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 'node: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。
不建议在调用 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 'node: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 'node: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 'node: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 'node: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)。
一般来说,只有在文件不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 'node: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 'node: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 'node: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 'node: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 'node: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.