*Please note that most of the necessary code adjustments were made in PR 7890.*
ESLint has a number of advantageous properties, compared to JSHint. Among those are:
- The ability to find subtle bugs, thanks to more rules (e.g. PR 7881).
- Much more customizable in general, and many rules allow fine-tuned behaviour rather than the just the on/off rules in JSHint.
- Many more rules that can help developers avoid bugs, and a lot of rules that can be used to enforce a consistent coding style. The latter should be particularily useful for new contributors (and reduce the amount of stylistic review comments necessary).
- The ability to easily specify exactly what rules to use/not to use, as opposed to JSHint which has a default set. *Note:* in future JSHint version some of the rules we depend on will be removed, according to warnings in http://jshint.com/docs/options/, so we wouldn't be able to update without losing lint coverage.
- More easily disable one, or more, rules temporarily. In JSHint this requires using a numeric code, which isn't very user friendly, whereas in ESLint the rule name is simply used instead.
By default there's no rules enabled in ESLint, but there are some default rule sets available. However, to prevent linting failures if we update ESLint in the future, it seemed easier to just explicitly specify what rules we want.
Obviously this makes the ESLint config file somewhat bigger than the old JSHint config file, but given how rarely that one has been updated over the years I don't think that matters too much.
I've tried, to the best of my ability, to ensure that we enable the same rules for ESLint that we had for JSHint. Furthermore, I've also enabled a number of rules that seemed to make sense, both to catch possible errors *and* various style guide violations.
Despite the ESLint README claiming that it's slower that JSHint, https://github.com/eslint/eslint#how-does-eslint-performance-compare-to-jshint, locally this patch actually reduces the runtime for `gulp` lint (by approximately 20-25%).
A couple of stylistic rules that would have been nice to enable, but where our code currently differs to much to make it feasible:
- `comma-dangle`, controls trailing commas in Objects and Arrays (among others).
- `object-curly-spacing`, controls spacing inside of Objects.
- `spaced-comment`, used to enforce spaces after `//` and `/*. (This is made difficult by the fact that there's still some usage of the old preprocessor left.)
Rules that I indend to look into possibly enabling in follow-ups, if it seems to make sense: `no-else-return`, `no-lonely-if`, `brace-style` with the `allowSingleLine` parameter removed.
Useful links:
- http://eslint.org/docs/user-guide/configuring
- http://eslint.org/docs/rules/
This allows us to remove the `try/catch` statements used in `src/core/stream.js` when parsing JPEG images.
As far as I can tell, the only reason for the current usage of plain `throw` is that `jpg.js` originally was external code. Given that this code now lives in our repo, this patch brings the JPEG code more in line with e.g. `src/core/jpx.js` and `src/core/jbig2.js`.
*A more robust solution for issue 6066.*
As a temporary work-around for (the upstream) [bug 1164199](https://bugzilla.mozilla.org/show_bug.cgi?id=1164199), we parsed *all* images in the Firefox addon during a short time.
Doing so uncovered an issue with our image handling (see 6066), for JPEG images with a `DeviceGray` ColorSpace *and* `bpc !== 1` (bits per component).
As long as we let the browser handle image decoding in this case, this isn't going to be an issue, but I do think that we should proactively fix this to avoid future issues if we change where the images are decoded (in `jpg.js` vs in browser).
Also, we currently don't seem to have a test-case for that kind of image data.
Profiling showed that receiveAndExtend is frequently called with the length of
one bit. This happens for example in decodeBaseline.
For a single bit, the loop and shift in receive, as well as the shifts in
receiveAndExtend are overhead.
This shortcut manually calculates the shifts by either returning 1 or -1 from
receiveAndExtend by reading the bit and deciding on the return value.
While it comes with an overhead for each non-one length, the speedup is at about
10% in the hot parse/decode path.
This change makes the 4 conversion loops look the same.
It optimizes access of the array length and access of the property
numComponents, which is known to be constant.