37 lines
1.4 KiB
TypeScript
37 lines
1.4 KiB
TypeScript
/**
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* Count the occurrences of each item in an array
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* based on a transformation function.
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*
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* This function takes an array and a transformation function
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* that converts each item in the array to a key. It then
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* counts the occurrences of each transformed item and returns
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* an object with the transformed items as keys and the counts
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* as values.
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*
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* @template T - The type of the items in the input array.
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* @template K - The type of keys.
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* @param {T[]} arr - The input array to count occurrences.
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* @param {(item: T, index: number, array: readonly T[]) => K} mapper - The transformation function that maps each item, its index, and the array to a key.
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* @returns {Record<K, number>} An object containing the transformed items as keys and the
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* counts as values.
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*
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* @example
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* const array = ['a', 'b', 'c', 'a', 'b', 'a'];
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* const result = countBy(array, x => x);
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* // result will be { a: 3, b: 2, c: 1 }
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*
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* @example
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* const array = [1, 2, 3, 4, 5];
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* const result = countBy(array, item => item % 2 === 0 ? 'even' : 'odd');
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* // result will be { odd: 3, even: 2 }
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*
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* @example
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* // Using index parameter
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* const array = ['a', 'b', 'c', 'd'];
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* const result = countBy(array, (item, index) => index < 2 ? 'first' : 'rest');
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* // result will be { first: 2, rest: 2 }
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*/
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declare function countBy<T, K extends PropertyKey>(arr: readonly T[], mapper: (item: T, index: number, array: readonly T[]) => K): Record<K, number>;
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export { countBy };
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