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groupby function
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| /** | |
| * Creates an object of groupings. | |
| * | |
| * @param {Function|String} f - The function that returns value to group by or object key. | |
| * @returns {Object} Returns the composed aggregate object. | |
| * @example | |
| * | |
| * array.groupBy('city'); | |
| * array.groupBy(function (obj) { return obj.city; }); | |
| * array.groupBy((obj) => { return obj.city; }); | |
| * array.groupBy(({ city }) => city); | |
| */ | |
| Array.prototype.groupBy = function (f) { | |
| return this.reduce((o, v) => { | |
| const key = typeof f === 'string' ? v[f] : f(v) | |
| o[key] || (o[key] = []) | |
| o[key].push(v) | |
| return o | |
| }, {}) | |
| } |
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