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ViewsJavascript FileSystem function not working as intended
I have a simple code to write a js list to a js file.
// Requiring fs module in which
// writeFile function is defined.
var fs = require('fs');
// first overwrite any pre-existing file with the start of the list
fs.writeFile('Output.js', 'export const addressPoints = [', function (err) {
if (err) throw err;
console.log('Create file Output.js');
});
for (let i = 0; i < 100; i++) {
// var string_row = "[" + latlon_datum1_row[i] + "],";
var string_row = "HI"
console.log(string_row);
fs.appendFile('Output.js', string_row, function (err) {
if (err) throw err;
console.log('Writing file contents line', i);
});
}
fs.appendFile('Output.js', '];', function (err) {
if (err) throw err;
console.log('Append file Output.js');
});
It can be thought of in three parts, one intial component creates the new js script and adds 'export const addressPoints = [' to it. After that, a for-loop runs to write the items of the list, and lastly, '];' is appended to the end of the file.
Very strangely, the code does not work as intended. One would expect the console log to look something like
Create file Output.js
Writing file contents line 1
Writing file contents line 2
Writing file contents line 3
...
Writing file contents line 99
Append file Output.js
However, the output is scrambled and written in the wrong order (showing last few lines):
...
Writing file contents line 96
Writing file contents line 97
Writing file contents line 98
Append file Output.js
Writing file contents line 99
Writing file contents line 91
Writing file contents line 79
Writing file contents line 68
Is there a method that can do this more quickly? If not, why is this error occurring? I assume it has something to do with fs closure but I am not too familiar with js.
1 answers
Answer question0
Extra points for your console.log detail.
As you see, the log has a wrong order. That is due to asynchronous nature of nodejs and javascript.
You need to use callbacks or promises to "wait" for the result of previous operation.
Sample: read a file and print its content
incorrect
fs.readFile('/tmp/foo.txt',function (err, data) {
});
console.log('data is...');
correct
fs.readFile('/tmp/foo.txt',function (err, data) {
console.log('data is:'+data);
});
As you can see, readFile and almost all nodejs functions are asynchronous. It means that we cannot think like java, c# or python in a sequential or synchronous mode. In this case, readFile needs a function to be executed when file is readed:
function (err, data) {
console.log('data is:'+data);
}
callbacks
Work passing function which will be executed after the async operation.
var fs = require('fs');
fs.writeFile('Output.js', 'export const addressPoints = [', function (err) {
if (err) throw err;
console.log('Create file Output.js');
//execute after file was updated
var newContent = "";
for (let i = 0; i < 100; i++) {
var string_row = "HI"
console.log(string_row);
newContent += string_row+"\n";
}
newContent += "];\n";
fs.appendFile('Output.js', newContent, function (err) {
if (err) throw err;
console.log('Output.js was updated');
});
});
Or a little more readable
var fs = require('fs');
fs.writeFile('Output.js', 'export const addressPoints = [', createCustomFile);
function createCustomFile(err, data){
//logic here
}
Disadvantage
If you need to execute more operations, you will have a nightmare
promises & await
A way to write more readable (which continues async) code vs nested callbacks
var fs = require('fs');
async function main(){
await fs.promises.writeFile('Output.js', 'export const addressPoints = [');
console.log('Create file Output.js');
for (let i = 0; i < 100; i++) {
var string_row = "HI"
console.log(string_row);
await fs.promises.appendFile('Output.js', string_row);
console.log('Writing file contents line', i);
}
await fs.promises.appendFile('Output.js', '];');
console.log('Append file Output.js');
}
//launch
//don't forget to catch async exceptions here!
main().catch(e => { console.error(e); process.exit(1) })