internal…

load-prefix.js
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			module.exports = loadPrefix

			var findPrefix = require('./find-prefix.js')
			var path = require('path')

			function loadPrefix (cb) {
			var cli = this.list[0]

			Object.defineProperty(this, 'prefix',
			{
			set: function (prefix) {
			var g = this.get('global')
			this[g ? 'globalPrefix' : 'localPrefix'] = prefix
			}.bind(this),
			get: function () {
			var g = this.get('global')
			return g ? this.globalPrefix : this.localPrefix
			}.bind(this),
			enumerable: true
			})

			Object.defineProperty(this, 'globalPrefix',
			{
			set: function (prefix) {
			this.set('prefix', prefix)
			}.bind(this),
			get: function () {
			return path.resolve(this.get('prefix'))
			}.bind(this),
			enumerable: true
			})

			var p
			Object.defineProperty(this, 'localPrefix',
			{ set: function (prefix) { p = prefix },
			get: function () { return p },
			enumerable: true })

			// try to guess at a good node_modules location.
			// If we are explicitly given a prefix on the cli, then
			// always use that. otherwise, infer local prefix from cwd.
			if (Object.prototype.hasOwnProperty.call(cli, 'prefix')) {
			p = path.resolve(cli.prefix)
			process.nextTick(cb)
			} else {
			findPrefix(process.cwd(), function (er, found) {
			p = found
			cb(er)
			})
			}
			}