How to reference to outer member

Given following JavaScript code:

({
  foo: 1,
  bar: 2,
  zoo: 3,
  test: function(i) {
    var g = function(i) {
        alert(i + zoo);
    }
    g(i);
  }
}).test(2);

Why is zoo undefined in alert() ? Which kind of syntax can I use to correctly reference to zoo and get alert display for value 5 ?

Update: I would prefer solution where only implementation of g needs to change, if at all possible.

---

zoo is not a free floating variable, it's a property of the object. Inside test you can reference the object using this (because of the way you call it). Inside g the this context will be lost though, so you need to explicitly preserve it:

test: function(i) {
    var g = function(i) {
        alert(i + this.zoo);
    }.bind(this);
    g(i);
}

or:

test: function(i) {
    var g = function(i) {
        alert(i + this.zoo);
    };
    g.call(this, i);
}

---

使用箭头功能保留的“外部”值this ，并用它来访问的价值zoo属性：

({
  foo: 1,
  bar: 2,
  zoo: 3,
  test: function(i) {
    var g = i => alert(i + this.zoo);
    g(i);
  }
}).test(2);

---

When you invoke a member function of an object, the this keyword is set to the object (unless you call the function using .call or .apply ). The function g is no longer a member of the object, so this is not set to the object. You have a couple of options if you want to keep using the g function.

Set a reference to this :

test: function(i) {
    var that = this;
    var g = function(i) {
        alert(i + that.zoo);
    }
    g(i);
} 

or manually set the value of this by using .call

test: function(i) {
    var g = function(i) {
        alert(i + this.zoo);
    }
    g.call(this, i);
} 

Here's a little more info about .call and .apply.

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