What is an "authoritative" way to detect if the _ variable is loaded with lodash or underscore?

I am using lodash for a project in an environment where underscore may sometimes also be loaded.

Currently, I've e up with this:

/** 
 * lodash defines a variable PLACEHOLDER = '__lodash_placeholder__'
 * so check if that is defined / contains the string "lodash"
 */
if ( typeof( _.PLACEHOLDER ) == 'undefined' || _.PLACEHOLDER.indexOf( 'lodash' ) < 0 ) {
    // _ is underscore, do what I need to access lodash
}

Important update: The above code does NOT work!

Is there an "authoritative" way to detect if _ is lodash or underscore?

Notes:
This is a specific request to find a way to determine if lodash or underscore is loaded in the _ variable:
1. It is outside of my control whether underscore is loaded or not. (lodash is within my control, and will always be loaded).
2. The load order of lodash / underscore can not be relied upon.
3. The version of underscore that is loaded is likely to change (it's part of a CMS framework that may be updated).
4. Lodash 4.17.x has 300+ functions. My code utilizes many of the functions in lodash.
5. Lodash contains many functions that underscore does not provide.
6. Some of the functions that do exist in both libraries have different implementations.

What is an "authoritative" way to detect if the _ variable is loaded with lodash or underscore?

I am using lodash for a project in an environment where underscore may sometimes also be loaded.

Currently, I've e up with this:

/** 
 * lodash defines a variable PLACEHOLDER = '__lodash_placeholder__'
 * so check if that is defined / contains the string "lodash"
 */
if ( typeof( _.PLACEHOLDER ) == 'undefined' || _.PLACEHOLDER.indexOf( 'lodash' ) < 0 ) {
    // _ is underscore, do what I need to access lodash
}

Important update: The above code does NOT work!

Is there an "authoritative" way to detect if _ is lodash or underscore?

Notes:
This is a specific request to find a way to determine if lodash or underscore is loaded in the _ variable:
1. It is outside of my control whether underscore is loaded or not. (lodash is within my control, and will always be loaded).
2. The load order of lodash / underscore can not be relied upon.
3. The version of underscore that is loaded is likely to change (it's part of a CMS framework that may be updated).
4. Lodash 4.17.x has 300+ functions. My code utilizes many of the functions in lodash.
5. Lodash contains many functions that underscore does not provide.
6. Some of the functions that do exist in both libraries have different implementations.

• javascript
• underscore.js
• lodash

• The only thing I can think of is _.VERSION present in lodash but not in underscore – bhantol Commented Dec 28, 2016 at 22:11
• 2 @bhantol: underscore has VERSION as well (at least the version on its website). – Felix Kling Commented Dec 28, 2016 at 22:14
• 1 isLodash = _.toString().indexOf('lodash') >=0 – bhantol Commented Dec 28, 2016 at 22:15
• 1 Can you elaborate a bit more on the overall problem. This seems to go in a similar direction as "browser vs feature detection". Do you need something specific from lodash that is not available in underscore? Or why do you need to know the difference? – Felix Kling Commented Dec 28, 2016 at 22:15
• 1 I'm with Felix here. I think you should be detecting features and not the source of the implementation. Due to the way you can do imports it could be individual functions in play, so sanity check those in advance if you must, then use them with the expectation that they'll work according to the documented contract. Normally you should unit test this aggressively with both libraries if that's how it will be run. – tadman Commented Dec 28, 2016 at 22:25

2 Answers 2

Going along similar lines as @bhantol already noted, there is a Migrating doc with a list of differences between lodash and underscore that were not patibilized with. Can't those be used? For example,

if ( typeof( _.invoke ) !== 'undefined' ){
    // it's lodash
}

But yeah, amplifying ments by @felix-kling and @tadman and others, if possible, it might be more reliable to restrict the question to the feature (e.g.: specific method) level rather than the whole library.

The code posted in the question does not work, as PLACEHOLDER is a private variable that gets renamed during minification.

Therefore, I've adapted the notion of "feature detection" as mentioned in the ments. Note that this method might break down if future versions of underscore roll in all of these functions, or if lodash deprecates any of these functions:

var isLodash = false;
// If _ is defined and the function _.forEach exists then we know underscore OR lodash are in place
if ( 'undefined' != typeof(_) && 'function' == typeof(_.forEach) ) {
  // A small sample of some of the functions that exist in lodash but not underscore
  var funcs = [ 'get', 'set', 'at', 'cloneDeep' ];
  // Simplest if assume exists to start
  isLodash  = true;
  funcs.forEach( function ( func ) {
    // If just one of the functions do not exist, then not lodash
    isLodash = ('function' != typeof(_[ func ])) ? false : isLodash;
  } );
}

if ( isLodash ) {
  // We know that lodash is loaded in the _ variable
  console.log( 'Is lodash: ' + isLodash );
} else {
  // We know that lodash is NOT loaded
}

<script src="https://cdnjs.cloudflare./ajax/libs/lodash.js/4.17.3/lodash.js"></script>