this doesn't act the same in lambda and without it in ecma6 javascript and nodejs

Question

var emitter = require('events').EventEmitter;
var util = require('util');

let Person = function (name) {
    this.name = name;

};

util.inherits(Person, emitter);
let p = new Person("jonny");

p.on('speak', (said) => {
    console.log(` ${this.name} said: ${said}`);

});

p.emit('speak', "You may delay but time will not!");

console will return

undefined said: You may delay but time will not!

if you change from lambda back

p.on('speak', function(said) {
    console.log(` ${this.name} said: ${said}`);

});

it works. can some explain to me why this acts differently with syntactic sugar changes?

Because is not only sugar. `=>` doesn't create new context and hasn't `this`. — Aikon Mogwai, Aug 01 '16 at 21:44
http://stackoverflow.com/documentation/javascript/186/functions/720/arrow-functions#t=201608012207550133428 — Kevin B, Aug 01 '16 at 22:08
@MichkeyPerlstein : I update my answer & i [suggest a solution](http://stackoverflow.com/a/38708833/747579) — Abdennour TOUMI, Aug 01 '16 at 22:17
Either use `p.name` or use a normal function. Arrow functions don't work the way you want them to. — loganfsmyth, Aug 01 '16 at 23:11
Also a duplicate of [Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?](http://stackoverflow.com/q/34361379/218196) — Felix Kling, Aug 03 '16 at 00:08

Abdennour TOUMI · Answer 1 · 2016-08-01T22:13:20.063

0

Arrow function (Lambda) has not neither this nor arguments. And if you find this or arguments , it belongs to its function caller .

Therefore , you need to use classic function until upgrading events package & supporting this feature .

Update:

IF you love arrow function & you insist to use it , i suggest to override on method AND convert arrow function to classic-function using eval

class Person extends require('events').EventEmitter{

     constructor(name){
      super();
       this.name=name;
     }
     on(event,arrow){
         super.on.call(this,event,eval(`(function${arrow.toString().replace('=>','')})`));
     }
}

let p = new Person("jonny");

p.on('speak', (said) => {
    console.log(` ${this.name} said: ${said}`);

});

p.emit('speak', "You may delay but time will not!");

DEMO :

edited Aug 01 '16 at 22:13

answered Aug 01 '16 at 22:02

Abdennour TOUMI

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Please don't recommend `eval`, that's not at all a general answer. If that function depended on anything besides `this` and `said`, it would break immediately. – loganfsmyth Aug 01 '16 at 23:10
i knew,,, hust for his requirements – Abdennour TOUMI Aug 01 '16 at 23:22
1

@AbdennourTOUMI thanks for your answer, but I did not require anything. I asked why javascript is behaving in a way that I do not understand. I come from C#, where "arrow functions" do not act in this manner and are in fact syntactic sugar. – Mickey Perlstein Aug 04 '16 at 12:45

this doesn't act the same in lambda and without it in ecma6 javascript and nodejs

1 Answers1

Update:

DEMO :