diff --git a/packages/zapshot/src/index.js b/packages/zapshot/src/index.ts
similarity index 94%
rename from packages/zapshot/src/index.js
rename to packages/zapshot/src/index.ts
index 32c8835..49cd5c2 100644
--- a/packages/zapshot/src/index.js
+++ b/packages/zapshot/src/index.ts
@@ -1,5 +1,3 @@
-// @flow
-
 export { Metric } from "./metric";
 export { Metrics } from "./metrics";
 export { measure } from "./measure";
diff --git a/packages/zapshot/src/measure.js b/packages/zapshot/src/measure.ts
similarity index 70%
rename from packages/zapshot/src/measure.js
rename to packages/zapshot/src/measure.ts
index fc461ee..ea8e22b 100644
--- a/packages/zapshot/src/measure.js
+++ b/packages/zapshot/src/measure.ts
@@ -1,31 +1,28 @@
-// @flow
-import path from "path";
-// $FlowFixMe(perf_hooks-exists)
-import { performance, PerformanceObserver } from "perf_hooks";
-import { Progress, type Logger } from "./progress";
-import { Metric, type Call } from "./metric";
+import path from "path"; // $FlowFixMe(perf_hooks-exists)
 
-// FIXME: Move to lib
-type Option = {|
-  +step: number,
-  +progressLogger: Logger
-|};
+import { performance, PerformanceObserver } from "perf_hooks";
+import { Progress, Logger } from "./progress";
+import { Metric, Call } from "./metric"; // FIXME: Move to lib
 
+type Option = {
+  readonly step: number;
+  readonly progressLogger: Logger;
+};
 type SetupFunction = (
   timerify: typeof performance.timerify
 ) => any | Promise<any>;
 type TestFunction = (arg: any) => void | Promise<void>;
-type Queue = Array<{|
-  name: string,
-  setup: SetupFunction,
-  fn: TestFunction
-|}>;
-
+type Queue = Array<{
+  name: string;
+  setup: SetupFunction;
+  fn: TestFunction;
+}>;
 export const measure = (
   target: string,
   option: Option
 ): Promise<Array<Metric>> => {
   const queue: Queue = [];
+
   global.bench = (name: string, setup: SetupFunction, fn: TestFunction) => {
     queue.push({
       name,
@@ -36,21 +33,29 @@ export const measure = (
 
   const run = async (queue: Queue): Promise<Array<Metric>> => {
     const metrics: Array<Metric> = [];
+
     for (let { name, fn, setup } of queue) {
       const calls: Array<Call> = [];
-      const markedCalls: { [string]: Array<Call> } = {};
-
+      const markedCalls: {
+        [x: string]: Array<Call>;
+      } = {};
       const observer = new PerformanceObserver(list => {
         list.getEntries().forEach(entry => {
           if (entry.entryType === "measure") {
-            calls.push({ duration: entry.duration });
+            calls.push({
+              duration: entry.duration
+            });
           } else {
             markedCalls[entry.name] = markedCalls[entry.name] || [];
-            markedCalls[entry.name].push({ duration: entry.duration });
+            markedCalls[entry.name].push({
+              duration: entry.duration
+            });
           }
         });
       });
-      observer.observe({ entryTypes: ["function", "measure"] });
+      observer.observe({
+        entryTypes: ["function", "measure"]
+      });
       const arg = setup(performance.timerify);
 
       for (let i = 0; i < option.step; i++) {
@@ -63,7 +68,6 @@ export const measure = (
       }
 
       observer.disconnect();
-
       metrics.push(
         new Metric({
           name,
@@ -74,15 +78,13 @@ export const measure = (
     }
 
     return metrics;
-  };
+  }; // $FlowFixMe(allow-dynamic-require)
 
-  // $FlowFixMe(allow-dynamic-require)
   require(target);
 
   const progress = new Progress(option.progressLogger);
   progress.setTotal(queue.length * option.step);
   progress.setTitle(path.relative(process.cwd(), target));
   progress.tick(0);
-
   return run(queue);
 };
diff --git a/packages/zapshot/src/metric.js b/packages/zapshot/src/metric.ts
similarity index 75%
rename from packages/zapshot/src/metric.js
rename to packages/zapshot/src/metric.ts
index 16e06ad..bd3d55d 100644
--- a/packages/zapshot/src/metric.js
+++ b/packages/zapshot/src/metric.ts
@@ -1,22 +1,22 @@
-// @flow
 import sumBy from "lodash/sumBy";
 import meanBy from "lodash/meanBy";
 import sortBy from "lodash/sortBy";
-
-export type Call = {|
-  +duration: number
-|};
-
-export type MetricProps = {|
-  +name: string,
-  +calls: Array<Call>,
-  +markedCalls: { [name: string]: Array<Call> }
-|};
-
+export type Call = {
+  readonly duration: number;
+};
+export type MetricProps = {
+  readonly name: string;
+  readonly calls: Array<Call>;
+  readonly markedCalls: {
+    [name: string]: Array<Call>;
+  };
+};
 export class Metric {
-  +name: string;
-  +calls: Array<Call>;
-  +markedCalls: { [marker: string]: Array<Call> };
+  name: string;
+  calls: Array<Call>;
+  markedCalls: {
+    [marker: string]: Array<Call>;
+  };
 
   constructor({ name, calls, markedCalls }: MetricProps) {
     this.name = name;
@@ -39,34 +39,41 @@ export class Metric {
     );
   }
 
-  getMarked(mark: string): ?Array<Call> {
+  getMarked(mark: string): Array<Call> | undefined | null {
     if (!this.markedCalls[mark]) {
       return null;
     }
+
     return this.markedCalls[mark];
   }
 
   getMarkedCount(mark: string): number {
     const marked = this.getMarked(mark);
+
     if (!marked) {
       return -1;
     }
+
     return marked.length;
   }
 
   getMarkedTotal(mark: string): number {
     const marked = this.getMarked(mark);
+
     if (!marked) {
       return -1;
     }
+
     return sumBy(marked, "duration");
   }
 
   getMarkedMean(mark: string): number {
     const marked = this.getMarked(mark);
+
     if (!marked) {
       return -1;
     }
+
     return meanBy(marked, "duration");
   }
 }
diff --git a/packages/zapshot/src/metrics.js b/packages/zapshot/src/metrics.ts
similarity index 64%
rename from packages/zapshot/src/metrics.js
rename to packages/zapshot/src/metrics.ts
index db28afb..3b64fa5 100644
--- a/packages/zapshot/src/metrics.js
+++ b/packages/zapshot/src/metrics.ts
@@ -1,20 +1,30 @@
-// @flow
-import { Metric, type MetricProps } from "./metric";
-
+import { Metric, MetricProps } from "./metric";
 export class Metrics {
-  metrics: { [name: string]: Metric };
+  metrics: {
+    [name: string]: Metric;
+  };
 
   static fromArray(metrics: Array<Metric>) {
     const metricsMap = {};
+
     for (let metric of metrics) {
       metricsMap[metric.name] = JSON.parse(JSON.stringify(metric));
     }
 
-    return new Metrics({ metrics: metricsMap });
+    return new Metrics({
+      metrics: metricsMap
+    });
   }
 
-  constructor({ metrics }: { metrics: { [name: string]: MetricProps } }) {
+  constructor({
+    metrics
+  }: {
+    metrics: {
+      [name: string]: MetricProps;
+    };
+  }) {
     this.metrics = {};
+
     for (let name in metrics) {
       this.metrics[name] = new Metric(metrics[name]);
     }
@@ -29,7 +39,7 @@ export class Metrics {
     return Object.values(this.metrics);
   }
 
-  getByName(name: string): ?Metric {
+  getByName(name: string): Metric | undefined | null {
     return this.metrics[name] || null;
   }
 }
diff --git a/packages/zapshot/src/progress.js b/packages/zapshot/src/progress.ts
similarity index 98%
rename from packages/zapshot/src/progress.js
rename to packages/zapshot/src/progress.ts
index bf86b7a..dddf9f7 100644
--- a/packages/zapshot/src/progress.js
+++ b/packages/zapshot/src/progress.ts
@@ -1,10 +1,7 @@
-// @flow
-
 export interface Logger {
   log: (title: string, tick: number, total: number, eta: number) => void;
   tearDown: (spent: number) => void;
 }
-
 export class Progress {
   logger: Logger;
   current: number;
diff --git a/packages/zapshot/src/report.js b/packages/zapshot/src/report.js
deleted file mode 100644
index c79b42a..0000000
--- a/packages/zapshot/src/report.js
+++ /dev/null
@@ -1,83 +0,0 @@
-// @flow
-import sortBy from "lodash/sortBy";
-import { type Metric } from "./metric";
-import { type Metrics } from "./metrics";
-
-export type Case = {
-  total: number,
-  mean: number,
-  diff: ?number,
-  diffPercentage: ?number,
-  marks: Array<{
-    name: string,
-    total: number,
-    times: number,
-    diff: ?number,
-    diffPercentage: ?number
-  }>
-};
-
-export class Report {
-  cases: { [caseName: string]: Case };
-
-  static fromMetricses(metrics: Metrics, beforeMetrics: ?Metrics): Report {
-    const cases: { [caseName: string]: Case } = metrics
-      .getItems()
-      .reduce((acc, metric) => {
-        const before: ?Metric = beforeMetrics
-          ? beforeMetrics.getByName(metric.name)
-          : null;
-        const total = metric.getTotal();
-        const totalDiff = diff(metric, before, m => m.getTotal());
-        const marks = metric.getMarks().reduce((marks, mark) => {
-          const subTotal = metric.getMarkedTotal(mark);
-          const subTotalDiff = diff(metric, before, m =>
-            m.getMarkedTotal(mark)
-          );
-
-          return marks.concat([
-            {
-              name: mark,
-              total: subTotal,
-              times: metric.getMarkedCount(mark),
-              diff: subTotalDiff,
-              diffPercentage: subTotalDiff
-                ? subTotalDiff / subTotal * 100
-                : null
-            }
-          ]);
-        }, []);
-
-        return {
-          ...acc,
-          [metric.name]: {
-            total: total,
-            mean: metric.getMean(),
-            diff: totalDiff,
-            diffPercentage: totalDiff ? totalDiff / total * 100 : null,
-            marks: sortBy(marks, mark => -mark.total)
-          }
-        };
-      }, {});
-
-    return new Report(cases);
-  }
-
-  constructor(cases: { [caseName: string]: Case }) {
-    this.cases = cases;
-  }
-}
-
-export const diff = (
-  a: Metric,
-  b: ?Metric,
-  accessor: Metric => number
-): ?number => {
-  if (!b) {
-    return null;
-  }
-
-  const aValue = accessor(a);
-  const bValue = accessor(b);
-  return aValue - bValue;
-};
diff --git a/packages/zapshot/src/report.ts b/packages/zapshot/src/report.ts
new file mode 100644
index 0000000..2e2d7eb
--- /dev/null
+++ b/packages/zapshot/src/report.ts
@@ -0,0 +1,77 @@
+import sortBy from "lodash/sortBy";
+import { Metric } from "./metric";
+import { Metrics } from "./metrics";
+export type Case = {
+  total: number;
+  mean: number;
+  diff: number | undefined | null;
+  diffPercentage: number | undefined | null;
+  marks: Array<{
+    name: string;
+    total: number;
+    times: number;
+    diff: number | undefined | null;
+    diffPercentage: number | undefined | null;
+  }>;
+};
+export class Report {
+  cases: {
+    [caseName: string]: Case;
+  };
+
+  static fromMetricses(
+    metrics: Metrics,
+    beforeMetrics: Metrics | undefined | null
+  ): Report {
+    const cases: {
+      [caseName: string]: Case;
+    } = metrics.getItems().reduce((acc, metric) => {
+      const before: Metric | undefined | null = beforeMetrics
+        ? beforeMetrics.getByName(metric.name)
+        : null;
+      const total = metric.getTotal();
+      const totalDiff = diff(metric, before, m => m.getTotal());
+      const marks = metric.getMarks().reduce((marks, mark) => {
+        const subTotal = metric.getMarkedTotal(mark);
+        const subTotalDiff = diff(metric, before, m => m.getMarkedTotal(mark));
+        return marks.concat([
+          {
+            name: mark,
+            total: subTotal,
+            times: metric.getMarkedCount(mark),
+            diff: subTotalDiff,
+            diffPercentage: subTotalDiff ? subTotalDiff / subTotal * 100 : null
+          }
+        ]);
+      }, []);
+      return {
+        ...acc,
+        [metric.name]: {
+          total: total,
+          mean: metric.getMean(),
+          diff: totalDiff,
+          diffPercentage: totalDiff ? totalDiff / total * 100 : null,
+          marks: sortBy(marks, mark => -mark.total)
+        }
+      };
+    }, {});
+    return new Report(cases);
+  }
+
+  constructor(cases: { [caseName: string]: Case }) {
+    this.cases = cases;
+  }
+}
+export const diff = (
+  a: Metric,
+  b: Metric | undefined | null,
+  accessor: (a: Metric) => number
+): number | undefined | null => {
+  if (!b) {
+    return null;
+  }
+
+  const aValue = accessor(a);
+  const bValue = accessor(b);
+  return aValue - bValue;
+};
diff --git a/packages/zapshot/src/reporter.js b/packages/zapshot/src/reporter.ts
similarity index 88%
rename from packages/zapshot/src/reporter.js
rename to packages/zapshot/src/reporter.ts
index acf658b..8da4a01 100644
--- a/packages/zapshot/src/reporter.js
+++ b/packages/zapshot/src/reporter.ts
@@ -1,15 +1,12 @@
-// @flow
 /* eslint-disable no-console */
 import maxBy from "lodash/maxBy";
 import chalk from "chalk";
-import { type Report, type Case } from "./report";
-
-export type ReportOptions = {|
-  +precision?: number,
-  +quiet: boolean,
-  +threshold: number
-|};
-
+import { Report, Case } from "./report";
+export type ReportOptions = {
+  readonly precision?: number;
+  readonly quiet: boolean;
+  readonly threshold: number;
+};
 export const report = (
   report: Report,
   { precision = 2, quiet, threshold }: ReportOptions
@@ -28,7 +25,6 @@ export const report = (
       );
     } else {
       const legend = `${indent(4)}Total`;
-
       console.log(`\n${indent(2)}${caseName}`);
       console.log(
         `${legend}: ${c.total.toFixed(precision)}ms (mean: ${c.mean.toFixed(
@@ -38,11 +34,11 @@ export const report = (
           threshold
         })}`
       );
-
       const longestMark = maxBy(c.marks, mark => mark.name.length);
       const longestMarkWidth = longestMark ? longestMark.name.length : 0;
       const totalMaxWidth =
         c.marks.length > 0 ? c.marks[0].total.toFixed(precision).length : 0;
+
       for (let mark of c.marks) {
         const percentage = (mark.total / c.total * 100).toFixed(0).padStart(5);
         console.log(
@@ -61,14 +57,18 @@ export const report = (
 
   return Promise.resolve();
 };
-
 export const indent = (num: number): string => " ".repeat(num);
-
 export const humanizeDiff = (
-  diff: ?number,
-  diffPercentage: ?number,
+  diff: number | undefined | null,
+  diffPercentage: number | undefined | null,
   unit?: string,
-  { precision, threshold }: { precision: number, threshold: number }
+  {
+    precision,
+    threshold
+  }: {
+    precision: number;
+    threshold: number;
+  }
 ) => {
   const unitStr = unit ? ` ${unit}` : "";
 
@@ -81,6 +81,7 @@ export const humanizeDiff = (
   const sign = diff > 0 ? "+" : "";
   let humanizedDiff = sign + diff.toFixed(precision);
   let humanizedPercentage = sign + diffPercentage.toFixed(precision);
+
   if (diffPercentage >= threshold) {
     humanizedDiff = chalk.red(humanizedDiff);
     humanizedPercentage = chalk.red(humanizedPercentage);