Question

Technologies:

javascript

reactjs

Tolerim

a day ago

After updating the state in useEffect, why is the React DOM not being updated?

In my React application, there is a parent component with multiple child components, each containing numerous fields. I use the Context API to maintain a global state object, and each child component has its own state within this global state. To update/set the global state, I employ the Data down Actions Up approach, only updating the state at the parent component and then rendering the fields in the child components based on this global state. The child components invoke the action via props to update their piece of the state. Here is an example of how individual child components call the action to merge the currentSectionResponse with the existingData:


let formattedData = formatSectionDataForms(currentSectionResponse, existingData);
props.updateSectionFromChild({
    sectionGroupOrder: formattedData.sectionGroupOrder,
    sectionGroups: {
        ...formattedData.sectionGroups,
        'Section 1': formattedData.sectionGroups['Section 1']
    }
});

Furthermore, each child component has a useEffect that sets a local state from the global context object based on myGlobalContextData, and then the fields are rendered to the UI. Here is an example:


useEffect(() => {   
    if (myGlobalContextData && myGlobalContextData.sectionGroups['Child Section 1'] && myGlobalContextData.sectionGroups['Child Section 1'].sections 
        && myGlobalContextData.sectionGroups['Child Section 1'].sections['child-section-1'].fields 
        && myGlobalContextData.sectionGroups['Child Section 1'].sections['child-section-1'].fields.length > 0) {
            data = myGlobalContextData;
            if (clientProfileFormData && clientProfileFormData.sectionGroups['Current Section'] && clientProfileFormData.sectionGroups['Current Section'].sections 
                    && clientProfileFormData.sectionGroups['Current Section'].sections['current-section-2'].fields 
                    && clientProfileFormData.sectionGroups['Current Section'].sections['current-section-2'].fields.length === 0) {
                getCurrentSectionData();
            }

            const section = myGlobalContextData?.sectionGroups[sectionGroupName].sections[sectionName];     

            setMyLoacalState({
                ...myLocalState,
                myField: section.fields.find(field => field.id == 'my-field')
            });
    }        
},[myGlobalContextData]);

However, in some cases, I do not observe proper existingData, and even though the section's state is updated in the useEffect, the same values are not reflected on the UI. How can I ensure that the DOM updates after setting the local state from the global context object?

Answers(1)

Accepted Answer

To ensure that the DOM gets updated after setting the local state, you can add a second argument to the setMyLocalState call, which is a callback function that will be executed after the state has been updated. This callback function can contain any code that needs to be executed after the state has been updated, such as triggering a re-render of the component. Here's an example:

setMyLocalState({
  ...myLocalState,
  myField: section.fields.find(field => field.id == 'my-field')
}, () => {
  // This code will be executed after the state has been updated
  console.log('State updated, triggering re-render...');
  forceUpdate(); // Or any other function that triggers a re-render
});

In this example, we pass a second argument to setMyLocalState, which is an anonymous function that logs a message and triggers a re-render. In this case, we used forceUpdate to trigger a re-render, but you can also use any other function that achieves the same result. Note that by default, React executes state updates asynchronously, so calling forceUpdate or any other method that triggers a re-render immediately after setMyLocalState may not always work. That's why we pass a callback function to setMyLocalState, which will be executed after the state has been updated and any pending re-renders have been queued.