every is the right call when you want a single boolean answer to an all-of question. It short-circuits on the first failure, which makes it cheap on large arrays where mismatches happen early. The empty-array case is the only real gotcha: [].every(predicate) returns true because there is nothing that fails the test. Guard with an explicit arr.length > 0 if you want "at least one item AND all match".

some is every's mirror image: it returns true as soon as one item passes and false if none do. It also short-circuits, so on a large array where matches cluster early it is essentially free. Use some when the predicate is non-trivial ((item) => item.role === 'admin' && item.active); use includes when you only need exact-value membership on primitives because it handles NaN correctly while indexOf does not. Empty arrays return false, which is the intuitive opposite of every's true.

filter(predicate).length is the readable answer for "how many items match?" The cost is one allocation for the intermediate array, which is fine for almost everything. For very large arrays or a tight loop, the reduce form sums booleans without allocating. Both are linear time. Note that this is a strict count, not a boolean; if you only need to know "at least one" or "none", reach for some or every from the previous accordions because they short-circuit and skip the rest of the array.

Naming predicates lets you read arr.every(isEven) instead of decoding an inline arrow at the call site, and predicate factories like atLeast(10) capture parameters in a closure so the call site stays clean. Combinators like all(...preds) (logical AND) and a similar any (logical OR) compose without changing how the underlying every/some work. This is the same pattern Lodash exposes through _.overEvery and _.overSome, but it is small enough to write by hand and it keeps your validation rules and permission checks readable.