 
object property, JavaScript will implicitly stringify the parameter value. In 
this case, since b and c are both objects, they will both be converted to 
"[object Object]". As a result, a[b] anda[c] are both equivalent to 
a["[object Object]"] and can be used interchangeably. Therefore, setting or 
referencing a[c] is precisely the same as setting or referencing a[b].

JavaScript object is essentially a hashtable of name-value pairs where the 
names (i.e., keys) are strings. And they are always strings. In fact, when an 
object other than a string is used as a key in JavaScript, no error occurs; 
rather, JavaScript silently converts it to a string and uses that value as the 
key instead. 

JavaScript automatically converts the key values to strings using each object’s 
toString() method. And since neither foo nor bar defines its own custom 
toString() method, they both use the same default implementation. That 
implementation simply generates the literal string “[object Object]” when it 
is invoked.

var foo = new Object();
var bar = new Object();
var map = new Object();

map[foo] = "foo";    // --> map["[Object object]"] = "foo";
map[bar] = "bar";    // --> map["[Object object]"] = "bar";
                     // NOTE: second mapping REPLACES first mapping!

alert(map[foo]);     // --> alert(map["[Object object]"]);
                     // and since map["[Object object]"] = "bar",
                     // this will alert "bar", not "foo"!!
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