migrate aio to eslint as tslint has been deprecated, the migration is restricted to the aio app and
its e2e tests and does not include the other tools, for such reason both tslint and codelyzer have not
been removed (to be done in a next PR)
some minor tweaks needed to be applied to the code so that it would adhere to the new ESLinting behaviour
most TSLint rules have been substituted with their ESLint equivalent, with some exceptions:
* [whitespace] does not have an ESLint equivalent (suggested to be handled by prettier)
* [import-spacing] does not have an ESLint equivalent (suggested to be handled by prettier)
* [ban] replaced with [no-restricted-syntax] as there is no (official/included) ESLint equivalent
some rules have minor different behaviours compared to their TSLint counterparts:
* @typescript-eslint/naming-convention:
- typescript-eslint does not enforce uppercase for const only.
* @typescript-eslint/no-unused-expressions:
- The TSLint optional config "allow-new" is the default ESLint behavior and will no longer be ignored.
* arrow-body-style:
- ESLint will throw an error if the function body is multiline yet has a one-line return on it.
* eqeqeq:
- Option "smart" allows for comparing two literal values, evaluating the value of typeof and null comparisons.
* no-console:
- Custom console methods, if they exist, will no longer be allowed.
* no-invalid-this:
- Functions in methods will no longer be ignored.
* no-underscore-dangle:
- Leading and trailing underscores (_) on identifiers will now be ignored.
* prefer-arrow/prefer-arrow-functions:
- ESLint does not support allowing standalone function declarations.
- ESLint does not support allowing named functions defined with the function keyword.
* space-before-function-paren:
- Option "constructor" is not supported by ESLint.
- Option "method" is not supported by ESLint.
additional notes:
* the current typescript version used by the aio app is 4.3.5, which is not supported by typescript-eslint (the supported
versions are >=3.3.1 and <4.3.0). this causes a warning message to appear during linting, this issue should
likely/hopefully disappear in the future as typescript-eslint catches up
* The new "no-console" rule is not completely equivalent to what we had prior the migration, this is because TSLint's "no-console"
rule let you specify the methods you did not want to allow, whilst ESLint's "no-console" lets you specify the methods that you do
want to allow, so and in order not to have a very long list of methods in the ESLint rule it's been decided for the time being
to simply only allow the "log", "warn" and "error" methods
* 4 dependencies have been added as they have been considered necessary (see: https://github.com/angular/angular/pull/42820#discussion_r669978232)
extra:
* the migration has been performed by following: https://github.com/angular-eslint/angular-eslint#migrating-an-angular-cli-project-from-codelyzer-and-tslin
* more on typescript-eslint at: https://github.com/typescript-eslint/typescript-eslint
PR Close#42820
dario-piotrowicz