How to use the npm/lib/cache.read function in npm

// now we know it's not a URL or file,
    // so handle it like a tag, version, or range.
    pkg = pkg.split("@");
    var name = pkg[0],
        defTag = npm.config.get("tag"),
        ver = pkg.slice(1).join("@").trim() || defTag,
        range = semver.validRange(ver),
        exact = semver.valid(ver),
        tag = !exact && !range && range !== "" && ver,
        pkg = pkg.join("@");

    // at this point, assume that it has to be debianized
    if (exact) {
        log.verbose("exact", pkg);
        // just pull the data out of the cache to ensure it's there
        return cache.read(name, ver, finisher(pkg, reg, pkglist, cb));
    }

    getData(name, function (er, data) {
        if (er) return cb(er);
        log.silly(data, pkg);
        if (tag) {
            log.verbose(tag, pkg + " tag");
            var tags = data["dist-tags"];
            if (!tags[tag]) return log.er(cb, "Tag not found: " + data.name + "@" + tag)(er);
            _source(data.name + "@" + tags[tag], reg, pkglist, cb);
        } else {
            log.verbose(tag, pkg + " range");
            // prefer the default tag version.
            var defTag = npm.config.get("tag"), satis;
            defTag = defTag && data["dist-tags"] && data["dist-tags"][defTag];
            if (semver.satisfies(defTag, range)) satis = defTag;

unpack(pkg, ver, debianPackageDir, function(err) {
                    if (err) throw err;

                    // read package data
                    cache.read(pkg, ver, function(err, packageData) {
                        if (err) throw err;

                        ctx.packageData = packageData;
                        ctx.shortdesc = packageData.description || '';
                        ctx.longdesc = 'This is a debianized npm package';

                        var deps = [],
                            nodeVer = (packageData && packageData.engines && packageData.engines.node) ? semver.clean(packageData.engines.node) : null;
                        nodeVer = (nodeVer && nodeVer != '*') ? '(' + packageData.engines.node + ')' : '(>= ' + semver.clean(process.version) + ')';
                        deps = ['nodejs ' + nodeVer];

                        if (packageData.dependencies) {
                            for (var dep in packageData.dependencies) {
                                deps.push(ctx.debianNamePrefix + dep + ' (' + packageData.dependencies[dep] + ')');
                            }
                            ctx.depends = deps.join(', ');

, exact = semver.valid(ver)
    , tag = !exact && !range && range !== "" && ver
  log.verbose([pkg, mustHave], "must install?")
  pkg = pkg.join("@")
  seen[name+"@"+ver] = true

  // if there is a satisfying version already, then simply move on.
  if (!tag && findSatisfying(pkg, name, range, mustHave, reg)) {
    return cb()
  }

  // at this point, assume that it has to be installed.
  if (exact) {
    log.verbose("exact", pkg)
    // just pull the data out of the cache to ensure it's there
    return cache.read(name, ver, finisher(pkg, reg, pkglist, cb))
  }

  getData(name, function (er, data) {
    if (er) return cb(er)
    log.silly(data, pkg)
    if (tag) {
      log.verbose(tag, pkg+" tag")
      var tags = data["dist-tags"]
      if (!tags[tag]) return log.er(cb, "Tag not found: "+data.name+"@"+tag)(er)
      install_(data.name+"@"+tags[tag], reg, seen, mustHave, pkglist, cb)
    } else {
      log.verbose(tag, pkg+" range")
      // prefer the default tag version.
      var defTag = npm.config.get("tag")
        , satis
      defTag = defTag && data["dist-tags"] && data["dist-tags"][defTag]

function unpack (pkg, ver, unpackTarget, cb) {
    cache.read(pkg, ver, function (er, data) {
        if (er) return log.er(cb, "Could not read data for "+pkg+"@"+ver)(er);
        unpackTar(path.join(npm.cache, pkg, ver, "package.tgz"), unpackTarget, cb);
    })
}