diff --git a/index.mjs b/index.mjs
index d97f47c..ce80540 100644
--- a/index.mjs
+++ b/index.mjs
@@ -48,7 +48,7 @@ const getLocal = mem => io => async ([root, file]) => {
   const write = buffer => io.append(tempFile, buffer)
   const error = await get({ read, write })(root)
   if (error !== null) {
-    console.error(error)
+    io.console.error(error)
     return -1
   }
   await io.rename(tempFile, file)
@@ -65,7 +65,7 @@ const getRemote = mem => host => io => async ([root, file]) => {
   const write = buffer => io.append(tempFile, buffer)
   const error = await get({ read, write })(root)
   if (error !== null) {
-    console.error(error)
+    io.console.error(error)
     return -1
   }
   await io.rename(tempFile, file)
diff --git a/io/io.mjs b/io/io.mjs
index 6efdc5c..77678b3 100644
--- a/io/io.mjs
+++ b/io/io.mjs
@@ -1,13 +1,23 @@
+/** @typedef {(log: string) => void} Logger */
+
+/**
+ * @typedef {{
+ *  readonly log: Logger,
+ *  readonly error: Logger
+ * }} Console
+ */
+
 /**
  * @typedef {{
-* readonly read: (path: string) => Promise<Uint8Array>,
-* readonly append: (path: string, buffer: Uint8Array) => Promise<void>,
-* readonly write: (path: string, buffer: Uint8Array) => Promise<void>,
-* readonly rename: (oldPath: string, newPath: string) => Promise<void>
-* readonly fetch: (url: string) => Promise<Response>
-* readonly document: Document | undefined
-* }} IO
-*/
+ *  readonly read: (path: string) => Promise<Uint8Array>,
+ *  readonly append: (path: string, buffer: Uint8Array) => Promise<void>,
+ *  readonly write: (path: string, buffer: Uint8Array) => Promise<void>,
+ *  readonly rename: (oldPath: string, newPath: string) => Promise<void>
+ *  readonly fetch: (url: string) => Promise<Response>
+ *  readonly console: Console
+ *  readonly document: Document | undefined
+ * }} IO
+ */
 
 export default {
 }
\ No newline at end of file
diff --git a/io/node.mjs b/io/node.mjs
index 6c6ef89..961bdec 100644
--- a/io/node.mjs
+++ b/io/node.mjs
@@ -8,6 +8,7 @@ const node = {
   append: fsPromises.appendFile,
   write: fsPromises.writeFile,
   rename: fsPromises.rename,
+  console: {log: console.log, error: console.error},
   fetch,
   document: undefined
 }
@@ -18,6 +19,7 @@ const nodeSync = {
   append: async(path, buffer) => fs.appendFileSync(path, buffer),
   write: async(path, buffer) => fs.writeFileSync(path, buffer),
   rename: async(oldPath, newPath) => fs.renameSync(oldPath, newPath),
+  console: {log: console.log, error: console.error},
   fetch,
   document: undefined
 }
diff --git a/io/virtual.mjs b/io/virtual.mjs
index adaa467..71a79e7 100644
--- a/io/virtual.mjs
+++ b/io/virtual.mjs
@@ -6,6 +6,13 @@ const notImplemented = () => { throw 'not implemented' }
  * @typedef {{[index: string]: Uint8Array}} FileSystem
  */
 
+/**
+ * @typedef {{
+ * readonly fs: FileSystem,
+ * readonly console: { log: string, error: string }
+ * }} MemIo
+ */
+
 /** @type {(fs: FileSystem) => (path: string) => Promise<Uint8Array>} */
 const read = fs => async (path) => {
   const buffer = fs[path]
@@ -39,18 +46,29 @@ const rename = fs => async (oldPath, newPath) => {
   fs[newPath] = buffer
 }
 
-/** @type {(fs: FileSystem) => IO} */
-const virtual = fs => {
+/** @type {(memIo: MemIo) => IO} */
+const virtual = ({ fs, console }) => {
   return {
     read: read(fs),
     append: append(fs),
     write: write(fs),
     rename: rename(fs),
     fetch: notImplemented,
+    console: { log: text => console.log += `${text}\n`, error: text => console.error += `${text}\n` },
     document: undefined
   }
 }
 
+/** @type {() => MemIo} */
+const createMemIo = () => ({
+  fs: {},
+  console: {
+    log: '',
+    error: ''
+  }
+})
+
 export default {
-  virtual
+  virtual,
+  createMemIo
 }
\ No newline at end of file
diff --git a/io/web.mjs b/io/web.mjs
index af4512e..e5c2bb1 100644
--- a/io/web.mjs
+++ b/io/web.mjs
@@ -1,15 +1,19 @@
 /** @typedef {import('./io.mjs').IO} IO */
+/** @typedef {import('./io.mjs').Console} Console */
 
 const notImplemented = () => { throw 'not implemented' }
 
-/** @type {IO} */
-const web = {
-  read: notImplemented,
-  append: notImplemented,
-  write: notImplemented,
-  rename: notImplemented,
-  fetch,
-  document
+/** @type {(createConsole: (d: Document) => Console) => IO} */
+const web = createConsole => {
+  return {
+    read: notImplemented,
+    append: notImplemented,
+    write: notImplemented,
+    rename: notImplemented,
+    console: createConsole(document),
+    fetch,
+    document
+  }
 }
 
 export default {
diff --git a/test.mjs b/test.mjs
index 3f7d640..23b29be 100644
--- a/test.mjs
+++ b/test.mjs
@@ -8,10 +8,11 @@ import ioNode from './io/node.mjs'
 import fs from 'node:fs'
 import fsPromises from 'node:fs/promises'
 import ioVirtual from './io/virtual.mjs'
+import assert from 'node:assert'
 /** @typedef {import('./cdt/sub-tree.mjs').State} StateSubTree */
 /** @typedef {import('./cdt/main-tree.mjs').State} StateTree */
 /** @typedef {import('./io/io.mjs').IO} IO */
-/** @typedef {import('./io/virtual.mjs').FileSystem} FileSystem */
+/** @typedef {import('./io/virtual.mjs').MemIo} MemIo */
 /** @typedef {import('./index.mjs').Cache} Cache */
 const { toBase32Hash, getParityBit } = base32
 const { compress } = sha224
@@ -20,7 +21,7 @@ const { highestOne256, height, push: pushSubTree } = subTree
 const { push: pushTree, end: endTree } = mainTree
 const { getLocal, getRemote } = index
 const { node, nodeSync } = ioNode
-const { virtual } = ioVirtual
+const { virtual, createMemIo } = ioVirtual
 
 console.log(`test start`)
 
@@ -206,9 +207,8 @@ console.log(`test start`)
 }
 
 const virtualFsTest = async () => {
-  /** @type {FileSystem} */
-  const fs = {}
-  const io = virtual(fs)
+  const memIo = createMemIo()
+  const io = virtual(memIo)
   await io.write('test', new Uint8Array([0, 1, 2]))
   let buffer = await io.read('test')
   if (buffer.toString() !== '0,1,2') { throw buffer }
diff --git a/web-test.mjs b/web-test.mjs
index 7f39d9f..e8b1840 100644
--- a/web-test.mjs
+++ b/web-test.mjs
@@ -6,7 +6,17 @@ const { get } = forest
 const { web } = ioWeb
 const { fetchRead } = index
 
-const d = web.document
+const webIo = web(d => ({
+  log: text => {
+    // @ts-ignore
+    d.getElementById('log').innerText += `${text}\n`
+  },
+  error: text => {
+    // @ts-ignore
+    d.getElementById('log').innerText += `${text}\n`
+  }
+}))
+const d = webIo.document
 // @ts-ignore
 d.getElementById('download').addEventListener('click', () => {
   reset()
@@ -15,23 +25,20 @@ d.getElementById('download').addEventListener('click', () => {
   // @ts-ignore
   const host = d.getElementById('input-host').value
   let buffer = new Uint8Array()
-  const fRead = fetchRead(host)(web)
+  const fRead = fetchRead(host)(webIo)
   /** @type {(forestNodeId: ForestNodeId) => Promise<Uint8Array>} */
   const read = forestNodeId => {
-    // @ts-ignore
-    d.getElementById('log').innerText += `read from ${forestNodeId}\n`
+    webIo.console.log(`read from ${forestNodeId}`)
     return fRead(forestNodeId)
   }
   /** @type {(b: Uint8Array) => Promise<void>} */
   const write = async (b) => {
-    // @ts-ignore
-    d.getElementById('log').innerText += `write ${b.length}\n`
-      buffer = new Uint8Array([...buffer, ...b])
+    webIo.console.log(`write ${b.length}`)
+    buffer = new Uint8Array([...buffer, ...b])
   }
   get({ read, write })(hash).then(exitCode => {
     if (exitCode !== null) {
-      // @ts-ignore
-      d.getElementById('log').innerText += `error exit code = ${exitCode}\n`
+      webIo.console.error(`error exit code = ${exitCode}`)
       return
     }
 
@@ -48,7 +55,7 @@ d.getElementById('download').addEventListener('click', () => {
     // @ts-ignore
     d.getElementById('output-text').style.display = 'block'
     // @ts-ignore
-    d.getElementById('output-text').innerText =  new TextDecoder().decode(buffer)
+    d.getElementById('output-text').innerText = new TextDecoder().decode(buffer)
   })
 });