There is a really cool way to do group by in an array using reduce.
The problem statement - you have an array of objects and need to create different arrays based on similar attributes. This is very similar to the SQL group by statement.
Consider the example array -
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const fruits = [
(apple = { color: "red", shape: "round" }),
(watermelon = { color: "green", shape: "square" }),
(orange = { color: "orange", shape: "round" }),
(pear = { color: "green", shape: "pear" })
];
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We have to group by color - all reds in one array, greens in next array and so forth. This can of course be done with a multiple, nested for loops. Or, you can push the array through a reduce statement that is fed an interesting function.
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const res = fruits.reduce((acc, item) => {
const index = fruit => fruit.color(item);
if (!acc[index]) {
acc[index] = [];
}
acc[index].push(item);
return acc;
}, {});
console.log(res);
/* output
{
red: [ { color: 'red', shape: 'round' } ],
green: [
{ color: 'green', shape: 'square' },
{ color: 'green', shape: 'pear' }
],
orange: [ { color: 'orange', shape: 'round' } ]
}
*/
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