loop inside react jsx

Using the Array map function is a very common way to loop through an Array of elements and create components according to them in React. This is a great way to do a loop which is a pretty efficient and is a tidy way to do your loops in JSX. It's not the only way to do it, but the preferred way. If you have nothing to .map() you can use Array.from() with the map function to repeat elements: If you want to assign unique key IDs into the rendered components, you can use React.Children.toArray as proposed in the React documentation

render: function() {
  const elements = ['one', 'two', 'three'];
  return (
    <ul>
      {elements.map((value, index) => {
        return <li key={index}>{value}</li>
      })}
    </ul>
  )
}
​

return tbody(
   for (let i = 0; i < numrows; i++) {
      ObjectRow()
   }
)

const rows = [];
for (let i = 0; i < numrows; i++) {
   rows.push(ObjectRow());
}
return tbody(rows);

<tbody>
    for (var i=0; i < objects.length; i++) {
        <ObjectRow obj={objects[i]} key={i}>
    }
</tbody>

<tbody>
    {objects.map(function(object, i){
        return <ObjectRow obj={object} key={i} />;
    })}
</tbody>

When I first started with React, I realized quite early that I did not know how to loop through an array and render a list of items. The most common way of doing that is with the map function that will return JSX. You will rarely need a loop other than this one. Below you can see how it works. As you can see there are quite a few different ways in which you can render a list of items in React. I hope that after reading this article you are now more confident with React and know how to use loops in JSX. Wondering how to create loops in React? Learn about JSX and how to use methods like the map function to loop inside React JSX and render a list of items.

// JSX example

return (
  <div>
    <button onClick={onClickHandler}>Submit</button>
  </div>
)

// The above will be converted to something like this

return React.createElement(
    "div",
    {},
    React.createElement(
      "button",
      {
        onClick: e => {}
      },
      "Submit"
    )
  );

import React from ‘react’

const RenderList = props => {
  const animals = ["Dog", "Bird", "Cat", "Mouse", "Horse"];

  return (
    <ul>
      {animals.map(animal => (
        <li>{animal}</li>
      ))}
    </ul>
  );
};

  return (
    <ul>
      {animals.map((animal, index) => (
        <li key={index}>{animal}</li>
      ))}
    </ul>
  );

  const renderAnimals = animals.map(item => (
    <li key={item.id}>{item.animal}</li>
  ));

  return <ul>{renderAnimals}</ul>;

  const getAnimalsContent = animals => animals.map(item => (
    <li key={item.id}>{item.animal}</li>
  ));

  return <ul>{getAnimalsContent(animals)}</ul>;

Assume for the moment that we have an array-style list of items. We can build a loop and add a JSX element to an array using the for a loop. You can make a loop, add JSX to an array, and then loop over a set of elements to generate a JSX partial: Without DOM queries, this would typically be hard to accomplish in a JavaScript app. Use component loops to produce sets of objects in a neat, effective, and legible way. The array’s length is adjusted using this technique to reflect the number of new elements added.

import React from 'react';
import ReactDOM from 'react-dom';
const name = "Olivier";
const element = Welcome {
      name
   },
   and thanks
for choosing Bosc Tech Labs Private Limited.;

ReactDOM.render(
   element,
   document.getElementById("root")
);

React.createElement(type, {
   props
}, children);

const elements = []
const items = []
for (const [index, value] of elements.entries()) {
   items.pugsh()
}

import React from‘ react’
const RenderList = props => {
   const Languages = ["French", "German", "English", "Spanish", "Mandarin"];

   return (

      {
         Languages.map(Languages => ({
            Languages
         }))
      }

   );
};

Keys in the React loop help identify which items have been changed, added or removed, and it is important to give the parent elements inside a loop unique keys to help give a stable identity to the element or component. In this short tutorial, we’ve covered the basics of looping in React JSX, how keys work, as well as how to add a unique key to iterable elements. For each element in the array, we map its text and status fields to content within a <p> element, whose key is mapped to the id field. This will generate the following markup result:

const todos = [{
      id: 1,
      text: "Learn React",
      status: "completed"
   },
   {
      id: 2,
      text: "Do something",
      status: "incomplete"
   },
   {
      id: 3,
      text: "Do something else",
      status: "incomplete"
   },
];

{
  todos.map((todo) => (
    <p key={todo.id}>
      {todo.text} - {todo.status}
    </p>
  ));
}

<p>Learn React - completed</p>
<p>Do something - incomplete</p>
<p>Do something else - incomplete</p>

{
  todos.map((todo) => (
    <div key={todo.id}>
      <p key={todo.text}>
        {todo.text} - {todo.status}
      </p>
    </div>
  ));
}

{
  todos.map((todo, index) => (
    <div key={index}>
      <p key={todo.text}>
        {todo.text} - {todo.status}
      </p>
    </div>
  ));
}

Suppose you have a React component and an items array you want to loop over, to print all the “items” you have. Inside this function we return a <li> (list item) with the value contained in the array, and with a key prop that is set to the index of the item in the array. This is needed by React. React: How to show a different component on click Inside this list, we add a JavaScript snippet using curly brackets {} and inside it we call items.map() to iterate over the items.

return (
<ul>

</ul>
)

return (
  <ul>
    {items.map((value, index) => {
      return <li key={index}>{value}</li>
    })}
  </ul>
)

return (
  <ul>
    {items.map((value, index) => <li key={index}>{value}</li>}
  </ul>
)

Let's try to understand what problem are we solving right here. We are trying to do something like the following in React JSX (where ObjectRow is a separate component): Regarding transpiling with Babel, its caveats page says that Array.from is required for spread, but at present ( v5.8.23 ) that does not seem to be the case when spreading an actual Array . We do have a documentation issue open to clarify that. But use at your own risk or polyfill. Demo on Plnkr Incidentally, the above JavaScript example is almost exactly what that example of JSX transforms into. Playing around with Babel REPL will help us to get a feel for how JSX works.

    for (var i = 0; i < numrows; i++) {

    }

return tbody(
   for (var i = 0; i < numrows; i++) {

      ObjectRow();
   }

)

var rows = [];
for (var i = 0; i < numrows; i++) {

   rows.push(ObjectRow());
}

return tbody(rows);

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

    }

{
   objects.map(function(object, i) {
      return;
   })
}

To do that, we can use few methods, one of the most popular is the map method, but we will cover the map in the separate section, and now we should focus on the other methods like loops or forEach method. This method, compared to the for-loops and map method, is the slowest one and doesn’t return the values like a map, so you need to have a special case to use it. It is very popular to use loops like for-loop (in most cases the fastest one), for-in, or for-of to iterate through elements. Anyway, we have a special reason why we should focus on the iteration methods when we develop apps for React.js.

// First one with For of loop
function renderComponent() {
  const products = ['orange', 'apple', 'watermelon'];

  const list = []

  for (const [i, product] of products.entries()) {
    list.push(<li>{product}</li>)
  }

  return (
    <div>
      {list}
    </div>
  )
}

function Parent(props) {
  return renderProducts();
}

// Second with forEach method
function renderComponent() {
  const products = ['orange', 'apple', 'watermelon'];

  const list = []

  products.forEach((product) => {
    list.push(<li>{product}</li>)
  })

  return (
    <div>
      {list}
    </div>
  )
}

function Parent(props) {
  return renderProducts();
}

function renderComponent() {
  const products = ['orange', 'apple', 'watermelon'];

  const list = products.map(product => <li key={product}>{product}</li>)

  return (
    <div>
      {list}
    </div>
  )
}

  const list = products.map((product, key) => <li key={key}>{product}</li>)

Keys in the React loop help identify which items have been changed, added, or removed, and it is important to give the parent elements inside a loop unique keys to help give a stable identity to the element or component. If the item you’re trying to loop through does not have a unique element, such as a unique id - it is a common convention to use the index returned by the map() function for each iterated element instead, ensuring unique element identification without changing your domain model: React allows you to easily write JavaScript code inside your components. This makes it easy for any developer to comfortably handle common programming techniques in React, such as looping through a set of items, creating and invoking functions, storing data in local variables, and so on.

const todos = [{
      id: 1,
      text: "Learn React",
      status: "completed"
   },
   {
      id: 2,
      text: "Do something",
      status: "incomplete"
   },
   {
      id: 3,
      text: "Do something else",
      status: "incomplete"
   },
];

{
  todos.map((todo) => (
    <p key={todo.id}>
      {todo.text} - {todo.status}
    </p>
  ));
}

<p>Learn React - completed</p>
<p>Do something - incomplete</p>
<p>Do something else - incomplete</p>

{
  todos.map((todo) => (
    <div key={todo.id}>
      <p key={todo.text}>
        {todo.text} - {todo.status}
      </p>
    </div>
  ));
}

{
  todos.map((todo, index) => (
    <div key={index}>
      <p key={todo.text}>
        {todo.text} - {todo.status}
      </p>
    </div>
  ));
}

React allows you to easily write JavaScript code inside your components. This makes it easy for any developer to comfortably handle common programming techniques in React, such as looping through a set of items, creating and invoking functions, storing data in local variables, and so on. Using component loops to output and manipulate data is a common development method in React. It allows you to group HTML elements with dynamic data together, as shown in this guide. This would generally be impossible to do in a pure JavaScript app without DOM queries. You should use component loops to output sets of items in a clean, efficient, and readable manner. Thus you can push your data along with its supporting HTML inside an array and then output that array inside your component's return statements enclosed in curly braces. There are several JavaScript loops that you can use for this purpose. Since map() is the most popular and easiest one, this guide extensively uses it in the examples.

1return(
2	<>
3	{data}
4	</>
5)

1import React from 'react';
2import './App.css';
3
4function App() {
5  return (
6    <>
7      <h2>This is a simple list of items</h2>
8      <ul>
9        <li>Item 1</li>
10        <li>Item 2</li>
11        <li>Item 3</li>
12        <li>Item 4</li>
13        <li>Item 5</li>
14      </ul>
15    </>
16  );
17}
18
19export default App;

1
let items = ['Item 1', 'Item 2', 'Item 3', 'Item 4', 'Item 5'];

1let itemList=items.map((item,index)=>{
2  return <li key={index}>{item}</li>
3})

1function App() {
2  return (
3    <>
4      <h2>This is a simple list of items</h2>
5      <ul>
6        {itemList}
7      </ul>
8    </>
9  );
10}

You can’t use the for loop within the return statements of React components for the same reasons you can’t use for loops within the return statement of a function in vanilla JavaScript. Variable declarations or complex statements are not allowed within return statements. And, to use a for loop, you need to define a variable with let or var: One shock beginners to React always face is finding out that they can’t use the beloved for loop (or its variations, ie ‘for of’, ‘for in’) within the return statements of React components. This always produces an error: “Syntax error: Unexpected token”.  If the element to be iterated is not an array, then you can either convert it to an array using Array.from() or carry out the required looping technique outside the return statement of the React component. 

const SampleComponent = () => {

   //I am a for loop. Can I stay here? YES!!!   

   return (

      //I am a for loop. Can I stay here? NO!!

   )
}

for (let i = 0; i < 5; i++) {
   ....
}

const SampleComponent = () => {

   const animals = [{
         id: 1,
         animal: "Dog"
      },
      {
         id: 2,
         animal: "Pig"
      },
      {
         id: 3,
         animal: "Cat"
      },
      {
         id: 4,
         animal: "Bird"
      },
      {
         id: 5,
         animal: "Snake"
      }
   ];

   return (

      {
         animals.map(item => ({
            item.animal
         }))
      }

   );
};