How to get the most specific X such that y instanceof X is true?

Question

Given some object y , how can I find out the most specific X , so the expression

 y instanceof X 

evaluates to true ?

For example, the following two expressions evaluate to true

 [] instanceof Object [] instanceof Array 

... but Array more specific than Object .

Answer 1

By MDN :

The instanceof operator checks whether an object in its prototype chain has a constructor prototype property.

So Kolink is correct that checking y.constructor will give you your answer. (Note that this assumes that you or someone else has not mutilated y.constructor in the meantime, i.e. y __proto__ (the beginning of its prototype chain) actually points to y.constructor.prototype )

Answer 2

Here is the code I just wrote that does the job. Read the comments for more details!

Note. I tested this piece of code in Google Chrome!

 function compareTypes(a,b){ if (a instanceof b) { if(b instanceof a) { /* same types */ return 0; } else { /* first is son */ return -1; } } if (b instanceof a) { /* second is son */ return 1; } return 0; /* not related to each other */ } function closestInstanceOf(obj, possibleTypesAsAdditionalArguments) { /* * How to call this function: * The first argument should be the object to check. * All other arguments are possible types. */ if (arguments.length === 0) return; /* undefined */ if (arguments.length === 1) return Object; /* If no possible type was passed, we returned Object as the base type */ /* converts the arguments to Javascript array */ var types = Array.prototype.slice.call(arguments, 0); var obj = types[0]; /* object to be checked */ types.splice(0, 1); /* possible types */ /* Sorts the types array from the more specific to the more generic type */ types.sort(compareTypes); /* find the first match */ for (var i = 0; i < types.length; i++) { var type = types[i]; if (obj instanceof type) { return type; } } return Object; } // examples // 1. Sorting types console.info([Array, Number, Object, String, Boolean].sort(compareTypes)); // results: //[function, function, function, function, function] // 0: function Array() { [native code] } // 1: function Number() { [native code] } // 2: function String() { [native code] } // 3: function Boolean() { [native code] } // 4: function Object() { [native code] } // 2. Find specific type console.info(closestInstanceOf([], Array, Number, Object, String, Boolean)); // results: // function Array() { [native code] } // 3. Find specific type console.info(closestInstanceOf(new String("Hello World"), Array, Number, Object, String, Boolean)); // results: // function String() { [native code] } // 4. Find specific type console.info(closestInstanceOf(new Number(19), Array, Number, Object, String, Boolean)); // results: // function Number() { [native code] } // 5. Find specific type console.info(closestInstanceOf("Hello world", Array, Number, Object, String, Boolean)); // results: // function Object() { [native code] } // Note: String iterals derived from object! // 6. Find specific type console.info(closestInstanceOf(18, Array, Number, Object, String, Boolean)); // results: // function Object() { [native code] } // The same applies for numbers // 7. Find specific type console.info(closestInstanceOf(false, Array, Number, Object, String, Boolean)); // results: // function Object() { [native code] } // And booleans... // 8. Find specific type console.info(closestInstanceOf(new Boolean(false), Array, Number, Object, String, Boolean)); // results: // function Object() { [native code] } // Again, using new Boolean does the job! // 9. Find specific type console.info(closestInstanceOf(function () { /* this is custom class */ }, Array, Number, Object, String, Boolean)); // results: // function Object() { [native code] } // This code works for custom class as well, try it!