Chai and ES6 polyfills

[olivr70]

Hello,

what are the guidelines in Chai regarding the runtime environment ? And more specifically for using some ES6 features :

• use ES5 only, include code for missing features and use that code exclusively (never the ES6 version which may available in the environment)
  • This to be sure to run exactly the same code on all runtimes
• use ES5 only, and include polyfilling code directly in the implementation.
  • This may lead to a less efficient code (environment adjustment is made in every call)
• target ES5, and polyfill localy in chai.js
• target ES5, and centralize all polyfills in a single util file
  • this could help remove polyfills later, or replace them by a third party polyfill component

I am asking because I need Number.isNaN() which is ES6 and has been natively available for a long time in Chrome (from v19) and Firefox (from v16).

I think this is may also be an interesting discussion for integration of ES6 specific features in chai (like isSymbol, String.isNormalized, Number.isFinite,closeTo using Number.EPSILON, testing typed arrays, iterators, computed property names...)

sorry if this has been discussed before, but I could not find any info in the site, the google group or the GitHub issues.

[keithamus]

Thanks for the issue @olivr70, it's a good question.

We don't really have an official stance, but in general we'll support ES6 features where we can. There's enough evidence mounting that we're keeping with the curve and supporting ES6:

• 3.0.0 added support for ES6 types, and Symbol.toString, e.g. expect(Symbol()).to.be.a('symbol')
• There's an open issue for using Proxies (where available) to throw on undefined property assertions.
• There's an open issue to support Object.is[Frozen/Sealed/Extensible] throwing in ES5 but returning false in ES6 - the aim to be following ES6 semantics.
• 4.0.0 will support getting keys on non-objects, rather than erroring - essentially following ES6 semantics.
• We've accepted patches before which fix quirks with ES6 or ES6-like tools, e.g. this one with babel.
• When we added support for propertyDescriptor assertions we discussed any problems with future ES6 support and made sure to avoid them.
• And (as you are very aware of @olivr70) there is an open issue for Number.isNaN semantics with the new isNaN assertions

Any new patches/PRs are reviewed on a case-per-case basis, generally with the thought toward forward and backward compat - but obviously some things slip through (like #498).

Hopefully this answers your question well enough. I'll close this issue now as its a question - feel free to continue the discussion though!