# JavaScript: Group the elements of a given array based on the given function

## JavaScript fundamental (ES6 Syntax): Exercise-52 with Solution

Write a JavaScript program to group the elements of a given array based on the given function.

• Use Array.prototype.map() to map the values of the 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
// Define a function 'group_By' to group elements of an array based on a specified function.
const group_By = (arr, fn) =>
// Map the array elements to keys using the provided function, then reduce into groups based on keys.
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]); // Create groups based on keys.
return acc;
}, {});

// Test cases
console.log(group_By([6.1, 4.2, 6.3], Math.sqrt)); // Output: { '2.463751439371663': [ 6.1, 6.3 ], '2.04939015319192': [ 4.2 ] }
console.log(group_By([6.1, 4.2, 6.3], Math.floor)); // Output: { '6': [ 6.1, 6.3 ], '4': [ 4.2 ] }
console.log(group_By(['one', 'two', 'three'], 'length')); // Output: { '3': [ 'one', 'two' ], '5': [ 'three' ] }
```
```

Output:

```{"2.4698178070456938":[6.1],"2.04939015319192":[4.2],"2.5099800796022267":[6.3]}
{"4":[4.2],"6":[6.1,6.3]}
{"3":["one","two"],"5":["three"]}
```

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-1-52 by w3resource (@w3resource) on CodePen.

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