fs.exists(path, callback)

• path <string> | <Buffer> | <URL>
• callback <Function>
  • exists <boolean>

通过检查文件系统来测试给定的路径是否存在。 然后使用 true 或 false 调用 callback 参数：

import { exists } from 'fs';

exists('/etc/passwd', (e) => {
  console.log(e ? 'it exists' : 'no passwd!');
});

此回调的参数与其他 Node.js 回调不一致。 通常，Node.js 回调的第一个参数是 err 参数，后跟可选的其他参数。 fs.exists() 回调只有一个布尔参数。 这是推荐 fs.access() 而不是 fs.exists() 的原因之一。

不推荐在调用 fs.open()、fs.readFile() 或 fs.writeFile() 之前使用 fs.exists() 检查文件是否存在。 这样做会引入竞争条件，因为其他进程可能会在两次调用之间更改文件的状态。 而是，用户代码应该直接打开/读取/写入文件，并在文件不存在时处理引发的错误。

写入（不推荐）

import { exists, open, close } from 'fs';

exists('myfile', (e) => {
  if (e) {
    console.error('myfile already exists');
  } else {
    open('myfile', 'wx', (err, fd) => {
      if (err) throw err;

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  }
});

写入（推荐）

import { open, close } from 'fs';
open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

读取（不推荐）

import { open, close, exists } from 'fs';

exists('myfile', (e) => {
  if (e) {
    open('myfile', 'r', (err, fd) => {
      if (err) throw err;

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  } else {
    console.error('myfile does not exist');
  }
});

读取（推荐）

import { open, close } from 'fs';

open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

上面的“不推荐”示例检查是否存在，然后使用该文件；“推荐”示例更好，因为它们直接使用文件并处理错误（如果有）。

通常，仅当文件不会被直接使用时才检查文件是否存在，例如当它的存在是来自另一个进程的信号时。

• path <string> | <Buffer> | <URL>
• callback <Function>
  • exists <boolean>

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false:

import { exists } from 'fs';

exists('/etc/passwd', (e) => {
  console.log(e ? 'it exists' : 'no passwd!');
});

The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

write (NOT RECOMMENDED)

import { exists, open, close } from 'fs';

exists('myfile', (e) => {
  if (e) {
    console.error('myfile already exists');
  } else {
    open('myfile', 'wx', (err, fd) => {
      if (err) throw err;

      try {
        writeMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  }
});

write (RECOMMENDED)

import { open, close } from 'fs';
open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  try {
    writeMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

read (NOT RECOMMENDED)

import { open, close, exists } from 'fs';

exists('myfile', (e) => {
  if (e) {
    open('myfile', 'r', (err, fd) => {
      if (err) throw err;

      try {
        readMyData(fd);
      } finally {
        close(fd, (err) => {
          if (err) throw err;
        });
      }
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

import { open, close } from 'fs';

open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  try {
    readMyData(fd);
  } finally {
    close(fd, (err) => {
      if (err) throw err;
    });
  }
});

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.