diff --git a/lib/steps/changelog.js b/lib/steps/changelog.js
index fcf50da..ddcfb57 100644
--- a/lib/steps/changelog.js
+++ b/lib/steps/changelog.js
@@ -32,6 +32,40 @@
 
 'use strict';
 
+/**
+ * @typedef PR
+ * @property {number} pullId
+ * @property {{name: string, href: string}} author
+ * @property {string} href
+ * @property {string} title
+ * @property {string[]} shas
+ * @property {Commit[]} commits
+ */
+
+/**
+ * @typedef Commit
+ * @extends PR
+ * @property {string} type
+ * @property {string} header
+ * @property {string} subject
+ * @property {string} sha
+ * @property {{title: string, text:string}[]} notes
+ * @property {{prefix: string, issue: number, href: string}[]} references
+ */
+
+/**
+ * @typedef RepoInfo
+ * @property {string} htmlBase
+ * @property {string} username
+ * @property {string} repository
+ */
+
+/**
+ * @typedef NlmOptions
+ * @property {{skip?: boolean, set: {[key: string]: string}}} emoji
+ * @property {{verbose?: boolean}} changelog
+ */
+
 const Github = require('../github/client');
 
 const parseRepository = require('../github/parse-repository');
@@ -73,11 +107,16 @@ const emojiMaps = new Map([
   ],
 ]);
 
+/**
+ * @param {string} type
+ * @param {{nlmOptions?: NlmOptions}} options
+ * @return {string}
+ */
 function getTypeCategory(type, options) {
   const { nlmOptions = {} } = options;
   const { emoji: emojiOpts = {} } = nlmOptions;
 
-  let descr = '';
+  let descr;
   const headlineLevel = '####';
 
   switch (type) {
@@ -129,6 +168,10 @@ function getTypeCategory(type, options) {
   );
 }
 
+/**
+ * @param {*[]} data
+ * @return {*[]}
+ */
 function sortByType(data) {
   if (!data.length) {
     return [];
@@ -159,6 +202,10 @@ function sortByType(data) {
   return sorted.concat(data);
 }
 
+/**
+ * @param {string} title
+ * @return {string}
+ */
 function getType(title) {
   const match = title.match(/^(\w+):/);
   let type = match && match[1];
@@ -168,13 +215,22 @@ function getType(title) {
   return type || 'internal';
 }
 
+/**
+ * @param {String} str
+ * @return {RegExpMatchArray|null}
+ */
 function findDependency(str) {
   return str.match(DEP_REGEXP);
 }
 
-function addPullRequestCommits(pkg, commits, pr) {
-  const github = Github.forRepository(pkg.repository);
-
+/**
+ *
+ * @param {Object} github
+ * @param {Commit[]} commits
+ * @param {PR} pr
+ * @return {Promise<void>}
+ */
+function addPullRequestCommits(github, commits, pr) {
   pr.commits = null;
   pr.shas = null;
 
@@ -197,10 +253,19 @@ function addPullRequestCommits(pkg, commits, pr) {
     });
 }
 
+/**
+ * @param {*[]} arrs
+ * @return {*[]}
+ */
 function flatten(arrs) {
   return [].concat.apply([], arrs);
 }
 
+/**
+ * @param {PR[]} prs
+ * @param {Commit[]} commits
+ * @return {*[]|Commit[]}
+ */
 function removePRCommits(prs, commits) {
   const prShas = flatten(prs.map(pr => pr.shas));
   return commits.filter(
@@ -208,8 +273,12 @@ function removePRCommits(prs, commits) {
   );
 }
 
+/**
+ * @param {Commit} commit
+ * @return {*[]|{commit: Commit, text: string}[]}
+ */
 function extractBreakingChanges(commit) {
-  if (!commit.notes || !commit.notes.length) {
+  if (!(commit.notes && commit.notes.length)) {
     return [];
   }
 
@@ -223,6 +292,9 @@ function extractBreakingChanges(commit) {
     });
 }
 
+/**
+ * @param {PR[]} prs
+ */
 function removeInvalidPRs(prs) {
   // Warning: We're doing something evil here and mutate the input array.
   const filtered = prs.filter(pr => {
@@ -232,18 +304,27 @@ function removeInvalidPRs(prs) {
   Object.assign(prs, filtered);
 }
 
+/**
+ * @param {{prefix: string, issue: number, href: string}[]} refs
+ * @return {string}
+ */
 function formatReferences(refs) {
   if (!refs || refs.length === 0) {
     return '';
   }
 
-  refs = refs.map(ref => {
+  const references = refs.map(ref => {
     return `[${ref.prefix}${ref.issue}](${ref.href})`;
   });
 
-  return ` - see: ${refs.join(', ')}`;
+  return ` - see: ${references.join(', ')}`;
 }
 
+/**
+ * @param {Commit} commit
+ * @param {RepoInfo} repoInfo
+ * @return {string}
+ */
 function getCommitLink(commit, repoInfo) {
   const abbr = commit.sha.substr(0, 7);
   const href = [
@@ -256,10 +337,21 @@ function getCommitLink(commit, repoInfo) {
   return `[\`${abbr}\`](${href})`;
 }
 
+/**
+ * @param {{text: string, commit: Commit}} change
+ * @param {RepoInfo} repoInfo
+ * @return {string}
+ */
 function formatBreakingChange(change, repoInfo) {
   return `${change.text}\n\n*See: ${getCommitLink(change.commit, repoInfo)}*`;
 }
 
+/**
+ * @param {Commit[]} commits
+ * @param {{nlmOptions?: NlmOptions}} options}} options
+ * @param {RepoInfo} repoInfo
+ * @return {*[]|(string)[][]}
+ */
 function getBreakingChanges(commits, options, repoInfo) {
   const breaking = flatten(commits.map(extractBreakingChanges));
   if (!breaking.length) {
@@ -273,6 +365,11 @@ function getBreakingChanges(commits, options, repoInfo) {
   return [['breaking', breakingChanges]];
 }
 
+/**
+ * @param {Commit} commit
+ * @param {RepoInfo} repoInfo
+ * @return {string}
+ */
 function formatCommit(commit, repoInfo) {
   let subject;
 
@@ -287,6 +384,12 @@ function formatCommit(commit, repoInfo) {
   )}`;
 }
 
+/**
+ * @param {PR} pr
+ * @param {{nlmOptions?: NlmOptions}} options
+ * @param {RepoInfo} repoInfo
+ * @return {string}
+ */
 function formatPR(pr, options, repoInfo) {
   const { nlmOptions = {} } = options;
   const changes =
@@ -301,6 +404,13 @@ function formatPR(pr, options, repoInfo) {
   return [titleLine].concat(changes).join('\n');
 }
 
+/**
+ * @param {PR[]} rawPRs
+ * @param {Commit[]} rawCommits
+ * @param {{nlmOptions?: NlmOptions}} options
+ * @param {RepoInfo} repoInfo
+ * @return {*[]|[string, string][]}
+ */
 function format(rawPRs, rawCommits, options, repoInfo) {
   const orphansCommits = rawCommits.map(commit => [
     getType(`${commit.type}: ${commit.subject}`),
@@ -313,6 +423,11 @@ function format(rawPRs, rawCommits, options, repoInfo) {
     .map(([type, line]) => [type, `* ${line}`]);
 }
 
+/**
+ * @param {[string, string][]|*[]} data
+ * @param {{nlmOptions?: NlmOptions}} options
+ * @return {string}
+ */
 function mergeChangelog(data, options) {
   const changelog = new Map();
   const sorted = sortByType(data);
@@ -330,15 +445,23 @@ function mergeChangelog(data, options) {
     .join('\n\n');
 }
 
+/**
+ * @param {*} cwd
+ * @param {{repository: string|{url: string}}} pkg
+ * @param {{commits: Commit[], nlmOptions?: NlmOptions, changelog?: string}} options
+ * @return {Promise<string>}
+ */
 async function generateChangeLog(cwd, pkg, options) {
   const { commits } = options;
   const repoInfo = parseRepository(pkg.repository);
+  const github = Github.forRepository(pkg.repository);
 
+  /** @type {PR[]} */
   const prs = commits.filter(c => c.type === 'pr');
 
   // step1: fetch PR commits data from GH & match with commits
   for (const pr of prs) {
-    await addPullRequestCommits(pkg, commits, pr);
+    await addPullRequestCommits(github, commits, pr);
   }
 
   // step2: remove PRs without commits