javascript prototype constructor and instanceof

Question

When I checked instanceof method, the results are not same .

function A(){}
function B(){};

First I assigned prototype ( reference ) property , into A

A.prototype = B.prototype;
var carA =  new A();

console.log( B.prototype.constructor );
console.log( A.prototype.constructor == B );
console.log( B.prototype.constructor == B );
console.log( carA  instanceof A );
console.log( carA  instanceof B );

The last 4 condition on above returns true .

But when I tried to assign constructor of B .. results are not same .

A.prototype.constructor = B.prototype.constructor;
var carA =  new A();

console.log( B.prototype.constructor );
console.log( A.prototype.constructor == B );
console.log( B.prototype.constructor == B );
console.log( carA  instanceof A );
console.log( carA  instanceof B );

On this case carA instanceof B returns false . Why it returns false

Community · Accepted Answer · 2017-05-23 12:05:29Z

1

I found answer from link .. https://stackoverflow.com/a/12874372/1722625

instanceof actually checking internal [[Prototype]] of left-hand object . Same like below

function _instanceof( obj , func ) {
    while(true) {
       obj = obj.__proto__; // [[prototype]] (hidden) property
       if( obj == null) return false;
       if( obj ==  func.prototype ) return true;
    }
}

// which always true 
console.log( _instanceof(carA , B ) == ( obj instanceof B ) )

if it returns true, obj is instanceof B

edited May 23, 2017 at 12:05

CommunityBot

11 silver badge

answered Jul 17, 2013 at 14:30

rab

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