Why Can’t I Add a Property to the Apex Result Array?

Question

I’m trying to add a new property called showDeleteButton to each object in the result array returned from an Apex call. However, when I include the result.forEach line, the code after it does not execute. It seems like the mutation is not working as expected, and the array remains unchanged. Here’s my code:

getLaborProductsByPartCategoryAndEquipmentType({ partCategories: partCategories, equipmentType: this.veteranequipmenttype })
    .then(result => {
        console.log('Result before modification:', JSON.stringify(result));

        result.forEach(item => {
            item.showDeleteButton = false; 
        });

        console.log('Result after modification:', JSON.stringify(result));

        this.laborProducts = result; // This array will just hold the labor product.
    })
    .catch(error => {
        console.error('Error fetching labor products:', error);
    });

Even though the console.log before modification works, the code after the forEach loop does not execute. What is causing this issue, and how can I properly modify the result array?

Answer

The issue arises because the results from both wired and imperative Apex calls in LWC are immutable. Salesforce enforces this immutability so that client-side caches remain consistent. Even though mutating these results may have worked before, this is no longer the case.

As per the documentation:

“Assume that Apex methods provision immutable data, even if it’s possible to mutate the data today. It’s a best practice to treat all data that a component receives as immutable. To mutate the data, make a shallow copy of the objects you want to mutate.”

Since the objects within the array are immutable, a shallow copy of the array is not enough. You need to deep copy each object within the array before modifying its properties.

Here’s the correct way to do it:

getLaborProductsByPartCategoryAndEquipmentType({ partCategories: partCategories, equipmentType: this.veteranequipmenttype })
    .then(result => {
        console.log('Result before modification:', JSON.stringify(result));

        // Create a new array with copies of objects, then modify the copies
        this.laborProducts = result.map(item => {
            return { ...item, showDeleteButton: false };
        });

        console.log('Result after modification:', JSON.stringify(this.laborProducts));
    })
    .catch(error => {
        console.error('Error fetching labor products:', error);
    });

The given code snippet makes an imperative Apex call to getLaborProductsByPartCategoryAndEquipmentType, fetching labor products based on partCategories and equipmentType. Since Apex results are immutable, directly modifying them would cause a runtime error. To work around this, the code first logs the original result for debugging. Then, it creates a new array using .map(), where each object is copied using the spread operator { ...item }, and a new property showDeleteButton is added with a value of false. This ensures that the original result remains unchanged while allowing modifications on a separate instance. After modifying the data, it logs the updated array to confirm the changes. If the Apex call fails, the .catch() block logs the error for debugging. This approach follows best practices in LWC development, ensuring data integrity and preventing unexpected errors caused by modifying immutable Apex responses.

This solution works because the spread operator ({ ...item }) creates a new object, preventing any direct modification of the immutable Apex result.

Another approach is to use JSON.parse(JSON.stringify(result)), which creates a deep copy:

getLaborProductsByPartCategoryAndEquipmentType({ partCategories: partCategories, equipmentType: this.veteranequipmenttype })
    .then(result => {
        console.log('Result before modification:', JSON.stringify(result));

        let clonedResult = JSON.parse(JSON.stringify(result));
        clonedResult.forEach(item => {
            item.showDeleteButton = false;
        });

        this.laborProducts = clonedResult;

        console.log('Result after modification:', JSON.stringify(this.laborProducts));
    })
    .catch(error => {
        console.error('Error fetching labor products:', error);
    });

The given code snippet retrieves labor products from an imperative Apex call and modifies the result while maintaining Salesforce’s immutability rules. Since Apex responses are immutable, directly modifying the result would cause a runtime error. To avoid this, the code first logs the original response for debugging. Instead of modifying the original array, it creates a deep copy of result using JSON.parse(JSON.stringify(result)), ensuring that both the array and its objects are fully cloned. The forEach loop then iterates over the cloned array, adding a new property showDeleteButton with a value of false to each object. The modified array is then assigned to this.laborProducts, allowing the component to use the updated data without affecting the original Apex response. Finally, the updated array is logged to confirm the changes, and any potential errors from the Apex call are caught and logged for debugging. This approach ensures data integrity and prevents unexpected issues caused by modifying immutable objects in Lightning Web Components (LWC).
Both approaches ensure that you are not mutating the original result, allowing your component to function correctly without runtime exceptions.

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