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README.md

babel-runtime

Meteor maintains a version of the runtime helpers needed by Babel-transpiled code. In most cases, the code is copied from Babel's helper implementations, though we have also made some changes.

Benefits of maintaining our own package include:

  • IE 8 support. Babel's helpers target IE 9 and do not work in IE 8, but generally IE 8 support can be achieved with only minor changes.

  • Backwards-compatibility. When the Babel compiler changes, the helpers sometimes change. Our Babel package can keep old helpers for back-compat. (If we change over to publishing original ES6 code in packages instead of transpiled code, this becomes less important.)

  • Client-side code size. We've made the helpers file as small as possible.

Helpers

Helpers needed for each transform as of Babel v5.6.15:

  • es3.propertyLiterals: None
  • es3.memberExpressionLiterals: None
  • es6.arrowFunctions: None
  • es6.templateLiterals
    • taggedTemplateLiteralLoose
  • es6.classes
    • inherits
    • classCallCheck
    • createClass (only for getter/setters)
    • Excluded because only for decorator support(2): createDecoratedClass, defineDecoratedPropertyDescriptor
  • es6.constants: None
  • es6.blockScoping: None
    • Excluded because only for spec mode(1): temporalUndefined, temporalAssertDefined
  • es6.properties.shorthand: None
  • es6.properties.computed: None
    • Excluded because only for non-loose mode(1): defineProperty
  • es6.parameters: None
  • es6.spread
    • bind (for new A(...b))
  • es6.forOf: None
  • es7.objectRestSpread
    • _extends
    • Everything in es6.destructuring
  • es6.destructuring
    • objectWithoutProperties
    • objectDestructuringEmpty
  • es7.trailingFunctionCommas: None
  • flow: None

Footnotes:

  1. A transform can be run in "loose," normal, or "spec" mode, with "loose" providing the fastest, most lightweight, and usually most browser-compatible transpilation, while "spec" mode tries extra hard to be spec-compliant at the expense of those things. We've found that "loose" mode is the best mode for production code for every transform we've looked at.

  2. Decorators are still a Stage 1 proposal and are only implemented in Babel as an experiment.