React: Map an array inside object

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7 Answers

objectreact
90%

If you want to map through an object you can use Object.keys.map.,You mean you have an array like the following and you are trying to access the elements in the arrays that are within the objects?,.map is an array function. You can’t use it on objects, so if you have an object nested inside an array you can use .map on the outer array but not the inner object.

I have an array of objects in my state called ‘ideas’. Each object in the array has a property called ‘premises’, and that property is an array. I’m trying to map through ideas, and within the function map through the premises of each idea, but it doesn’t seem to be working - idea.premises is supposedly undefined. What am I doing wrong? Should I be doing this a different way anyway?

{this.state.ideas.map(idea => {
  <div className="card">
    <div className="card-body">
      <h1>{idea.title}</h1>
      {idea.premises.map(premise => {
        <p>{premise}</p>
      })}
      <p>{idea.conclusion}</p>
    </div>
  </div>
})}
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88%

I spent the last couple of hours trying to figure out how to get to the data in the Parts array within the Courses array of objects inside my react app and googled extensively without finding anything that could help.,I wanted to return the names of all the books within the parts array inside the courses array.,I will be using the following books array of objects to explain how I solved this problem.  We want to return only the categories of the books on the shelves.

Pretag
 Pretag team - issue, fix, solve, resolve
72%

Pretag
 Pretag team - issue, fix, solve, resolve
65%

The map() function is used to iterate over an array and manipulate or change data items. In React, the map() function is most commonly used for rendering a list of data to the DOM.,The map() function lets you manipulate the items in an array by iterating and accessing individual items. In this guide, you will learn how to use the map() function and export it.,Other than rendering, the map() function can also be used in a utility function, which can be imported from another file.

1
let new_array = arr.map(function callback(currentValue[, index[, array]]) {
   2 // return element for new_array
   3
} [, thisArg])
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75%

In React, transforming arrays into lists of elements is nearly identical.,Given the code below, we use the map() function to take an array of numbers and double their values. We assign the new array returned by map() to the variable doubled and log it:,We include the entire listItems array inside a <ul> element, and render it to the DOM:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map((number) => number * 2);
console.log(doubled);
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40%

The Array.map() method is key to rendering objects and data in react and it is a prototype function on all arrays in JavaScript. This means you can call it on any array with the following syntax: [].map(callback).,Removing all spaces from strings in JavaScript is a pretty common task that we need to do, and there are many ways to do it, find out how in this post!,Learn everything you need to know how to render an array of objects with Array.map() in React with any data - Array.prototype.map().

const arrayOfObjects = [
  { coffee: "Americano", size: "Medium" },
  { coffee: "Espresso", size: "Single" },
];

export default function MyReactComponent() {
  return (
    <>
      {arrayOfObjects.map(({ coffee, size }) => (
        <p key={coffee}>Coffee type {coffee} in a {size} size.</p>
      ))}
    </>
  );
}
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22%

callbackFn is invoked with three arguments: the value of the element, the index of the element, and the array object being mapped. ,The index of the current element being processed in the array.,A new array with each element being the result of the callback function.

// Arrow function
map((element) => {
   ...
})
map((element, index) => {
   ...
})
map((element, index, array) => {
   ...
})

// Callback function
map(callbackFn)
map(callbackFn, thisArg)

// Inline callback function
map(function callbackFn(element) {
   ...
})
map(function callbackFn(element, index) {
   ...
})
map(function callbackFn(element, index, array) {
   ...
})
map(function callbackFn(element, index, array) {
   ...
}, thisArg)
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