Node.js fs.watchFile() Method
The fs.watchFile() method is used to continuously watch for changes on the given file. It has a callback listener that is called every time the file is accessed. It has an optional options parameter that can be used to specify properties like the interval on how often the file has to be polled and whether the process would continue as long as the file is being watched.
The listener has two arguments, that is the current stat object and the previous stat object. This can be used to compare the changes to the file. The modified file access time can be found from the fs.Stat object’s mtime property.
While watching a file, if it disappears and reappears, the previousStat of the disappearance callback will be the same as the previousStat of the appearance callback. This happens when the file is renamed and then renamed a second time back to its original name. It can also happen when the file is deleted and then restored back.
fs.watchFile(filename[, options], listener)
Parameters: This method accepts three parameters as mentioned above and described below:
- filename: It is a String, Buffer or URL that denotes the filename of the file to be watched.
- options: It is an Object that can be used to modify the behavior of the method. It is an optional parameter. It has the following parameters:
- bigint: It is a Boolean value that is used to specify the numeric values of the fs.Stat object as BigInt values. The default value is false.
- persistent: It is a Boolean value that is used to specify if the process should continue as long as the file is being watched. The default value is true.
- interval: It is an integer that specifies the time interval between each poll to the target. It is specified in milliseconds. The default value is 5007.
- listener: It is function that is invoked when the file is accessed or modified.
- current: It is a fs.Stats object that denotes the current state of the file after being accessed or modified.
- previous: It is a fs.Stats object that denotes the previous state of the file before being accessed or modified.
Return Value: It returns a fs.StatWatcher object when the function is successfully called.
Below examples illustrate the fs.watchFile() method in Node.js:
Example 1: This example shows the usage of the watchFile() method along with its parameters.
The file was edited Previous Modified Time 2020-05-30T07:52:14.587Z Current Modified Time 2020-05-30T08:01:40.948Z The contents of the current file are: File Contents are Edited The file was edited Previous Modified Time 2020-05-30T08:01:40.948Z Current Modified Time 2020-05-30T08:01:45.950Z The contents of the current file are: File Contents are Edited Again
Example 2: This example shows the file modification time when a file is renamed and then renamed back to its original name, causing a file disappearance and reappearance.
Previous Modification Time 2020-05-30T08:01:45.950Z Current Modification Time 1970-01-01T00:00:00.000Z The File was modified Previous Modification Time 2020-05-30T08:01:45.950Z Current Modification Time 2020-05-30T08:01:45.950Z