JavaScript's Iterator.concat() Chains Iterables Lazily Without Intermediate Arrays

A DEV Community article explains that Iterator.concat() lets developers loop over several iterables in sequence, returning a lazy iterator. The common spread-into-array approach copies every item and fully runs generators even when only a few values are needed. The built-in method works like a hand-written yield* generator and accepts arrays, Sets, Maps, and generators. Its result can be chained with iterator helpers such as filter, map, and toArray. The author notes it became Baseline Newly available in March 2026 and points out that the result is single-use and that breaking early closes the underlying iterators.