Everything you need to know about Ruby 4.0
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Ruby 4.0 was released on December 25, 2025, marking the language's 30th anniversary. The upgrade is relatively painless with mostly backwards-compatible changes. Key features include Ruby::Box for isolated namespaces within processes, ZJIT as a new method-based JIT compiler alongside YJIT, improvements to Ractors with a new Ractor::Port API, changes to nil splatting behavior, and the ability to start lines with logical operators. Set and Pathname are now built-in core classes, and Array gains new methods like rfind. The article provides a step-by-step upgrade guide covering release notes review, deprecation warnings, testing, bundler updates, and version switching.
Table of contents
What is Ruby::Box?Why ZJIT mattersRuby 4 has some improvements to Ractors*.nil changes in Ruby 4Logical operators at the beginning of a line in Ruby 4Some class updates in Ruby 4How to upgrade to Ruby 4Sort: