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Arrays and Keys

Whenever we are transforming data into an array of elements and put it in our React tree, we need to make sure to give every element an unique identifier to help React distinguish elements for each render. This page will explain the key attribute and how to apply it whenever we need to map data to React.elements.

Keys & Rendering Arrays

Keys help React identify which elements have been changed, added, or removed throughout each render. Keys should be given to elements inside the array to give the elements a stable identity:

RES
let numbers = [1, 2, 3, 4, 5];

let items = Belt.Array.map(numbers, (number) => {
  <li key={Belt.Int.toString(number)}> {React.int(number)} </li>
})

The best way to pick a key is to use a string that uniquely identifies a list item among its siblings. Most often you would use IDs from your data as keys:

RES
type todo = {id: string, text: string}

let todos = [
  {id: "todo1", text: "Todo 1"},
  {id: "todo2", text: "Todo 2"}
]

let items = Belt.Array.map(todos, todo => {
  <li key={todo.id}> {React.string(todo.text)} </li>
})

If you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:

RES
let items = Belt.Array.mapWithIndex(todos, (i, todo) => {
  // Only do this if items have no stable id
  <li key={Belt.Int.toString(i)}>
    {todo.text}
  </li>
});

Keys Must Only Be Unique Among Siblings

Keys used within arrays should be unique among their siblings. However they don’t need to be globally unique. We can use the same keys when we produce two different arrays:

RES
type post = {id: string, title: string, content: string}

module Blog = {
  @react.component
  let make = (~posts: array<post>) => {
    let sidebar =
      <ul>
        {
          Belt.Array.map(posts, (post) => {
            <li key={post.id}>
              {React.string(post.title)}
            </li>
          })->React.array
        }
      </ul>

    let content = Belt.Array.map(posts, (post) => {
        <div key={post.id}>
          <h3>{React.string(post.title)}</h3>
          <p>{React.string(post.content)}</p>
        </div>
    });
      
      <div>
      {sidebar}
      <hr />
      {React.array(content)}
    </div>
  }
}

let posts = [
  {id: "1", title: "Hello World", content: "Welcome to learning ReScript & React!"},
  {id: "2", title: "Installation", content: "You can install reason-react from npm."}
]

let blog = <Blog posts/>

Rendering list Values

In case you ever want to render a list of items, you can do something like this:

RES
type todo = {id: string, text: string}

@react.component
let make = () => {
  let todoList = list{
    {id: "todo1", text: "Todo 1"},
    {id: "todo2", text: "Todo 2"},
  }

  let items =
    todoList
    ->Belt.List.toArray
    ->Belt.Array.map(todo => {
      <li key={todo.id}> {React.string(todo.text)} </li>
    })

  <div> {React.array(items)} </div>
}

We use Belt.List.toArray to convert our list to an array before creating our array<React.element>. Please note that using list has performance impact due to extra conversion costs.

99% of the time you'll want to use arrays (seamless interop, faster JS code), but in some cases it might make sense to use a list to leverage advanced pattern matching features etc.

Components and PropsRefs and the DOM