How to Sum Values of Duplicate Keys in a JavaScript Array of Objects?-JS Tutorial-php.cn

How to Sum Values of Duplicate Keys in a JavaScript Array of Objects?

Linda Hamilton

Release： 2024-12-17 07:41:24

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How to Sum Values of Duplicate Keys in a JavaScript Array of Objects?

Combining Same-Key Property Values in an Array of Objects

Given an array of JavaScript objects like the one below, how can we combine duplicate keys by summing their corresponding values?

objArr = [
    {key:&quot;Mon Sep 23 2013 00:00:00 GMT-0400&quot;, val:42},
    {key:&quot;Mon Sep 24 2013 00:00:00 GMT-0400&quot;, val:78},
    {key:&quot;Mon Sep 25 2013 00:00:00 GMT-0400&quot;, val:23},
    {key:&quot;Mon Sep 23 2013 00:00:00 GMT-0400&quot;, val:54} // <- duplicate key
]

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Our goal is to obtain an array with unique keys and summed values, like this:

reducedObjArr = [
    {key:&quot;Mon Sep 23 2013 00:00:00 GMT-0400&quot;, val:96},
    {key:&quot;Mon Sep 24 2013 00:00:00 GMT-0400&quot;, val:78},
    {key:&quot;Mon Sep 25 2013 00:00:00 GMT-0400&quot;, val:23}
]

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Solution Using Map and Reduce

Instead of iterating and pushing values, we can utilize the powerful combination of map and reduce:

const objArr = [
  {key: 'Mon Sep 23 2013 00:00:00 GMT-0400', val: 42},
  {key: 'Mon Sep 24 2013 00:00:00 GMT-0400', val: 78},
  {key: 'Mon Sep 25 2013 00:00:00 GMT-0400', val: 23},
  {key: 'Mon Sep 23 2013 00:00:00 GMT-0400', val: 54} // <- duplicate key
];

// Convert data into a Map with reduce
const counts = objArr.reduce((prev, curr) => {
  const count = prev.get(curr.key) || 0;
  prev.set(curr.key, curr.val + count);
  return prev;
}, new Map());

// Map counts object back to an array
const reducedObjArr = [...counts].map(([key, value]) => {
  return {key, value}
});

console.log(reducedObjArr);

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This approach effectively aggregates and combines values with the same keys, resulting in the desired reduced array.

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