2023-11-22, Version 20.10.0 'Iron' (LTS), @targos

Notable Changes

--experimental-default-type flag to flip module defaults

The new flag --experimental-default-type can be used to flip the default module system used by Node.js. Input that is already explicitly defined as ES modules or CommonJS, such as by a package.json "type" field or .mjs/.cjs file extension or the --input-type flag, is unaffected. What is currently implicitly CommonJS would instead be interpreted as ES modules under --experimental-default-type=module:

• String input provided via --eval or STDIN, if --input-type is unspecified.

• Files ending in .js or with no extension, if there is no package.json file present in the same folder or any parent folder.

• Files ending in .js or with no extension, if the nearest parent package.json field lacks a type field; unless the folder is inside a node_modules folder.

In addition, extensionless files are interpreted as Wasm if --experimental-wasm-modules is passed and the file contains the "magic bytes" Wasm header.

Contributed by Geoffrey Booth in #49869.

Detect ESM syntax in ambiguous JavaScript

The new flag --experimental-detect-module can be used to automatically run ES modules when their syntax can be detected. For “ambiguous” files, which are .js or extensionless files with no package.json with a type field, Node.js will parse the file to detect ES module syntax; if found, it will run the file as an ES module, otherwise it will run the file as a CommonJS module. The same applies to string input via --eval or STDIN.

We hope to make detection enabled by default in a future version of Node.js. Detection increases startup time, so we encourage everyone—especially package authors—to add a type field to package.json, even for the default "type": "commonjs". The presence of a type field, or explicit extensions such as .mjs or .cjs, will opt out of detection.

Contributed by Geoffrey Booth in #50096.

New flush option in file system functions

When writing to files, it is possible that data is not immediately flushed to permanent storage. This allows subsequent read operations to see stale data. This PR adds a 'flush' option to the fs.writeFile family of functions which forces the data to be flushed at the end of a successful write operation.

Contributed by Colin Ihrig in #50009 and #50095.

Experimental WebSocket client

Adds a --experimental-websocket flag that adds a WebSocket global, as standardized by WHATWG.

Contributed by Matthew Aitken in #49830.

vm: fix V8 compilation cache support for vm.Script

Previously repeated compilation of the same source code using vm.Script stopped hitting the V8 compilation cache after v16.x when support for importModuleDynamically was added to vm.Script, resulting in a performance regression that blocked users (in particular Jest users) from upgrading from v16.x.

The recent fixes allow the compilation cache to be hit again for vm.Script when --experimental-vm-modules is not used even in the presence of the importModuleDynamically option, so that users affected by the performance regression can now upgrade. Ongoing work is also being done to enable compilation cache support for vm.CompileFunction.

Contributed by Joyee Cheung in #49950 and #50137.

Other notable changes

• [21453ae555] - (SEMVER-MINOR) deps: update uvwasi to 0.0.19 (Node.js GitHub Bot) #49908
• [ee65e44c31] - esm: use import attributes instead of import assertions (Antoine du Hamel) #50140
• [ffdc357167] - (SEMVER-MINOR) stream: allow passing stream class to stream.compose (Alex Yang) #50187
• [4861ad6431] - stream: improve performance of readable stream reads (Raz Luvaton) #50173
• [4b27087b30] - stream: optimize Writable (Robert Nagy) #50012
• [709c6c0cab] - (SEMVER-MINOR) test_runner, cli: add --test-concurrency flag (Colin Ihrig) #49996
• [57efd5292c] - (SEMVER-MINOR) vm: use import attributes instead of import assertions (Antoine du Hamel) #50141