File.writeAsString

Learn to write files with File.writeAsString.

Writing a string to a file

File.writeAsStringSync('text') writes a string to a file. By default it writes from scratch (overwrites). With FileMode.append you add to the end.

Example: f.writeAsStringSync('new line', mode: FileMode.append) appends 'new line' to the file. It does not overwrite.

flush: true makes the write hit the disk immediately. By default Dart buffers writes.

Overwrite and append

Overwrite erases the old content and places the new. Append keeps the old content and writes at the end. Choosing the right mode prevents data loss.

For example when logging you use FileMode.append to add each day's record at the end. Overwrite is wanted only when building the file from scratch.

When writing twice to the same file use await or a sync method; avoid partial writes.

Exercise Append two lines

Import the library with import 'dart:io';. In main create File('log.txt'). First write with writeAsStringSync('first'). Then append with writeAsStringSync(' second', mode: FileMode.append). Then read with readAsStringSync() and print it with print.

Show example solution

import 'dart:io';

void main() {
  var f = File('log.txt');
  f.writeAsStringSync('first');
  f.writeAsStringSync(' second', mode: FileMode.append);
  print(f.readAsStringSync());
}

Test yourself

Append mode

f.writeAsStringSync('metin', mode: ____);

Show answer

f.writeAsStringSync('metin', mode: FileMode.append);

What is the default behavior of writeAsStringSync?
- Overwrites
- Throws
- Appends
- Deletes the file
Show answer

Overwrites — by default it overwrites the file.
Writing parts

writeAsStringSync()

Sync writing
FileMode.append

Appending
FileMode.write

Overwriting
flush: true

Flush buffer

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