Runtime

A runtime is the environment that executes a program after it has been written and compiled or interpreted. It supplies what the language itself does not: memory management, a garbage collector, an event loop, standard library functions, and the interface to the operating system for files, networking, and processes. The same source file can behave differently across runtimes because those services differ. In JavaScript, Node.js has long been the default server runtime, Deno adds permission-based sandboxing and native TypeScript, and Bun bundles a runtime, package manager, and test runner while targeting faster start-up. Edge runtimes such as Cloudflare Workers expose only a Web-standard subset of APIs, so code that expects Node's file system will fail there. Python has CPython and PyPy, and the JVM runs Java, Kotlin, and Scala. The word also names the phase of execution, as in a runtime error, which appears only when the code runs rather than when it is checked. The common trap is assuming portability. A library that depends on native modules or Node-specific globals often will not run unchanged on an edge runtime.