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Table of contents
1.
Introduction
2.
Why This Function is Used
3.
Syntax, Parameter and Return Value
3.1.
Syntax:
3.2.
Parameters:
3.3.
Return Value: 
4.
Examples 
4.1.
Finding the Last Key Based on a Condition:
4.2.
JavaScript
4.3.
Using a Shorthand with Property Value:
4.4.
JavaScript
4.5.
Using a Property Name Shorthand:
4.6.
JavaScript
4.7.
Matching Against a Property Value:
4.8.
JavaScript
5.
Frequently Asked Questions
5.1.
How does _.findLastKey() differ from _.findKey()?
5.2.
What happens if no element satisfies the predicate?
5.3.
Can _.findLastKey() be used with arrays?
6.
Conclusion
Last Updated: Mar 27, 2024
Easy

Lodash _.findLastKey() Method

Author Gaurav Gandhi
0 upvote

Introduction

In JavaScript programming, especially when dealing with objects, finding the key of the last element that satisfies a specific condition can be an essential task. Lodash simplifies this process with the _.findLastKey() method. This method iterates over the properties of an object in reverse, returning the key of the first element the predicate returns truthy for.

Lodash _.findLastKey() Method

It's particularly useful for locating a key based on a value's properties when the order of properties matters or when the most recent (last) match is required.

Why This Function is Used

The _.findLastKey() function is used to find the key of the last element in an object that satisfies a given condition, defined by a predicate function. This method is especially useful in scenarios where the properties of an object are ordered or have a sense of recency, and you need to find the last key that meets certain criteria.

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Syntax, Parameter and Return Value

Syntax:

 _.findLastKey(object, [predicate=_.identity])

Parameters:

  • object (Object): The object to inspect.
     
  • [predicate=_.identity] (Function): The function invoked per iteration.

Return Value: 

(string|undefined) - Returns the key of the matched element, else undefined.

Examples 

Finding the Last Key Based on a Condition:

  • JavaScript

JavaScript

var _ = require('lodash');

var users = {

 'barney':  { 'age': 36, 'active': false },

 'fred':  { 'age': 40, 'active': false },

 'pebbles': { 'age': 1,  'active': true }

};

var key = _.findLastKey(users, function(o) { return o.active; });

console.log(key);

Output:

 'pebbles'


Demonstrates finding the key of the last user who is active.

Using a Shorthand with Property Value:

  • JavaScript

JavaScript

var key = _.findLastKey(users, { 'age': 36, 'active': false });

console.log(key);

Output:

 'barney'


Shows how to use object literal shorthand to find the last matching key.

Using a Property Name Shorthand:

  • JavaScript

JavaScript

var key = _.findLastKey(users, 'active');

console.log(key);

Output:

 'pebbles'


An example of using property name shorthand to find the key of the last active user.

Matching Against a Property Value:

  • JavaScript

JavaScript

var key = _.findLastKey(users, ['active', false]);

console.log(key);

 Output:

 'fred'


Demonstrates finding the key of the last user where the 'active' property is false.

Frequently Asked Questions

How does _.findLastKey() differ from _.findKey()?

_.findKey() returns the key of the first element that satisfies the predicate, while _.findLastKey() returns the key of the last element that satisfies the predicate.

What happens if no element satisfies the predicate?

If no element satisfies the predicate, _.findLastKey() returns undefined.

Can _.findLastKey() be used with arrays?

While _.findLastKey() is primarily designed for objects, it can be used with arrays. However, since array indices are less informative than object keys, array-specific methods are usually more suitable for arrays.

Conclusion

Lodash's _.findLastKey() method offers an effective way to search for the last key in an object that meets a specific condition. It is particularly useful for ordered objects or when the most recent (last) match based on certain criteria is required.

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