javascript - Binding different this scope to ES6 => function operator -

after experimenting inheriting contexts => feature es6 gives noticed context can never changed. example:

var othercontext = {   a: 2 }; function foo() {   this.a = 1;    this.bar = () => this.a;  }  var instance = new foo; instance.bar(); // returns 1 instance.bar.bind(othercontext)(); // returns 1 

without => operator , using function keyword:

function foo() {   this.a = 1;    this.bar = function () {     return this.a;   } } var instance = new foo; instance.bar(); // returns 1 instance.bar.bind(othercontext)(); // returns 2 

therefore, if receive function external call or have function in variable, how can sure if going able bind different or if inherit somewhere?

it sounds dangerous javascript not tell anything, 1 might fall subtle , difficult bug.

it new syntax bind, doesn't introduce new in way of gotchas.

var othercontext = {    a: 2  };  function foo() {    this.a = 1;    this.bar = function () { return this.a }.bind(this);  }    var instance = new foo;  log(instance.bar()); // returns 1  log(instance.bar.bind(othercontext)()); // returns 1    function log(value) {     document.body.appendchild(      document.createtextnode(value)    );  }

therefore, if receive function external call or have function in variable, how can sure if going able bind different or if inherit somewhere?

because either:

1. you'll have written function in first place or
2. you'll have written specification how call function people know pass in function makes use of this context choose.