Understanding the functionality of the Array.prototype.map() method when used with JavaScript Objects

Question

Understanding the functionality of the Array.prototype.map() method when used with JavaScript Objects

Experimenting with code:

 let obj ={};
    let newItems = ["Apple", "Banana", "Carrot", "Donut"].map(item => {
        obj.count= Math.random()*item.length;
        obj.itemName= item;
        return obj;
    });
    console.log(newItems);

The output generated is:

(4) [{…}, {…}, {…}, {…}]
0: {count: 1.4648989727265578, itemName: "Donut"}
1: {count: 1.4648989727265578, itemName: "Donut"}
2: {count: 1.4648989727265578, itemName: "Donut"}
3: {count: 1.4648989727265578, itemName: "Donut"}
length: 4
__proto__: Array(0)

This result is not as expected. However, when I modify the code to:

let obj;
let newItems = ["Apple", "Banana", "Carrot", "Donut"].map(item => {
      obj = {
               count: Math.random()*item.length,
               itemName: item
            } ;
      return obj;
 });
 console.log(newItems);

The desired output is achieved:

(4) [{…}, {…}, {…}, {…}]
0: {count: 4.2813861024052615, itemName: "Apple"}
1: {count: 5.850654082147917, itemName: "Banana"}
2: {count: 6.646446034466489, itemName: "Carrot"}
3: {count: 1.7062874250924214, itemName: "Donut"}
length: 4
__proto__: Array(0)

Can someone explain why there is a difference in behavior between these two code snippets?

javascript arrays

Answer 1

Answer №1

When looking at your initial example, it is evident that only a single object was created and then added to the array four times:

let obj ={}; // <===== Creates the one object
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
    obj.count= Math.random()*el.length;
    obj.ingredient= el;
    return obj; // <===== Puts it in the array four times
});
console.log(newIngredients);

This approach resulted in each instance of obj.ingredient = el; updating the property value, overwriting the previous one.

The memory representation would resemble something like this:

...

Your second scenario involves creating a distinct object for each invocation of the map callback function, which is the correct behavior. It's worth noting that the use of the variable obj may be unnecessary and can be removed without affecting functionality. This revised version looks like this:

let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
    return {
        count: Math.random()*el.length,
        ingredient: el
    };
});
console.log(newIngredients);

Alternatively, you could utilize a concise arrow function like this:

let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => ({
    count: Math.random()*el.length,
    ingredient: el
}));
console.log(newIngredients);

Answer 2

When looking at your initial example, it is evident that only a single object was created and then added to the array four times:

let obj ={}; // <===== Creates the one object
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
    obj.count= Math.random()*el.length;
    obj.ingredient= el;
    return obj; // <===== Puts it in the array four times
});
console.log(newIngredients);

This approach resulted in each instance of obj.ingredient = el; updating the property value, overwriting the previous one.

The memory representation would resemble something like this:

...

Your second scenario involves creating a distinct object for each invocation of the map callback function, which is the correct behavior. It's worth noting that the use of the variable obj may be unnecessary and can be removed without affecting functionality. This revised version looks like this:

let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
    return {
        count: Math.random()*el.length,
        ingredient: el
    };
});
console.log(newIngredients);

Alternatively, you could utilize a concise arrow function like this:

let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => ({
    count: Math.random()*el.length,
    ingredient: el
}));
console.log(newIngredients);

Answer 3

Answer №2

This discussion isn't about Array.prototype.map(), but rather focuses on the behavior of Objects in JavaScript.

Take a look at this example:

let obj = {};
let prev = obj;
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
  obj = {
    count: Math.random() * el.length,
    ingredient: el
  };
  if (obj === prev) {
    console.log(true);
  } else {
    console.log(false)
  }
  prev = obj;
  return obj;
});
console.log(newIngredients);

Now, look at this variation:

let obj = {};
let prev = obj;
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
  obj.count = Math.random() * el.length;
  obj.ingredient = el;
  if (obj === prev) {
    console.log(true);
  } else {
    console.log(false)
  }
  prev = obj;
  return obj;
});
console.log(newIngredients);

In the first example, you can see that it returns false due to the difference in Object creation syntax - using let a = { } compared to let a = new Object();.

In the correct version, you are simply updating the existing object referenced by all elements in the array, resulting in them all being updated simultaneously.

I hope this clarifies things for you.

Answer 4

This discussion isn't about Array.prototype.map(), but rather focuses on the behavior of Objects in JavaScript.

Take a look at this example:

let obj = {};
let prev = obj;
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
  obj = {
    count: Math.random() * el.length,
    ingredient: el
  };
  if (obj === prev) {
    console.log(true);
  } else {
    console.log(false)
  }
  prev = obj;
  return obj;
});
console.log(newIngredients);

Now, look at this variation:

let obj = {};
let prev = obj;
let newIngredients = ["Hello", "Distraction", "Nothing", "Love"].map(el => {
  obj.count = Math.random() * el.length;
  obj.ingredient = el;
  if (obj === prev) {
    console.log(true);
  } else {
    console.log(false)
  }
  prev = obj;
  return obj;
});
console.log(newIngredients);

In the first example, you can see that it returns false due to the difference in Object creation syntax - using let a = { } compared to let a = new Object();.

In the correct version, you are simply updating the existing object referenced by all elements in the array, resulting in them all being updated simultaneously.

I hope this clarifies things for you.

Understanding the functionality of the Array.prototype.map() method when used with JavaScript Objects

Answer №1

Answer №2

Similar questions

Incorporate zoom feature into the jQuery polaroid gallery

Duplicate a DOM element and incorporate animation into it

What is the best method to determine the mean score by utilizing the ID values obtained from API responses?

Issue accessing member value in inherited class constructor in Typescript

The perplexing behavior of RxJS Observables with Mongo Cursors

JavaScript rearrange array elements

Ruby organizes columns when generating a CSV file from Mongoid documents

Error TS2322: The object with properties "ready: false" and "session: null" cannot be assigned to the type "Readonly<S & withAuthState>"

Associations in Typescript Sequelize

Display issue with Google Chart

Ensure that only distinct elements are added to an ArrayList in Java

Guide to configuring the initial lookAt/target for a Control

Guide to retrieving variables stored in Local Storage using javascript and jquery while building an HTML form

Invalid sequencing of Nest.js async onModuleInit causing issues

Set a unique class for several elements depending on a given condition

How can I trigger a function by clicking on a link that was generated dynamically with jQuery?

Alter the background of cards in real-time

Error: React JS is unable to access the property 'name' because it is undefined

Remembering position in JSP using Java Beans

Just ran $npm install and encountered an error message: "Module '../lib/utils/unsupported.js' not found."