Consider the following array of arrays
const data = [[150, 1], [300, 2], [430, 1]]If I were to use Lodash's countBy, I could achieve the same result with ES6 syntax by writing
{ ...data.reduce((acc, row) => ({ ...acc, [row[1]]: (acc[row[1]] || 0) + 1 }), {}) }Using map reduce makes this task sound quite simple. The provided function takes an array and a selector parameter as input. The selector parameter is utilized in the map function to fetch the items for counting. With reduce, we can aggregate the results by initializing an empty object and incrementing the value for each key on every iteration.
function countBy(ary, selector) {
return ary.map(selector)
.reduce((acc, cur) => { acc[cur] = (acc[cur] || 0) + 1; return acc; } ,{});
// Alternatively use spread operator:
// .reduce((acc, cur) => ({ ...acc, [cur]: (acc[cur] || 0) + 1 }) ,{});
}
// See console.log for the outcome
console.log(countBy([[150, 1], [300, 2], [430, 1]], x => x[1]));To create a counting function that utilizes a simple loop, one can retrieve the key by invoking a callback and then assigning key/value pairs to an object:
function generateCount(data, getKey) {
let count = {};
for (let item of data) {
let key = getKey(item);
count[key] = (count[key] || 0) + 1;
}
return count;
}
const data = [[150, 1], [300, 2], [430, 1]]
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