CODEWITHSUNDEEP
javascriptunderscore.js
TheRealFakeNews
TheRealFakeNews

Reputation: 8153

Need clarification understanding _.some()

It may be that I'm just very groggy this morning, but I'm having trouble understanding why this returns as true:

_.some([null, 0, 'yes', false]); // true

I know that _.some() returns true if at least one of the elements passes the predicate test as true. But from my understanding, if no predicate is provided, _.identity() is used. But console.log-ing each of those elements individually with _.identity() didn't return true for any of them. So why does it return true?

Upvotes: 0

Views: 134

Answers (3)

cl3m
cl3m

Reputation: 2801

'yes' is truthy:

_.some([null])  // false
_.some([0])     // false
_.some(['yes']) // true
_.some([false]) // false

From the Truth, equality in javascript link:

The construct if ( Expression ) Statement will coerce the result of evaluating the Expression to a boolean using the abstract method ToBoolean for which the ES5 spec defines the following algorithm:

string: The result is false if the argument is the empty String (its length is zero); otherwise the result is true.

Upvotes: 2

ssube
ssube

Reputation: 48277

Without a predicate, some uses identity, which uses the value itself, and 'yes' is truthy.

A quick dive through the annotated source (paying special attention to cb and the handling of missing predicates there) leaves you with, essentially, a coercion to boolean when they do:

if (predicate(obj[currentKey], currentKey, obj)) return true;

No predicate means you're working with the original value there, so if ('yes'), which is true.

You're not seeing true in the console for any of those values because _.identity will return the value itself (so 'yes') rather than coercing it to a boolean. If you were to do !!'yes' (coercion and double-not), you will see true.

Upvotes: 2

Bergi
Bergi

Reputation: 664547

It doesn't need to return the literal value true, it only needs to return a truthy value (although you always should return only booleans).

The non-empty string 'yes' is truthy (you can test by Boolean('yes') or !!'yes').

Upvotes: 1

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