JavaScript / Constructor Functions

Question

I'm still learning JavaScript. Not able to get past this:

  function Fruits(category, berries, notberries) {
      this.category = category;
      this.berries = [];
      this.notberries = [];
  }

  let f = new Fruits("fresh", ["strawberry", "raspberry"], ["apple", "mango"]);

  console.log(f); // Fruits {category: "fresh", berries: Array(0), notberries: Array(0)}

  f.category;  //"fresh"

  f.berries; //[]

Why is it not logging the values of berries and instead returning an empty array ?

By calling this.berries = []; you've declared berries' value as an empty array, regardless of the constructor's parameters — GalAbra
– GalAbra, Commented Feb 26, 2018 at 23:28

Ele · Accepted Answer · 2018-02-27 01:13:49Z

2

You need to assign the arguments to the proper attributes.

function Fruits(category, berries, notberries) {
  this.category = category;
  this.berries = berries;
  this.notberries = notberries;
}

let f = new Fruits("fresh", ["strawberry", "raspberry"], ["apple", "mango"]);

console.log(f); // Fruits {category: "fresh", berries: Array(0), 
f.category;
f.berries;

edited Feb 27, 2018 at 1:13

answered Feb 26, 2018 at 23:29

Ele

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1 Comment

Bergi Over a year ago

Which local variables? The parameters are local variables.

T.J. Crowder · Accepted Answer · 2018-02-26 23:41:36Z

Nothing in your constructor ever assigns the parameter berries to the property this.berries (or copies its content); the same for notberries.

To assign:

function Fruits(category,berries,notberries) {
    this.category = category;
    this.berries = berries;          // ***
    this.notberries = notberries;    // ***
}

To copy (shallow, which is fine, these are arrays of strings):

function Fruits(category,berries,notberries) {
    this.category = category;
    this.berries = berries.slice();          // ***
    this.notberries = notberries.slice();    // ***
}

The difference is that if you assign, then the object has a reference to the array that was passed in, and so does the calling code; if the calling code modifies that array, it affects the object (since they share the array). If you copy the contents of the array, then the calling code and the object each have their own array, so changes to one don't affect the other.

Both can be correct, you just need to be aware of the distinction.

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