CODEWITHSUNDEEP
javascript
Max
Max

Reputation: 13

Why doesn't a for (...in...) loop return the key when iterating through an object method

Why is it that when I run employee.report() on this object method I get alerts for "Undefined is John Smith", etc.

var employee = {
    name: "John Smith",
    job: "Programmer",
    age: 31,
    report: function() {
        for (k in employee) {
            alert(this.k + " is " + employee[k])
        }
    }
}

but when I run the loop the console like this

for (k in employee) {
    alert(this.k + " is " + employee[k])
}

it returns the proper alerts ("name is John Smith", etc.)

Upvotes: 1

Views: 53

Answers (4)

namth
namth

Reputation: 3117

@Barmar explain very well, I just add more detail

In your object method report

this in this.k points to employee object and you get Undefined because employee does not have a property named k. You should use this[k] instead.

And in te loop outside of object

for (k in employee){
  alert(this.k+ " is " +employee[k])
}

this in this.k points to global Window object Your code actually is

for (var k in employee){
  alert(this.k+ " is " +employee[k])
}

k will become a global variable. this.k is same as window.k or k

BTW: You should use const/let and use strict for more clarify your variable

Upvotes: 0

Ahmed ElMetwally
Ahmed ElMetwally

Reputation: 2383

To access k you should use k instead of this.k. so replace alert(this.k+ " is " +employee[k]) with alert(k+ " is " +employee[k]) then your code will work successfully.

References for understand this in js.

  1. w3schools
  2. mozilla
  3. geeksforgeeks

Upvotes: 1

Barmar
Barmar

Reputation: 780974

In the report function this is the employee object, which doesn't have a k property. So this.k is undefined. If you want to print the property name, just use k:

alert(k + " is " + employee[k]);

Outside the function, this is the global window object. Global variables are automatically made properties of window, so this.k is the same as k.

Upvotes: 0

fig
fig

Reputation: 374

Since you haven't defined employee in the scope of the employee variable, you must use the keyword this to reference the object itself.

var employee = {
  name: "John Smith",
  job: "Programmer",
  age: 31,
  report: function(){
      for (k in this){
          alert(k+ " is " +this[k])
      }
    }
  }

Upvotes: 0

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