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JavaScript Understanding "this" in JavaScript

Harsh Khandelwal
Harsh Khandelwal
5,539 Points
Posted by Harsh Khandelwal
Harsh Khandelwal
5,539 Points

Cannot understand behavior of this code.

For the below code

var Portland = {
    bridges: 12,
    airport: 1,
    soccerTeams: 1,
    logNumberOfBridges: function() {
        console.log("There are " + this.bridges + " bridges in Portland!")
    },
    logTeams: function(){
      console.log(this.logNumberOfBridges());
    }
}

Portland.logTeams();

Why do I get undefined in the output?

Output:

There are 12 bridges in Portland!                                                                                                                 
undefined

3 Answers

Dino Paškvan
Dino Paškvan
Courses Plus Student 44,108 Points

The developer tools in browsers like to be helpful, so they log the return value of a function/method in the console. When there is no return statement in the function, the return value is undefined, as is the case here. That's why the console shows undefined.

Nothing is wrong with your code, it's just your browser trying to be helpful. You might notice that there's a little arrow in front of undefined — this signifies that it's a return value.

If you are interested in a much longer explanation, please take a look here.

Seth Kroger
Seth Kroger
56,413 Points
Seth Kroger
Seth Kroger
56,413 Points

You're calling logNumberOfBridges() but the function doesn't return anything, so the second log has nothing to log, or undefined.

David Kanwisher
David Kanwisher
9,751 Points
David Kanwisher
David Kanwisher
9,751 Points

Wanted to add 3 examples of what they are talking about:

These will all have the desired result:

//option 1:

var Portland = {
    bridges: 12,
    airport: 1,
    soccerTeams: 1,
    logNumberOfBridges: function() {
        console.log("There are " + this.bridges + " bridges in Portland!")
    },
    logTeams: function(){
      return this.logNumberOfBridges();
    }
}

Portland.logTeams();




//option 2:

var Portland = {
    bridges: 12,
    airport: 1,
    soccerTeams: 1,
    logNumberOfBridges: function() {
        return "There are " + this.bridges + " bridges in Portland!"
    },
    logTeams: function(){
      console.log(this.logNumberOfBridges());
    }
}

Portland.logTeams();



//option 3


var Portland = {
    bridges: 12,
    airport: 1,
    soccerTeams: 1,
    logNumberOfBridges: function() {
        return "There are " + this.bridges + " bridges in Portland!"
    },
    logTeams: function(){
      return this.logNumberOfBridges();
    }
}

console.log(Portland.logTeams());
ywang04
ywang04
6,762 Points
ywang04
ywang04
6,762 Points

You can just simply print out There are 12 bridges in Portland! using the following way:

var Portland = {
    bridges: 12,
    airport: 1,
    soccerTeams: 1,
    logNumberOfBridges: function() {
        console.log("There are " + this.bridges + " bridges in Portland!")
    },
    logTeams: function(){
      this.logNumberOfBridges();  //print out the message directly without console.log.
    }
}
Portland.logTeams();