Loop inside React JSX

Question

I'm trying to do something like the following in React JSX (where ObjectRow is a separate component):

<tbody>
    for (var i=0; i < numrows; i++) {
        <ObjectRow/>
    } 
</tbody>

I realize and understand why this isn't valid JSX, since JSX maps to function calls. However, coming from template land and being new to JSX, I am unsure how I would achieve the above (adding a component multiple times).

Answer 1

function TableComponent({ numRows }) {
 return (
 <tbody>
  {Array.from({ length: numRows }).map((_, index) => (
    <ObjectRow key={index} />
  ))}
</tbody>
);
}

function ObjectRow() {
 return (
  <tr>
   <td>Row Content</td>
  </tr>
);
}

export default TableComponent;

How It Works

Array.from({ length: numRows }) creates an array with numRows elements. .map() iterates over the array, returning an ObjectRow component for each element. The key prop is added to each row for efficient rendering and reconciliation by React.

Why map and Not for?

JSX expects expressions, not statements. for is a statement, whereas map() is an expression that directly returns an array of elements. This functional approach is the most idiomatic in React.

Answer 2

In JSX, you can't directly use for loops inside the JSX code because JSX expects expressions, not statements. To render a component multiple times, you can use a combination of JavaScript's array methods (like map) and JSX.

You can achieve this with:

<tbody>
    {[...Array(numrows)].map((_, i) => (
        <ObjectRow key={i} />
    ))}
</tbody>

Keys: Always provide a unique key prop when rendering lists of components. It helps React optimize rendering.

Readable Code: This approach leverages the functional style of React, keeping it clean and idiomatic.

This will achieve your desired behavior in a React-friendly way.

Answer 3

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 pretty efficient and tidy loop in JSX. It's not the only way to do it, but the preferred way.

Also, don't forget to have a unique Key for each iteration as required. The map function creates a unique index from 0, but it's not recommended to use the produced index, but if your value is unique or if there is a unique key, you can use them:

<tbody>
  {numrows.map(x=> <ObjectRow key={x.id} />)}
</tbody>

Also, a few lines from MDN if you not familiar with the map function on Array:

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback is invoked only for indexes of the array which have assigned values, including undefined. It is not called for missing elements of the array (that is, indexes that have never been set, which have been deleted or which have never been assigned a value).

callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

If a thisArg parameter is provided to the map, it will be used as callback's this value. Otherwise, the value undefined will be used as its this value. This value ultimately observable by the callback is determined according to the usual rules for determining the this seen by a function.

map does not mutate the array on which it is called (although callback, if invoked, may do so).

Answer 4

Adding memoization - it depends on use case.

If Class based components

 shouldComponentUpdate(nextProps) {
    // Only re-render if numRows changes
    return nextProps.numRows !== this.props.numRows;
  }

  const rows = Array.from({ length: numRows }, (_, i) => <ObjectRow key={i} />);    
    return <tbody>{rows}</tbody>;

If functional components, try useMemo hook.

 const rows = React.useMemo(() => {
        return Array.from({ length: numRows }, (_, i) => <ObjectRow key={i} />);
      }, [numRows]);
    
    return <tbody>{rows}</tbody>;

Answer 5

You might want to checkout React Templates, which does let you use JSX-style templates in React, with a few directives (such as rt-repeat).

Your example, if you used react-templates, would be:

<tbody>
    <ObjectRow rt-repeat="obj in objects"/>
</tbody>

Answer 6

Simply using map Array method with ES6 syntax:

<tbody>
  {items.map(item => <ObjectRow key={item.id} name={item.name} />)} 
</tbody>

Don't forget the key property.

Answer 7

In JSX, you cannot directly use a loop statement like for within the JSX code. Instead, you can use JavaScript expressions and functions to achieve the desired result. Here are a few approaches you can take:

1. Using Array.map(): You can create an array of the desired length and use the map function to render the <ObjectRow /> component multiple times. Here's an example:

<tbody>
  {Array(numrows).fill().map((_, i) => (
    <ObjectRow key={i} />
  ))}
</tbody>

In this approach, Array(numrows).fill() creates an array of length numrows and then map is used to iterate over each element. The _ argument is the current element (which we don't need in this case), and i is the index. We set a unique key prop for each rendered component to satisfy React's requirements.

2. Using a loop outside JSX: You can use a regular JavaScript loop outside the JSX code and store the resulting components in an array. Then, you can render the array of components within the JSX. Here's an example:

// Outside JSX
const rows = [];
for (let i = 0; i < numrows; i++) {
  rows.push(<ObjectRow key={i} />);
}

// Inside JSX
<tbody>
  {rows}
</tbody>

In this approach, you create an empty array called rows and use a for loop to push the <ObjectRow /> components into it. Then, you can simply render the rows array within the JSX.

Both these approaches achieve the same result of rendering multiple instances of the <ObjectRow /> component based on the value of numrows.

Answer 8

This is not directly possible in React JSX.

You can do the following: First generate a new array with the length you need;

let filledArray = new Array(numRoiws).fill(0); // 0 here for example, choose what you want

Then loop over them in the React JSX

<tbody>
  {filledArray.map((arrayItem) => (
    <Component />
  ))}
</tbody>

Answer 9

Hello my developer friend! you can use this for make a loop looks like "for" inside the JSX react for essay! just use crt c plus crt p inside your project and enjoy!

<>{Array(6).fill({}).map((d, i) => (<p>Hi from item number {i}</p>)))}</>

Answer 10

This can be done in multiple ways.

1. As suggested above, before return store all elements in the array

2. Loop inside return

   Method 1:

   let container = [];
   let arr = [1, 2, 3] //can be anything array, object
   arr.forEach((val, index) => {
     container.push(
     <div key={index}>
       val
     </div>)
     /**
     * 1. All loop generated elements require a key
     * 2. only one parent element can be placed in Array
     * e.g. container.push(
     *         <div key={index}>
                 val
               </div>
               <div>
               this will throw error
               </div>
           )
     **/
   });
   return (
     <div>
       <div>any things goes here</div>
       <div>{container}</div>
     </div>
   )
   

   Method 2:

   return (
     <div>
       <div>any things goes here</div>
       <div>
         {
           (() => {
             let container = [];
             let arr = [1, 2, 3] //can be anything array, object
             arr.forEach((val, index) => {
               container.push(
                 <div key={index}>
                   val
                 </div>)
             });
             return container;
           })()
         }
       </div>
     </div>
   )
   

Answer 11

It is looping through a set of data, creating a new React Component for each item in the data set. This can be achieved using the map() method of JavaScript.

For example, if you have an array of objects called "data" and a React Component called "ObjectRow":

const rows = data.map((item, index) => <ObjectRow key={index} {...item} />);

Then you can render the rows inside the tbody tag like this:

<tbody>{rows}</tbody>

Answer 12

You can not use any loop or other extra methods directly in JSX. Instead what you can do is make separate function for that and make loop statement and in that function return your component Example for above code:

function renderObjectRow (){
   for (var i=0; i < numrows; i++) {
       return  <ObjectRow/>
     } 
}

<tbody>
{renderObjectRow()}
</tbody>

Answer 13

A one liner (assuming numrows is a number):

<tbody>
  {
    Array(numrows).fill().map(function (v, i) {
      return <ObjectRow/>
    })
  }
</tbody>

Answer 14

you can use .map() for loop in reactjs

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

Answer 15

Think of it like you're just calling JavaScript functions. You can't use a for loop where the arguments to a function call would go:

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

See how the function tbody is being passed a for loop as an argument – leading to a syntax error.

But you can make an array, and then pass that in as an argument:

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

---

You can basically use the same structure when working with JSX:

const rows = [];
for (let i = 0; i < numrows; i++) {
    // note: we are adding a key prop here to allow react to uniquely identify each
    // element in this array. see: https://reactjs.org/docs/lists-and-keys.html
    rows.push(<ObjectRow key={i} />);
}
return <tbody>{rows}</tbody>;

Incidentally, my JavaScript example is almost exactly what that example of JSX transforms into. Play around with Babel REPL to get a feel for how JSX works.

Answer 16

There are multiple ways to go about doing this. JSX eventually gets compiled to JavaScript, so as long as you're writing valid JavaScript, you'll be good.

My answer aims to consolidate all the wonderful ways already presented here:

If you do not have an array of object, simply the number of rows:

Within the return block, creating an Array and using Array.prototype.map:

render() {
  return (
    <tbody>
      {Array(numrows).fill(null).map((value, index) => (
        <ObjectRow key={index}>
      ))}
    </tbody>
  );
}

Outside the return block, simply use a normal JavaScript for loop:

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

Immediately invoked function expression:

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

If you have an array of objects

Within the return block, .map() each object to a <ObjectRow> component:

render() {
  return (
    <tbody>
      {objectRows.map((row, index) => (
        <ObjectRow key={index} data={row} />
      ))}
    </tbody>
  );
}

Outside the return block, simply use a normal JavaScript for loop:

render() {
  let rows = [];
  for (let i = 0; i < objectRows.length; i++) {
    rows.push(<ObjectRow key={i} data={objectRows[i]} />);
  }
  return (
    <tbody>{rows}</tbody>
  );
}

Immediately invoked function expression:

render() {
  return (
    <tbody>
      {(() => {
        const rows = [];
        for (let i = 0; i < objectRows.length; i++) {
          rows.push(<ObjectRow key={i} data={objectRows[i]} />);
        }
        return rows;
      })()}
    </tbody>
  );
}

Answer 17

try this one please

<tbody>
   {Array.apply(0, Array(numrows)).map(function (x, i) {
     return <ObjectRow/>;
   })}
</tbody>

or

{[...Array(numrows)].map((x, i) =>
   <ObjectRow/>
)}

Answer 18

This is what I used in most of my projects up to now:

const length = 5;
...
<tbody>
    {Array.from({ length }).map((_,i) => (
        <ObjectRow key={i}/>
    ))}
</tbody>

Answer 19

use map to looping

 Array.map(arrayItem => {
return <ObjectRow arrayItem={arrayItem}/> // pass array item in component
})

Answer 20

If you opt to convert this inside return() of the render method, the easiest option would be using the map( ) method. Map your array into JSX syntax using the map() function, as shown below (ES6 syntax is used).

---

Inside the parent component:

<tbody>
   { objectArray.map(object => <ObjectRow key={object.id} object={object.value} />) }
</tbody>

Please note the key attribute is added to your child component. If you didn't provide a key attribute, you can see the following warning on your console.

Warning: Each child in an array or iterator should have a unique "key" prop.

Note: One common mistake people do is using index as the key when iterating. Using index of the element as a key is an antipattern, and you can read more about it here. In short, if it's not a static list, never use index as the key.

---

Now at the ObjectRow component, you can access the object from its properties.

Inside the ObjectRow component

const { object } = this.props

Or

const object = this.props.object

This should fetch you the object you passed from the parent component to the variable object in the ObjectRow component. Now you can spit out the values in that object according to your purpose.

---

References:

map() method in JavaScript

ECMAScript 6 or ES6

Answer 21

try this

<tbody>
  {new Array(numrows).map((row,index)=><ObjectRow key={***someThingUniqe***}/>)} //don't use index as key 
</tbody>

if you wanna know why you shouldn't use indices as keys in react check this

Answer 22

You could do the following to repeat a component numrows times

<tbody>{Array(numrows).fill(<ObjectRow />)}</tbody>

Answer 23

Just do something like this:

<tbody>
    {new Array(numRows).fill("", 0, numRows).map((p, i) => <YourRaw key={i} />)}
</tbody>

Answer 24

If you really want a for loop equivalent (you have a single number, not an array), just use range from Lodash.

Don't reinvent the wheel and don't obfuscate your code. Just use the standard utility library.

import range from 'lodash/range'

range(4);
// => [0, 1, 2, 3]

range(1, 5);
// => [1, 2, 3, 4]

Answer 25

You may use .map() in a React for loop.

<tbody>
    { newArray.map(() => <ObjectRow />) }
</tbody>

Answer 26

Here is a sample from the React documentation, JavaScript Expressions as Children:

function Item(props) {
  return <li>{props.message}</li>;
}

function TodoList() {
  const todos = ['finish doc', 'submit pr', 'nag dan to review'];
  return (
    <ul>
      {todos.map((message) => <Item key={message} message={message} />)}
    </ul>
  );
}

As for your case, I suggest writing like this:

function render() {
  return (
    <tbody>
      {numrows.map((roe, index) => <ObjectRow key={index} />)}
    </tbody>
  );
}

Please notice the key is very important, because React use the key to differ data in array.

Answer 27

Use the map function.

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

Answer 28

React elements are simple JavaScript, so you can follow the rule of JavaScript. You can use a for loop in JavaScript like this:-

<tbody>
    for (var i=0; i < numrows; i++) {
        <ObjectRow/>
    } 
</tbody>

But the valid and best way is to use the .map function. Shown below:-

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

Here, one thing is necessary: to define the key. Otherwise it will throw a warning like this:-

Warning.js:36 Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of ComponentName. See "link" for more information.

Answer 29

Since you are writing JavaScript syntax inside JSX code, you need to wrap your JavaScript code in curly braces.

row = () => {
   var rows = [];
   for (let i = 0; i<numrows; i++) {
       rows.push(<ObjectRow/>);
   }
   return rows;
}
<tbody>
{this.row()}
</tbody>

Answer 30

If numrows is an array, it's very simple:

<tbody>
   {numrows.map(item => <ObjectRow />)}
</tbody>

The array data type in React is much better. An array can back a new array, and support filter, reduce, etc.