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Below is the coding exercise from code academy whose objective is to print out name property. Can you please explain the error and how do i fix it. My code looks like:

 // Our Person constructor
 function Person(name, age) {
 this.name = name;
 this.age = age;
 }

var family = new Array();
family[0] = new Person("alice", 40);
family[1] = new Person("bob", 42);
family[2] = new Person("michelle", 8);
family[3] = new Person("timmy", 6);

for (var i = 0; i <= family.length; i++) {
console.log (family[i].name);
};  
// Now we can make an array of people

// loop through our new array

OUTPUT :
alice
bob
michelle
timmy

---
We're running a test below to make sure your code works.
alicebobmichelletimmy
TypeError: Cannot read property 'name' of undefined

------------------------------------------
Many thanks.
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5
  • 1
    Check that you aren't running outside the bounds of the array. Commented Jun 26, 2016 at 2:25
  • @mparnisari: Yes, OP is running out of the bounds. i < family.length. Commented Jun 26, 2016 at 2:26
  • 3
    I know, I wanted him/her to notice that :) Commented Jun 26, 2016 at 2:26
  • Jup, just remove that equal (=) Commented Jun 26, 2016 at 2:31
  • Possible duplicate of Javascript While Looping Through Array And Outputting in .innerhtml — it’s hard to find a good dupe target for this. Commented Jun 26, 2016 at 3:18

3 Answers 3

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In the loop, you are running out of the bounds of the array. Consider the following.

I have an array of 3 items, indices are 0, 1, 2. If I use the <= operator for the loop conditional, it will yield this result: 0, 1, 2, 3. Since 3 doesn't exist, it's undefined and it can't find the property name.

Arrays are 0-based which means that indices start at zero. If we have an array with 3 elements, the length is 3, but the largest index is 2 (0, 1, 2). If the for loop continues looping even if the index is equal the length, it will return undefined because that element in the array doesn't exist.

In your case, you have 4 elements in an array, all Person objects. They are accessed by the indices 0, 1, 2, and 3. If the loop continues after 3 and checks if i is equal to the length (4), it will try to check for the 4th element of the array, which doesn't exist because the array length is only 4 with elements, not five.

Rewrite the for loop like this:

for (var i = 0; i < family.length; i++) {
    console.log (family[i].name);
};

This will prevent the loop from iterating to an unknown index. It stops the loop when all elements have been loop through.

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Comments

1

as in the comments, but to explain the error (so that you learn from the mistake and understand why its causing an error): - you are running a loop that has :

for (var i = 0; i <= family.length; i++) {

This is causing the issue since arrays are zero indexed - meaning that if you have 

family[0] = new Person("alice", 40);
family[1] = new Person("bob", 42);
family[2] = new Person("michelle", 8);
family[3] = new Person("timmy", 6);

then you will have the family array of length 4 but its indexed 0 -3.

This means that when you have i <= family.length, you are attempting to find an element with an index of 4 - which doesn't exist and so throws the error.

As mentioned you simply need to remove the = in the for loop and all will be well since i will never get to 4 and will stop at 3 - the highest index in the array.

 for (var i = 0; i < family.length; i++) {
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Comments

1

Your line that says: for (var i = 0; i <= family.length; i++) {

Should say: for (var i = 0; i < family.length; i++) {

Here is the corrected code:

 // Our Person constructor
 function Person(name, age) {
 this.name = name;
 this.age = age;
 }

var family = new Array();
family[0] = new Person("alice", 40);
family[1] = new Person("bob", 42);
family[2] = new Person("michelle", 8);
family[3] = new Person("timmy", 6);

for (var i = 0; i < family.length; i++) {
console.log (family[i].name);
};

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2 Comments

Thanks all of you guyz. Missed it totally, made an error where expected the least :)
No worries, that's why StackOverflow is here! Happy coding!

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