Not able to understand the behavior of WeakMap in ECMA6

Question

I am facing a weird scenario while playing with WeakMap in ECMA6. I am writing a class which is as follows

'use strict';
class WeekMaptest {

    constructor(options){
        console.log("constructor");
        this.weekMap = new WeakMap();
        this._init(options);
    }
    _init(options) {
        console.log("init called");
        var privateProps = {
            name: options.name,
            email: options.email
        };
        this.weekMap.set(this, privateProps);
    }

    getName(){
        return this.weekMap.get(this).name;
    }


}

Now calling this class to instantiate an object

var obj = new WeekMaptest({name: 'Rohit', email: 'rohit.choudhar@gmail.com'});

Here comes the out put

console.log(obj.getName());
Output : Rohit

console.log(obj.weekMap.get(obj).name);
Output : Rohit

console.log(obj.weekMap.set(obj).name = 'I mena');
Output : I mena

console.log(obj.weekMap);
Output: WeakMap { name: 'I mena' }

console.log(obj.weekMap.get(obj).name);
Error:
/home/bll/bll-jb/server/lib/ease/testweak.js:35
console.log(obj.weekMap.get(obj).name);
                                ^

TypeError: Cannot read property 'name' of undefined
    at Object.<anonymous> (/home/bll/bll-jb/server/lib/ease/testweak.js:35:33)
    at Module._compile (module.js:434:26)
    at Object.Module._extensions..js (module.js:452:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:475:10)
    at startup (node.js:117:18)
    at node.js:951:3

I am not able to get myself clear about this behavior of WeakMap.

Answer 1

I guess you confused set and get .

console.log(obj.weekMap.set(obj).name = 'I mena');

Entries before this call: obj => obj

Entries after this call: obj => undefined

set expects arguments for key and value. You don't provide a value, so this code sets the value of the entry with the key obj to undefined . Consequently the next call to obj.weekMap.get(obj) returns undefined.