diff --git a/src/config.js b/src/config.js
new file mode 100644
index 0000000..664c6f9
--- /dev/null
+++ b/src/config.js
@@ -0,0 +1,15 @@
+const emoji = {
+  directory: '📁',
+  file: '📄',
+}
+const characters = {
+	border: '|',
+	contain: '├',
+	line: '─',
+	last: '└'
+}
+
+module.exports = {
+  emoji,
+  characters,
+}
\ No newline at end of file
diff --git a/src/generate.js b/src/generate.js
new file mode 100644
index 0000000..19d60f3
--- /dev/null
+++ b/src/generate.js
@@ -0,0 +1,28 @@
+const { emoji, characters } = require('./config')
+const { isNumber } = require('./utils')
+
+function generate(data, options = {}, deep = 0) {
+  if (isNumber(options.level) && deep >= options.level) {
+    return ''
+  }
+  let output = ''
+
+  data.forEach((item, index) => {
+    const borderPrefix = (characters.border + ' '.repeat(3)).repeat(deep)
+    let contentPrefix = index === data.length - 1 ? characters.last : characters.contain 
+    contentPrefix += characters.line.repeat(2)
+    const content = options.icon ? `${emoji[item.type]}${item.name}` : `${item.name}`
+
+    let currentLineStr = `${borderPrefix}${contentPrefix}${content}`
+    currentLineStr += '\n'
+    output += currentLineStr
+    
+    if (item.children) { // directory
+      output += generate(item.children, options, deep + 1)
+    }
+  })
+
+  return output
+}
+
+module.exports = generate
\ No newline at end of file
diff --git a/src/index.js b/src/index.js
index 1d38bea..f92cfa6 100644
--- a/src/index.js
+++ b/src/index.js
@@ -2,20 +2,11 @@
 
 const fs = require('fs')
 const { program } = require('commander')
-const { fileExistSync, isDirectory } = require('./utils')
+const { fileExistSync } = require('./utils')
+const toTree = require('./toTree')
+const generate = require('./generate')
 const package = require('../package.json')
 
-const map = {
-  directory: '📁',
-  file: '📄',
-}
-const characters = {
-	border: '|',
-	contain: '├',
-	line: '─',
-	last: '└'
-}
-
 program
   .name('treei')
   .version(package.version)
@@ -30,85 +21,34 @@ program
 
 const options = program.opts()
 
-let ignoreList = []
-if (options.ignore) {
-  ignoreList = options.ignore.replace(/\s*/g, '').split(/,|\|/)
-}
-
-// The difference between statSync and lstatSync
-// https://stackoverflow.com/questions/32478698/what-is-the-different-between-stat-fstat-and-lstat-functions-in-node-js
+const onExits = []
 
-function toTree(path, deep = 0) {
-  if (isDirectory(path)) {
-    let dir = fs.readdirSync(path)
-
-    if (ignoreList.length) {
-      dir = dir.filter(child => !ignoreList.includes(child))
-    }
-    if (options.onlyFolder) {
-      dir = dir.filter(child => isDirectory(`${path}/${child}`))
-    }
-    const children = dir.map(child => toTree(`${path}/${child}`, deep + 1))
-    const dirName = path.split('/').pop()
-
-    return {
-      type: 'directory',
-      name: dirName,
-      children,
-    }
-  } else {
-    return {
-      type: 'file',
-      name: path.split('/').pop(),
-    }
+function handleOptions() {
+  if (options.ignore) {
+    options.ignore = options.ignore.replace(/\s*/g, '').split(/,|\|/)
+  }
+  if (options.level && !Number.isNaN(parseInt(options.level))) {
+    options.level = parseInt(options.level)
+  }
+  if (options.output) {
+    onExits.push((result) => {
+      let outputString = result
+      if (fileExistSync(options.output)) { // appending mode
+        outputString = '\n' + result
+      }
+      fs.appendFile(options.output, outputString, (err) => {
+        if (err) throw err
+      })
+    })
   }
 }
 
-const root = toTree(options.directory)
-
-let level
-if (options.level && !Number.isNaN(parseInt(options.level))) {
-  level = parseInt(options.level)
-}
-
-function isReachedDepth(deep) {
-  return typeof level === 'number' && deep >= level
-}
-
-let output = ''
-
-function generateTreeStructure(data, deep = 0) {
-  if (isReachedDepth(deep)) return
+handleOptions()
 
-  data.forEach((item, index) => {
-    const borderPrefix = (characters.border + ' '.repeat(3)).repeat(deep)
-    let contentPrefix = index === data.length - 1 ? characters.last : characters.contain 
-    contentPrefix += characters.line.repeat(2)
-    const content = options.icon ? `${map[item.type]}${item.name}` : `${item.name}`
-    let currentLineStr = `${borderPrefix}${contentPrefix}${content}`
+const root = toTree(options.directory, options)
+const result = generate(root.children, options)
 
-    currentLineStr += '\n'
-    
-    if (item.children) { // directory
-      output += currentLineStr
-      generateTreeStructure(item.children, deep + 1)
-    } else { // file
-      output += currentLineStr
-    }
-  })
-}
-
-generateTreeStructure(root.children)
-
-if (options.output) {
-  let outputString = output
-  if (fileExistSync(options.output)) { // appending mode
-    outputString = '\n' + output
-  }
-  fs.appendFile(options.output, outputString, (err) => {
-    if (err) throw err
-  })
-}
+onExits.forEach(onExit => onExit(result))
 
 console.log(root.name)
-console.log(output)
\ No newline at end of file
+console.log(result)
\ No newline at end of file
diff --git a/src/toTree.js b/src/toTree.js
new file mode 100644
index 0000000..a279501
--- /dev/null
+++ b/src/toTree.js
@@ -0,0 +1,34 @@
+const fs = require('fs')
+const { isDirectory } = require('./utils')
+
+// The difference between statSync and lstatSync
+// https://stackoverflow.com/questions/32478698/what-is-the-different-between-stat-fstat-and-lstat-functions-in-node-js
+
+function toTree(path, options = {}) {
+  if (isDirectory(path)) {
+    let dir = fs.readdirSync(path)
+    const ignoreList = options.ignore || []
+
+    if (ignoreList.length) {
+      dir = dir.filter(child => !ignoreList.includes(child))
+    }
+    if (options.onlyFolder) {
+      dir = dir.filter(child => isDirectory(`${path}/${child}`))
+    }
+    const children = dir.map(child => toTree(`${path}/${child}`))
+    const dirName = path.split('/').pop()
+
+    return {
+      type: 'directory',
+      name: dirName,
+      children,
+    }
+  } else {
+    return {
+      type: 'file',
+      name: path.split('/').pop(),
+    }
+  }
+}
+
+module.exports = toTree
\ No newline at end of file
diff --git a/src/utils.js b/src/utils.js
index 82256f4..da28ccf 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -15,7 +15,12 @@ function isDirectory(path) {
   return stats.isDirectory()
 }
 
+function isNumber(value) {
+  return typeof value === 'number'
+}
+
 module.exports = {
   fileExistSync,
   isDirectory,
+  isNumber,
 }
\ No newline at end of file