So I've recently delved into trying to understand promises and the purpose behind them due to javascripts asynchronous behavior. While I "think" I understand, I still struggle with how to promisify something to return the future value, then execute a new block of code to do something else. Two main node modules I'm using:
What I'd like to do is read a file, then once fully read, iterate of each worksheet executing DB commands. Then once all worksheets are processed, go back and delete the original file I read. Here is the code I have. I have it working to the point everything writes into the database just fine, even when there are multiple worksheets. What I don't have working is setting it up to identify when all the worksheets have been fully processed, then to go remove the file
workbook.csv.readFile(fileName)
.then(function () {
// this array I was going to use to somehow populate a true/false array.
// Then when done with each sheet, push a true into the array.
// When all elements were true could signify all the processing is done...
// but have no idea how to utilize this!
// So left it in to take up space because wtf...
var arrWorksheetComplete = [];
workbook.eachSheet(function (worksheet) {
console.log(worksheet.name);
db.tx(function (t) {
var insertStatements = [];
for (var i = 2; i <= worksheet._rows.length; i++) {
// here we create a new array from the worksheet, as we need a 0 index based array.
// the worksheet values actually begins at element 1. We will splice to dump the undefined element at index 0.
// This will allow the batch promises to work correctly... otherwise everything will be offset by 1
var arrValues = Array.from(worksheet.getRow(i).values);
arrValues.splice(0, 1);
// these queries are upsert. Inserts will occur first, however if they error on the constraint, an update will occur instead.
insertStatements.push(t.one('insert into rq_data' +
'(col1, col2, col3) ' +
'values($1, $2, $3) ' +
'ON CONFLICT ON CONSTRAINT key_constraint DO UPDATE SET ' +
'(prodname) = ' +
'($3) RETURNING autokey',
arrValues));
}
return t.batch(insertStatements);
})
.then(function (data) {
console.log('Success:', 'Inserted/Updated ' + data.length + ' records');
})
.catch(function (error) {
console.log('ERROR:', error.message || error);
});
});
});
I would like to be able to say
.then(function(){
// everything processed!
removeFile(fileName)
// this probably also wouldn't work as by now fileName is out of context?
});
But I'm super confused when having a promise inside a promise.. I have the db.tx call which is essentially a promise nested inside the .eachSheet function. Please help a dumb programmer understand! Been beating head against wall for hours on this one. :)
If i understand correctly, you're trying to chain promises.
I suggest you to read this great article on Promises anti-pattern (see 'The Collection Kerfuffle' section)
If you need to execute promises in series, this article suggests to use reduce.
I'll rewrite your snippet to:
workbook.csv.readFile(fileName).then(function () {
processWorksheets().then(function() {
// all worksheets processed!
});
});
function processWorksheets() {
var worksheets = [];
// first, build an array of worksheet
workbook.eachSheet(function (worksheet) {
worksheets.push(worksheet);
});
// then chain promises using Array.reduce
return worksheets.reduce(function(promise, item) {
// promise is the the value previously returned in the last invocation of the callback.
// item is a worksheet
// when the previous promise will be resolved, call saveWorksheet on the next worksheet
return promise.then(function(result) {
return saveWorksheet(item, result);
});
}, Promise.resolve()); // start chain with a 'fake' promise
}
// this method returns a promise
function saveWorksheet(worksheet, result) {
return db.tx(function (t) {
var insertStatements = [];
for (var i = 2; i <= worksheet._rows.length; i++) {
// here we create a new array from the worksheet, as we need a 0 index based array.
// the worksheet values actually begins at element 1. We will splice to dump the undefined element at index 0.
// This will allow the batch promises to work correctly... otherwise everything will be offset by 1
var arrValues = Array.from(worksheet.getRow(i).values);
arrValues.splice(0, 1);
// these queries are upsert. Inserts will occur first, however if they error on the constraint, an update will occur instead.
insertStatements.push(t.one('insert into rq_data' +
'(col1, col2, col3) ' +
'values($1, $2, $3) ' +
'ON CONFLICT ON CONSTRAINT key_constraint DO UPDATE SET ' +
'(prodname) = ' +
'($3) RETURNING autokey',
arrValues));
}
return t.batch(insertStatements);
})
// this two below can be removed...
.then(function (data) {
return new Promise((resolve, reject) => {
console.log('Success:', 'Inserted/Updated ' + data.length + ' records');
resolve();
});
})
.catch(function (error) {
return new Promise((resolve, reject) => {
console.log('ERROR:', error.message || error);
reject();
});
});
}
Don't forget to include the promise module:
var Promise = require('promise');
I haven't tested my code, could contains some typo errors.