Array.find()that can even be used as expressions in JSX (if surrounded by curly brackets
Array.map()enables us to not only work with data but it can return modified items which in turn can also contain JSX. This adds further flexibility and allows us to transform data sets into React elements.
keyprop for all of the values returned by arrays or iterators. The primary reasoning behind this rule is to make it easier for React's Reconciler (the React Comparison algorithm) to identify and compare list elements. The Reconciler can spot whether an array element was added, removed or modified if the
keyprop has been given. This prop needs to be a unique identifier which only appears once in the array. Normally, the id of a data set is used for this.
CryptoListcomponent elsewhere using the same key - even within the same component. Just don't use the key again in the same loop.
keyprop is always present in the top-level component or array element within the iterator but not in the JSX returned by the component.
keyprop which we added beforehand is no longer present. Our
map()call would now look like this:
<li></li>element under the hood, the
<CryptoListItem />needs to contain the
keyprop as it is the top level component within our
Array.map()call in this JSX snippet.
CryptoListcomponent by using Object spread syntax:
currencyobjects are passed as props to the
Array.map(), a list would need to be explicitly defined like this:
render()method without a surrounding element, leading to an error. We do not always want to create new elements though, especially if the surrounding element is found in a parent component and the child element is found in its own component.
dl, ...) do not allow for
divelements to be used as an intermediary wrapper (ignoring the fact that we would also litter the markup by using
divs). As we are only permitted to ever return a single root element from a component, Fragments can be incredibly useful. We could transform our example from above into the following:
keyprop, still holds. Using a Fragment this is still possible. Let's illustrate this further by examining another slightly more complex yet more realistic example:
spanor another element around the
<dd></dd>. Trying to do that would result in this:
dlelement only permits
ddas its child element. The Fragment helps us to alleviate these situations and creates valid JSX without creating invalid markup. It was only introduced in React 16.3 and meant that some React components were unnecessarily complex to deal with this problem to avoid violating JSX or HTML rules.
<React.Fragment>can be used saving us multiple key strokes.
keyprop thus forcing us to use
render()function of Class components as well as Functional components can return a React element (also in form of JSX), a string, a number,
null(if nothing should be rendered) or an array of these types.
render()method nice and clean which I will explain now.
NotificationListcomponent receives a list of items in the form of props. If the list contains any entries at all, they are rendered as a list item of an unordered list. If however the list is empty, a message is returned informing the user that no new notifications are available.
view. Depending on which mode we are currently in, we either want to simply show text (View mode) or be able to see a text field containing the previously entered value (edit mode).
render()method of the component. The state is tested against its property value in
editis the current value in state, we directly return the input field with an "early return". If this is not the case, we assume that the "standard case" is taking place meaning that the current mode is
elsepart of this condition is not actually necessary here and would only add unnecessary complexity. Both times, the text is rendered with the difference that it is an editable
inputfield in one case, and a simple text node in the other. A button is included to switch between the different modes of
elseconstructs are common if you want to create output based on state and props within a component. I will explain them in more detail in just a moment.
nullis the most simple case of conditional rendering. If the
render()method of a component returns
null, nothing is rendered and also does not appear in the DOM. This can be useful for displaying error components that should only be displayed if the error has actually occurred.
nullis being returned. Otherwise, an error message containing the error in state is returned in a
div. We achieve this by using a simple
ifcondition similar to what we have used above.
condition ? met : not met. For example:
isLoggedIn ? 'Logout' : 'Login';
isLoggedInprop it can either include the message: Logout or Login.
is-disabledclass, whereas active users are denoted with
de(for Germany) as our country. In all other cases a simple text input is shown to user in which they can enter their state freely. However, careful consideration should be given when to use the ternary operator: it can become a little hard to read quickly if complex JSX is used.
&&) and Logical OR (
undefinedresulting in no visible markup for the user interface:
isMenuVisibleprop and check if it is
true. If that is the case, it will return the
Menucomponent. If the result is
undefinedis returned and nothing else will be rendered to the screen.
true, or Login if the user is logged out.
render()method to separate
renderXY()methods. The regular
render()method still forms the core of the component and decides which parts of the user interface to show to the user. Thus, this method should not become overly complex or contain any unnecessary logic.
render()methods into much smaller, more digestible chunks and implement these as custom class methods. If proper naming is used, this technique usually aids readability and understanding. Often these custom
render()blocks are combined with
render()method but also increases the complexity of the component slightly. Many people recommend to move parts of the code into their own Function components instead though (myself included).
render()methods, we can create new Function components. These will receive props from their parent component and then take care of displaying their own data as an independent, self-governing and testable component.
render()method in one component has become too complex or if the same elements are used repetitively within a component.
typeattribute. The label also needs to be equipped with an id that is unique to each field that it represents.
TextFieldcomponent and then cut the recurring JSX from the form component into this one:
idwhich is needed to link the label to the text input. Using Object Rest/Spread, we add all remaining props to the
inputelement as attributes, transforming the component into the following:
render()method which only consists of very few components. If we ever wanted to add a change which should affect all of our text fields (for example adding a class), this can be added with ease by only changing data in one place — in the new