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

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

Last Key Satisfying Condition

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:

// Define a function 'findLastKey' to find the last key in an object that satisfies a provided testing function
const findLastKey = (obj, fn) =>
  Object.keys(obj) // Get the keys of the object
    .reverse() // Reverse the array of keys to search from end to start
    .find(key => fn(obj[key], key, obj)); // Find the last key that satisfies the condition

// Test the 'findLastKey' function with a sample object and testing function
console.log(findLastKey(
  {
    barney: { age: 36, active: true },
    fred: { age: 40, active: false },
    pebbles: { age: 1, active: true }
  },
  o => o['active'] // Check if the 'active' property of the object is true
)); // Output: 'pebbles'

Output:

pebbles

Flowchart:

Live Demo:

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

For more Practice: Solve these Related Problems:

• Write a JavaScript program that returns the last key in an object for which a provided test function returns true.
• Write a JavaScript function that iterates over an object’s keys in reverse and returns the first key that satisfies a condition.
• Write a JavaScript program that searches an object for keys matching a predicate and returns the last matching key.
• Write a JavaScript function that uses Object.keys(), reverses the key array, and finds the last key meeting a criterion.

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