babel-runtime

Source code of released version | Source code of development version

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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.