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JavaScript: Get the last key that satisfies the provided testing function, otherwise undefined is returned

JavaScript fundamental (ES6 Syntax): Exercise-235 with Solution

Write a JavaScript program to get the last key that satisfies the provided testing function, otherwise undefined is returned.

  • Use Object.keys(obj) to get all the properties of the object.
  • Use Array.prototype.reverse() to reverse the order and Array.prototype.find() to test the provided function for each key-value pair.
  • The callback receives three arguments - the value, the key and the object.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const findLastKey = (obj, fn) =>
  Object.keys(obj)
    .reverse()
    .find(key => fn(obj[key], key, obj));

console.log(findLastKey(
  {
    barney: { age: 36, active: true },
    fred: { age: 40, active: false },
    pebbles: { age: 1, active: true }
  },
  o => o['active']
));

Sample Output:

pebbles

Flowchart:

flowchart: Get the last key that satisfies the provided testing function, otherwise undefined is returned.

Live Demo:

See the Pen javascript-basic-exercise-235-1 by w3resource (@w3resource) on CodePen.


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JavaScript: Tips of the Day

Creates an object composed of the properties the given function returns truthy for. The function is invoked with two arguments: (value, key)

Example:

const tips_composed = (obj, fn) =>
  Object.keys(obj)
    .filter(k => fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

console.log(tips_composed({ p: 2, q: '4', r: 6 }, x => typeof x === 'number'));

Output:

[object Object] {
  p: 2,
  r: 6
}