Summary

Restore Webpack 4 / ES2018 compatibility for lib/adapters/fetch.js. The fetch adapter was using the nullish coalescing operator (??), which is ES2020 syntax and breaks consumers bundling axios source with Webpack 4 (e.g. Vue 2.7 projects). axios's lib/ source has always been intended to stay ES2018-compatible; the lint config already enforces ecmaVersion: 2018, but the rule was never wired into CI, so the regression slipped through.

Linked issue

Closes #10851

Changes

• lib/adapters/fetch.js: replace utils.global ?? globalThis with an explicit !== undefined && !== null ternary so the source parses under ES2018.
• .github/workflows/run-ci.yml: run npm run lint in the build-and-run-vitest job, so the existing ecmaVersion: 2018 rule is actually enforced on PRs.
• tests/unit/syntaxCompat.test.js: new vitest unit test that parses every lib/**/*.js file with acorn at ecmaVersion: 2018 defence in depth so any future ES2019+ syntax in lib/ fails a named test, not just a generic lint error.
• eslint.config.js: add globalThis: 'readonly' to the lib globals blocks (the bare globalThis reference was previously masked by the ?? parse error; once parsing succeeds, no-undef flags it).
• package.json / package-lock.json: add acorn ^8.16.0 as a devDependency for the new test.

Checklist

• Tests added or updated (or N/A with reason) — new tests/unit/syntaxCompat.test.js covers the regression class
• Docs / types updated if public API changed (index.d.ts and index.d.cts) — N/A, no public API change
• No breaking changes (or called out explicitly above) runtime behaviour preserved; the ternary preserves the original nullish semantics

Summary by cubic

Restores ES2018 compatibility in the fetch adapter to fix “Unexpected token ??” build errors when bundling axios with webpack 4 / vue 2.7. CI now enforces ES2018 syntax and adds a parsing test to prevent regressions. Addresses #10851.

Description

• Summary of changes

  • Replaced ?? in lib/adapters/fetch.js with an ES2018-safe ternary.
  • Added npm run lint to CI to enforce ecmaVersion: 2018.
  • New test tests/unit/syntaxCompat.test.js parses all lib/**/*.js with acorn at ES2018.
  • Declared globalThis: 'readonly' in eslint.config.js for lib targets.
  • Added acorn as a devDependency.

• Reasoning

  • lib/ is intended to stay ES2018. ?? is ES2020 and breaks webpack 4 builds.
  • Lint and parsing tests catch future syntax regressions early.

• Additional context

  • No runtime behavior change; ternary preserves nullish semantics.
  • Closes Build Error: Unexpected token '??' in lib/adapters/fetch.js (Compatibility issue with Webpack 4 / Vue 2.7) #10851.

Docs

• Add a note in /docs/ about lib/ targeting ES2018.
• Mention compatibility with webpack 4 and vue 2.7 when bundling axios source.

Testing

• Added tests/unit/syntaxCompat.test.js (acorn ES2018 parse over all lib/**/*.js).
• CI now runs npm run lint.
• No additional unit tests needed for runtime behavior; existing suites should remain green.

Semantic version impact

• Patch: internal syntax fix and CI/test hardening. No API changes.

^{Written for commit ab8ecfb. Summary will update on new commits.}