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Basic usage of Array.reduce
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// Basic usage of reduce to find sum of numbers in an array | |
const numbers = [1, 2, 3, 4, 5]; | |
const sum = numbers.reduce((prev, curr) => prev + curr, 0); | |
console.log('sum', sum); | |
// Basic usage of reduce to convert an array of objects into an object. | |
// For e.g. | |
// Input -> [{ id: 1, key: 'a', value: 'a' }, { id: 2, key: 'b', value: 'b' }, { id: 3, key: 'c', value: 'c' }]; | |
// Output -> { 1: { key: 'a', value: 'a' }, 2: { key: 'b', value: 'b' }, 3: { key: 'c', value: 'c' } }; | |
// Now you might ask, what's the use of this and where we might use this? | |
// Imagine this usecase: | |
// This was a huge array and time complexity is important for us. | |
// We have to find the key/value pairs multiple times based on `id`. | |
// We have memory to spare but not CPU cycles. | |
// Once it's an object, searching a certain object by `id` takes just O(1) time as opposed to O(n) when it's an array (no need to use Array.find/Array.filter etc.) | |
// Just `output[id]` is sufficient. | |
// In this case, time complexity of reduce is also O(n) but we do that just once and then finding any key/value by `id` is going to take constant time even if we have to find it 100x times. | |
const input = [{ | |
id: 1, | |
key: 'a', | |
value: 'a' | |
}, | |
{ | |
id: 2, | |
key: 'b', | |
value: 'b' | |
}, | |
{ | |
id: 3, | |
key: 'c', | |
value: 'c' | |
}]; | |
const output = input.reduce((prev, curr) => ({ | |
...prev, | |
[curr.id]: { | |
key: curr.key, | |
value: curr.value | |
} | |
}), {}); | |
console.log('output', output); | |
console.log('find by id=2 (array)', input.find(i => i.id === 2)); // O(n) 🐢 | |
console.log('find by id=2 (object)', output['2']); // O(1) 🔥 |
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