Add resolvePeerDependenciesFrom option.

[rwjblue]

Pass this option if you know the base directory (the dir containing the package.json) that should be used as the starting point of peer dependency resolution.

For example, given the following dependencies:

• Package parent depends on child and sibling
• Package child has a peer dependency on sibling package
• Package child uses validate-peer-dependencies to confirm that sibling is provided

Most of the time in the Node ecosystem you can not actually know the path to parent (it could be hoisted / deduplicated to any number of possible locations), but in some (some what special) circumstances you can. For example, in the ember-cli addon ecosystem an addon is instantiated with access to the root path of the package that included it (parent in the example above).

The main benefit of specifying resolvePeerDependenciesFrom is that while locally developing child you might npm link/yarn link it into parent manually. In that case the default behavior (using the directory that contains child's package.json) is not correct! When linking (and not specifying resolvePeerDependenciesFrom) the invocation to validatePeerDependencies would always find the peer dependencies (even if the parent didn't have them installed) because the locally linked copy of child would have specified them in its devDependencies and therefore the peer dependency would be resolvable from child's on disk location.

Here is an example of what usage by an ember-cli addon would look like:

'use strict';

const validatePeerDependencies = require('validate-peer-dependencies');

module.exports = {
  // ...snip...
  init() {
    this._super.init.apply(this, arguments);

    validatePeerDependencies(__dirname, {
      resolvePeerDependenciesFrom: this.parent.root,
    });
  }
};

[rwjblue]

/cc @hjdivad

[hjdivad]

@rwjblue this looks great. As discussed OOB I would emphasize that failure to pass resolvePeerDependenciesFrom will in some cases result in incorrect behaviour (not just the linking case) but

1. you will never have false negatives (i.e. we will not throw incorrectly)
2. you can have false positives (i.e. we will sometimes fail to throw when we should)

Really just to highlight that if you know the parent dir you should absolutely pass it to catch all error cases, and if you don't we will still catch most error cases, but there are some known ones we cannot detect.