Update an Item in a List in React

 by Robin Wieruch
 - Edit this Post

It's a common task in React to update an item in a list. Here I want to show you briefly how this works. Every time you want to modify something in React, for example a list where you want to change an item, you have to use . We will be using React's useState Hook here, for the sake of keeping the first example simple, however, you can also use React's useReducer Hook, as you will see later.

We will start with a typical where we provide a for each rendered list item:

import React from 'react';
const list = [
{
id: 'a',
task: 'Learn React',
isComplete: false,
},
{
id: 'b',
task: 'Learn GraphQL',
isComplete: true,
},
];
const App = () => {
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span>
{item.task}
</span>
</li>
))}
</ul>
);
};
export default App;

In addition, the list item is either struck through or untouched based on its isComplete boolean flag. We are using for quick prototyping here:

import React from 'react';
const list = [
{
id: 'a',
task: 'Learn React',
isComplete: false,
},
{
id: 'b',
task: 'Learn GraphQL',
isComplete: true,
},
];
const App = () => {
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete
? 'line-through'
: 'none',
}}
>
{item.task}
</span>
</li>
))}
</ul>
);
};
export default App;

So far, the list is just a JavaScript variable and not stateful yet. In order to modify it, in this case to edit an item in it, we need to make the list stateful with React's state and its :

const initialList = [
{
id: 'a',
task: 'Learn React',
isComplete: false,
},
{
id: 'b',
task: 'Learn GraphQL',
isComplete: true,
},
];
const App = () => {
const [list, setList] = React.useState(initialList);
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete
? 'line-through'
: 'none',
}}
>
{item.task}
</span>
</li>
))}
</ul>
);
};

Now we have a stateful list and we are able to alter it. Let's add a button with a which deals with the click event for each item in the list. In this case, the button should be there for editing an item:

const App = () => {
const [list, setList] = React.useState(initialList);
function handleToggleComplete() {
// toggle item's complete flag
}
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete
? 'line-through'
: 'none',
}}
>
{item.task}
</span>
<button type="button" onClick={handleToggleComplete}>
{item.isComplete ? 'Undo' : 'Done'}
</button>
</li>
))}
</ul>
);
};

Since we are in a mapped list, we need to figure how to pass the specific item, or the item's identifier, which we want to change in the list, to the handler function. The most straightforward approach to this would be using an inline handler to sneak in the item, or item identifier in this case, as a parameter:

const App = () => {
const [list, setList] = React.useState(initialList);
function handleToggleComplete(id) {
console.log(id);
// toggle item's complete flag
}
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete
? 'line-through'
: 'none',
}}
>
{item.task}
</span>
<button
type="button"
onClick={() => handleToggleComplete(item.id)}
>
{item.isComplete ? 'Undo' : 'Done'}
</button>
</li>
))}
</ul>
);
};

The only thing missing is updating the specific item in the list whenever a click on the button happens. We will do this by modifying the current stateful list with a map function:

const App = () => {
const [list, setList] = React.useState(initialList);
function handleToggleComplete(id) {
const newList = list.map((item) => {
if (item.id === id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
setList(newList);
}
return (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete
? 'line-through'
: 'none',
}}
>
{item.task}
</span>
<button
type="button"
onClick={() => handleToggleComplete(item.id)}
>
{item.isComplete ? 'Undo' : 'Done'}
</button>
</li>
))}
</ul>
);
};

Rather than mutating the list, we keep it as and therefore create a new list based on the mapped list where we change every item which meets the condition. If an item meets the condition, we are using all the item's properties for the new item with and change the property that we want to modify. It's because the map function doesn't modify the list but only returns a new list.

Now, when our state updater function from React's useState Hook is called, the list with the changed item is set as new state and the component re-renders to display all items again. That's everything there is to know about changing an entry in an array in React. But there is more ...

For example, in our case everything happens in one component. What would happen if you would want to update an item from the list from a child component? Let's continue with splitting the component into multiple components. We will need a callback handler to pass the functionality as in order to change an item:

const App = () => {
const [list, setList] = React.useState(initialList);
function handleToggleComplete(id) {
const newList = list.map((item) => {
if (item.id === id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
setList(newList);
}
return <List list={list} onToggleComplete={handleToggleComplete} />;
};
const List = ({ list, onToggleComplete }) => (
<ul>
{list.map((item) => (
<li key={item.id}>
<span
style={{
textDecoration: item.isComplete ? 'line-through' : 'none',
}}
>
{item.task}
</span>
<button
type="button"
onClick={() => onToggleComplete(item.id)}
>
{item.isComplete ? 'Undo' : 'Done'}
</button>
</li>
))}
</ul>
);

That's it. You are able to update an item from a child component whereas the list is managed as state somewhere up in a parent component. If you would want to manage the list as state in the List component instead of managing it in the App component, you would have to .

Now, we will continue by exchanging React's useState with . The reducer hook can be used in React for complex state and complex state transitions. This is not the case for our state at the moment, but it could be of interest for your particular case in the future. Let's start by defining a for managing the stateful list:

const listReducer = (state, action) => {
switch (action.type) {
case 'UPDATE_ITEM':
return state.map((item) => {
if (item.id === action.id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
default:
throw new Error();
}
};

Essentially a reducer function takes a state and action as input and returns a new state based on this information as output. In addition, it has a branch for each action type. In this case, there is only one action type and thus one branch to edit an item. The actual logic to update the item from the list moved from our handler function into this reducer now.

Next, we will exchange the component's useState hook with a useReducer hook. This hook returns the state and a dispatch function as array which we conveniently access again via . The dispatch function is then used in our handler function by passing an appropriate action to it:

const App = () => {
const [list, dispatchList] = React.useReducer(
listReducer,
initialList
);
function handleToggleComplete(id) {
dispatchList({ type: 'UPDATE_ITEM', id });
}
return <List list={list} onToggleComplete={handleToggleComplete} />;
};

That's it for using useReducer instead of useState. Both state hooks are useful in React, so you should decide based on your needs whether you need a .

Last but not least, it may not always be the case that your state is only the list. Often you will have a more complex state object and the list is only one property of this object. How would you change an item from this list in the object then? Let's go through this example first with React's useState Hook again. Let's say next to the list there is a boolean flag to either show or hide the list with a :

const App = () => {
const [listData, setListData] = React.useState({
list: initialList,
isShowList: true,
});
function handleToggleComplete(id) {
// this doesn't work yet
const newList = list.map((item) => {
if (item.id === id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
// this doesn't work yet
setList(newList);
}
if (!listData.isShowList) {
return null;
}
return (
<List
list={listData.list}
onToggleComplete={handleToggleComplete}
/>
);
};

We start off with a complex state object which has the list as one of its properties. Wherever we want to use the list (or the boolean flag), we need to access the property from the object first. The only thing missing is fixing the handler function, because it cannot operate solely on the list anymore, but needs to take the object into account:

const App = () => {
const [listData, setListData] = React.useState({
list: initialList,
isShowList: true,
});
function handleToggleComplete(id) {
const newList = listData.list.map((item) => {
if (item.id === id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
setListData({ ...listData, list: newList });
}
if (!listData.isShowList) {
return null;
}
return (
<List
list={listData.list}
onToggleComplete={handleToggleComplete}
/>
);
};

Again, we access the list property from the object to edit the list item based on the incoming identifier. Then, we have to update the state with the complex state object again. We could set both, the new list and the boolean flag -- which didn't change -- explicitly, but in this case we are using JavaScript's spread operator to spread all key/value pairs from the state object into the new state object while overriding the list property with the new list. Let's apply the same technique for the example with the reducer function:

const listReducer = (state, action) => {
switch (action.type) {
case 'UPDATE_ITEM': {
const newList = state.list.map((item) => {
if (item.id === action.id) {
const updatedItem = {
...item,
isComplete: !item.isComplete,
};
return updatedItem;
}
return item;
});
return { ...state, list: newList };
}
default:
throw new Error();
}
};
const App = () => {
const [listData, dispatchListData] = React.useReducer(listReducer, {
list: initialList,
isShowList: true,
});
function handleToggleComplete(id) {
dispatchListData({ type: 'UPDATE_ITEM', id });
}
if (!listData.isShowList) {
return null;
}
return (
<List
list={listData.list}
onToggleComplete={handleToggleComplete}
/>
);
};

That's it. Similar to the previous version, we are just applying all the changes to the complex state object which has the list as property rather than using the list directly as state. The updating of the item in the list stays the same.


All the shown examples for changing an item in a list in React can be seen in this GitHub repository. If you have any feedback about how to update items in lists in React, just ping me.

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