Confused about arrow functions

Question

would you please tell me what's wrong with snippet 1? My expected output for obj.discover() is 3. How can't I bind this to an arrow function method of an object?

'use strict'

// Snippet 1
var obj = {
  data: 3,
  discover: () => {
    return this.data
  }
}
obj.discover() // -> undefined 
obj.discover.bind(obj)() // undefined

// but if I don't use the arrow notation, everything works
// Snippet 2
var obj2 = {
  data: 3,
  discover: function(){
    return this.data
  }
}
obj2.discover() // -> 3

Arrow functions don't have their own `this`, use regular functions for that — George Kagan, Nov 16 '16 at 11:05

score 3 · Accepted Answer · answered Nov 16 '16 at 11:06

3

Arrow functions are not just a syntactic sugar but differ in some behaviour as well. In arrow functions, this always point to the object at which it was pointing at the time of function definition.

answered Nov 16 '16 at 11:06

Falloutcoder

991
6
19

1

Worth also pointing out that in the OPs case, `this` is the global object when `discover` is created in `obj`. If it were an object created using `new` etc, it'd be fine to use `this` in an arrow function in that way. – James Thorpe Nov 16 '16 at 11:08
@JamesThorpe Thank you for your thorough explanation. – anhldbk Nov 16 '16 at 11:28
1

@dejavu_cmd_delt: Thank you so much. – anhldbk Nov 16 '16 at 11:28

score 0 · Answer 2 · answered Nov 16 '16 at 11:06

Arrow functions don't allow for binding. Check this reference on MDN:

An arrow function expression has a shorter syntax compared to function expressions and does not bind its own this, arguments, super, or new.target.

Arrow function will resolve the this to its lexical scope.

score 0 · Answer 3 · answered Nov 16 '16 at 11:08

0

Arrow functions aren't the same as normal functions and shouldn't be used as object methods. They don't bind 'this', among other things. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

answered Nov 16 '16 at 11:08

Paul

35,689
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Confused about arrow functions

3 Answers3