internal…

writable.js
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			'use strict'
			var path = require('path')
			var fs = require('fs')
			var inflight = require('inflight')
			var accessError = require('./access-error.js')
			var andIgnoreErrors = require('./and-ignore-errors.js')
			var isFsAccessAvailable = require('./is-fs-access-available.js')

			if (isFsAccessAvailable) {
			module.exports = fsAccessImplementation
			} else {
			module.exports = fsOpenImplementation
			}

			// exposed only for testing purposes
			module.exports.fsAccessImplementation = fsAccessImplementation
			module.exports.fsOpenImplementation = fsOpenImplementation

			function fsAccessImplementation (dir, done) {
			done = inflight('writable:' + dir, done)
			if (!done) return
			fs.access(dir, fs.W_OK, done)
			}

			function fsOpenImplementation (dir, done) {
			done = inflight('writable:' + dir, done)
			if (!done) return
			var tmp = path.join(dir, '.npm.check.permissions')
			fs.open(tmp, 'w', function (er, fd) {
			if (er) return done(accessError(dir, er))
			fs.close(fd, function () {
			fs.unlink(tmp, andIgnoreErrors(done))
			})
			})
			}