tamda/dist/list/groupBy.js

Practical functional programming library for TypeScript

import { infer } from '../function/infer';
export function groupBy() {
    return inferred.apply(undefined, arguments);
}
const inferred = infer((array, keyFn) => array.reduce((groups, item, index) => {
    const key = `${keyFn(item, index, array)}`;
    // Initializes the array if undefined for the key.
    if (!groups[key]) {
        groups[key] = [];
    }
    // Faster than spreading the object and array, safe here
    // i.e. { ...groups, [key]: [...groups[key], item] }
    groups[key].push(item);
    return groups;
}, {}));
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