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JavaScript: Invert the key-value pairs of an object, without mutating it

JavaScript fundamental (ES6 Syntax): Exercise-78 with Solution

Write a JavaScript program to invert the key-value pairs of an object, without mutating it. The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.

  • Use Object.keys() and Array.prototype.reduce() to invert the key-value pairs of an object and apply the function provided (if any).
  • Omit the second argument, fn, to get the inverted keys without applying a function to them.
  • The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const invertKeyValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, key) => {
    const val = fn ? fn(obj[key]) : obj[key];
    acc[val] = acc[val] || [];
    acc[val].push(key);
    return acc;
  }, {});
console.log(invertKeyValues({ a: 1, b: 2, c: 1 }));
console.log(invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value));

Sample Output:

{"1":["a","c"],"2":["b"]}
{"group1":["a","c"],"group2":["b"]}

Flowchart:

flowchart: Invert the key-value pairs of an object, without mutating it

Live Demo:

See the Pen javascript-basic-exercise-78-1 by w3resource (@w3resource) on CodePen.


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Previous: Write a JavaScript program to iterate over all own properties of an object, running a callback for each one.
Next: Write a JavaScript program to take any number of iterable objects or objects with a length property and returns the longest one.

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JavaScript: Tips of the Day

How to remove a specific item from an array?

Find the index of the array element you want to remove using indexOf, and then remove that index with splice.

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

const array = [2, 5, 9];

console.log(array);

const index = array.indexOf(5);
if (index > -1) {
  array.splice(index, 1);
}

// array = [2, 9]
console.log(array);

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

For the reason of completeness, here are functions. The first function removes only a single occurrence (i.e. removing the first match of 5 from [2,5,9,1,5,8,5]), while the second function removes all occurrences:

function removeItemOnce(arr, value) {
  var index = arr.indexOf(value);
  if (index > -1) {
    arr.splice(index, 1);
  }
  return arr;
}

function removeItemAll(arr, value) {
  var i = 0;
  while (i < arr.length) {
    if (arr[i] === value) {
      arr.splice(i, 1);
    } else {
      ++i;
    }
  }
  return arr;
}
//Usage
console.log(removeItemOnce([2,5,9,1,5,8,5], 5))
console.log(removeItemAll([2,5,9,1,5,8,5], 5))

Ref: https://bit.ly/2N9nKRp