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Tolerim
a month ago

How can I perform an additive merge of two arrays of objects by matching on non-matching keys?

The task is to merge two arrays (A and B) of objects into a new array C. If an object appears in A but not in B, it must be copied into C without modification. If an object appears in B but not in A, it should be copied into C without modification. If an object appears in both A and B, all properties from A should be copied into B. If any properties appear in B but not in A, they should remain in B. If any properties are shared between A and B, the B properties should overwrite the A properties. Initially, the approach was to use Object.assign and Lodash merge, but it resulted in some issues where certain fields were not copied over, and some fields were merged in unexpected ways. The expected output array C is given as an example. To achieve the goals 1-3 above, a new approach is needed.
Answers(2)
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Vogi
a month ago
Verified Answer
To merge two arrays of objects in JavaScript, you can use either the spread operator or the concat() method. The spread operator is a more optimal solution if you are sure that all inputs to merge are arrays. In case you are unsure, use the concat() method instead. Here's an example of how you can use the spread operator to merge two arrays of objects: 
const arr1 = [{ id: 1, name: 'John' }, { id: 2, name: 'Jane' }];
const arr2 = [{ id: 2, age: 25 }, { id: 3, age: 30 }];

const mergedArr = [...arr1, ...arr2];

console.log(mergedArr);
// Output: [{ id: 1, name: 'John' }, { id: 2, name: 'Jane' }, { id: 2, age: 25 }, { id: 3, age: 30 }]
To achieve your goals of copying objects from A and B into C without modification and overwriting properties in B with properties in A when they share the same name, you can use a combination of Array.prototype.filter(), Array.prototype.map(), and Object.assign() methods. Here's an example: 
const arrA = [{ id: 1, name: 'John', age: 25 }, { id: 2, name: 'Jane', age: 30 }];
const arrB = [{ id: 2, age: 35 }, { id: 3, age: 40 }];

const mergedArr = [
    ...arrA.filter(a => !arrB.some(b => b.id === a.id)),
    ...arrB.filter(b => !arrA.some(a => a.id === b.id)),
    ...arrA.map(a => Object.assign({}, arrB.find(b => b.id === a.id) || {}, a))
];

console.log(mergedArr);
// Output:
// [
//     { id: 1, name: 'John', age: 25 },
//     { id: 2, age: 35 },
//     { id: 3, age: 40 }
// ]
I hope this helps! Let me know if you have any other questions.

Sources

How to merge two objects in JavaScript - Atta-Ur-Rehman Shah3 Ways to Merge Arrays in JavaScript - Dmitri Pavlutin BlogHow to merge two arrays in JavaScript and de-duplicate itemsjavascript - Merge 2 arrays of objects - Stack OverflowArray.prototype.concat() - JavaScript | MDN - Mozilla Developer
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Tolerim
a month ago
To achieve the goals of combining and merging two arrays of objects into a new array C, the following approach can be used: 
const A = [...form.layout];
const B = [...updates.layout];
const C = [];

// loop through array A and add objects to new array C
A.forEach(objA => {
  const objInB = B.find(objB => objB.key === objA.key);
  if (!objInB) {
    C.push(objA);
  } else {
    const mergedObj = Object.assign(objA, objInB);
    C.push(mergedObj);
  }
});

// loop through array B and add objects to new array C that were not in A
B.forEach(objB => {
  const objInA = A.find(objA => objA.key === objB.key);
  if (!objInA) {
    C.push(objB);
  }
});

console.log(C);
This code first creates new copies of the A and B arrays to ensure they remain unmodified. It then creates a new array C and loops through each object in A. For each object, it checks if there is an object with the same key property in B. If there is not, the object is added to C without modification. If there is, the properties of objA are merged with objB using Object.assign() and the merged object is added to C. This ensures that any properties in B that are not in A remain in the merged object and that any shared properties have the B value overwrite the A value. After looping through array A, the code then loops through array B and adds any objects that were not already added to C during the loop through A. This ensures that all objects from both arrays are included in the final array C. This approach should result in array C matching the desired output as outlined in the question.
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