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JavaScript: Group the elements of an array based on the given function and returns the count of elements in each group

JavaScript fundamental (ES6 Syntax): Exercise-69 with Solution

Write a JavaScript program to group the elements of an array based on the given function and returns the count of elements in each group.

  • Use Array.prototype.map() to map the values of an array to a function or property name.
  • Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const countBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});
console.log(countBy([6, 10, 100, 10], Math.sqrt));
console.log(countBy([6.1, 4.2, 6.3], Math.floor));
console.log(countBy(['one', 'two', 'three'], 'length'));

Sample Output:

{"10":1,"2.449489742783178":1,"3.1622776601683795":2}
{"4":1,"6":2}
{"3":2,"5":1}

Flowchart:

flowchart: Group the elements of an array based on the given function and returns the count of elements in each group

Live Demo:

See the Pen javascript-basic-exercise-69-1 by w3resource (@w3resource) on CodePen.


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Previous: Write a JavaScript program that accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
Next: Write a JavaScript program to count the occurrences of a value in an array.

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JavaScript: Tips of the Day

Using the window.location object

JavaScript can access the current URL using the window.location object. Pretty neat, but even cooler is that this object contains certain parts of the URL as well.
Get access to the protocol/host/pathname/search/and more!

// JavaScript can access the current URL in parts. For this URL:
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window.location.protocol == 'https:'
window.location.host == 'thatsanegg.com'
window.location.pathname == '/example/index.html'
window.location.search == '?s=article'

Ref: https://bit.ly/2LBj213