Count By

JSTS

Implement a function countBy(array, iteratee) that creates an object composed of keys generated from the results of running each element of array through iteratee. The corresponding value of each key is the number of times the key was returned by iteratee.

countBy(array, iteratee);

Arguments

array (Array): The array to iterate over.
iteratee (Function): The iteratee function to transform elements. The function is invoked with one argument: (value).

Returns

(Object): Returns the composed aggregate object.

Examples

countBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': 1, '6': 2 }

countBy([{ n: 3 }, { n: 5 }, { n: 3 }], (o) => o.n);
// => { '3': 2, '5': 1 }

The function should return when array is empty and treat null / undefined keys after going through iteratee as it is.

countBy([], (o) => o); // => {}

countBy([{ n: 1 }, { n: 2 }], (o) => o.m); // => { undefined: 2 }

Resources

Lodash _.countBy

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Count By

JSTS

countBy(array, iteratee);

Arguments

array (Array): The array to iterate over.
iteratee (Function): The iteratee function to transform elements. The function is invoked with one argument: (value).

Returns

(Object): Returns the composed aggregate object.

Examples

countBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': 1, '6': 2 }

countBy([{ n: 3 }, { n: 5 }, { n: 3 }], (o) => o.n);
// => { '3': 2, '5': 1 }

The function should return when array is empty and treat null / undefined keys after going through iteratee as it is.

countBy([], (o) => o); // => {}

countBy([{ n: 1 }, { n: 2 }], (o) => o.m); // => { undefined: 2 }

Resources

Lodash _.countBy

Similar Questions

Group By

Solution

Solution 1: Standard method of basic functions

Create an empty result object to store the count of occurrences of each key.
Iterate through the array and determine the key for each element by calling iteratee(element). If the key does not exist within the result object, set the value for that key to 0. Next we can increment the value for that key.

/**
 * @param {Array} array The array to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {Object} Returns the composed aggregate object.
 */
export default function countBy(array, iteratee) {
  const result = {};

  for (const element of array) {
    const key = String(iteratee(element));
    if (!Object.prototype.hasOwnProperty.call(result, key)) {
      result[key] = 0;
    }

    result[key]++;
  }

  return result;
}

Solution 2: Using nullish coalescing assignment operator

An alternative way to increment the result counter is to use the nullish coalescing assignment operator to set the value to 0 if key doesn't exist within result. Note that using nullish coalescing assignment operator means you might be accessing inherited properties, which is not desired, but since the object is created via Object.create(null), there will not be inherited properties and is safe to use.

/**
 * @param {Array} array The array to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {Object} Returns the composed aggregate object.
 */
export default function countBy(array, iteratee) {
  const result = Object.create(null);

  for (const element of array) {
    const key = String(iteratee(element));
    result[key] ??= 0;
    result[key]++;
  }

  return result;
}

Resources

Lodash _.countBy

Count By

Arguments

Returns

Examples

Resources

Count By

Arguments

Returns

Examples

Resources

Similar Questions

Solution

Solution 1: Standard method of basic functions

Solution 2: Using nullish coalescing assignment operator

Resources

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