JS Prototype error "Prototype is not defined"

Question

I am new to JS, I write below code, but I got the error "Prototype is not defined".

var proto = {
    describe: function () {
        return 'name: ' + this.name;
    }
};

var obj = {                
    [[Prototype]]: proto, //error in this line
    name:'obj'
};

console.log(proto.describe());
console.log(obj.describe());

[[prototype]] is an internal property, not a valid name for a key. You want to use Object.create — Bergi
– Bergi, Commented Sep 6, 2016 at 12:57

nils · Accepted Answer · 2016-09-06 14:42:02Z

2

[[Prototype]] is only specification-speech for an internal property (the link in the prototype chain). To link obj to proto through the prototype chain, you can use Object.create:

   var obj = Object.create(proto);
   obj.name = 'obj';

Or Object.setPrototypeOf in ES6/ES2015:

    var obj = {                
        name:'obj'
    };

    Object.setPrototypeOf(obj, proto);

Alternatively, there is the legacy property __proto__, which is not necessarily recommended though:

    var obj = {                
        __proto__: proto,
        name:'obj'
    };

edited Sep 6, 2016 at 14:42

answered Sep 6, 2016 at 12:57

nils

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3 Comments

Bergi Over a year ago

Don't use __proto__. It's been officially deprecated with ES6. Use Object.setPrototypeOf instead if you have to.

nils Over a year ago

Oh, I wasn't aware of that, Allen Wirfs-Brock even "recommended" using it: twitter.com/awbjs/status/730789464021159938. Object.setPrototypeOfmodifies the prototype of a live object, so there will be a performance hit. __proto__ in the object literal should have avoided that. Looks like there is no clean method to link object literals anymore…

Bergi Over a year ago

If Object.setPrototypeOf is used immediately after the object creation, it shouldn't have a performance hit.

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