ReQL command: eachAsync

Command syntax

sequence.eachAsync(function[, errorFunction]) → promise

Description

Lazily iterate over a cursor, array, or feed one element at a time. eachAsync always returns a promise that will be resolved once all rows are returned.

The first, required function passed to eachAsync takes either one or two functions as arguments. The first is a callback to process each row as it is emitted; the second is an optional callback which will be executed when all row processing is completed.

function(rowProcess[, final])

The rowProcess callback receives the row as its first argument; it may also take an optional second argument, which is a callback function to be executed after each row has been processed.

function(row[, rowFinished])

If you accept the rowFinished callback, it must be called at the end of each row. If you call rowFinished with any value, iteration will stop, and the value will be wrapped in error.message for the error handler.

If you do not use rowFinished, the rowProcess callback can end iteration early by returning any value other than a Promise. If it returns a non-Promise value (other than undefined), then final will receive an error object. If it returns a Promise, the Promise will be resolved before iteration continues; if the resolved Promise returns a value, iteration will be stopped and an error object will be passed to final.

If you provide a final callback, it will always be executed when row processing is completed (the end of the sequence is hit, iteration is stopped prematurely, or an error occurs). The final callback will receive an error object if an error is thrown or rowProcess returns any value (other than a Promise). If final returns any value it will be ignored.

To summarize all of the above in code:

// process each row asynchronously
cursor.eachAsync(function (row) {
    doSomethingWith(row);
});

// as above, but using rowFinished callback
cursor.eachAsync(function (row, rowFinished) {
    doSomethingWith(row);
    rowFinished();
});

// as above, but using final callback
cursor.eachAsync(function (row, rowFinished) {
    doSomethingWith(row);
    rowFinished();
}, function (final) {
    // the 'final' argument will only be defined when there is an error
    console.log('Final called with:', final);
});

Example: Process all the elements in a stream, using then and catch for handling the end of the stream and any errors. Note that iteration may be stopped in the first callback (rowProcess) by returning any non-Promise value.

cursor.eachAsync(function (row) {
    var ok = processRowData(row);
    if (!ok) {
        throw new Error('Bad row: ' + row);
    } 
}).then(function () {
    console.log('done processing'); 
}).catch(function (error) {
    console.log('Error:', error.message);
});

Example: As above, but using the rowFinished and final callbacks rather than the Promise returned from eachAsync.

cursor.eachAsync(
    function (row, rowFinished) {
        var ok = processRowData(row);
        if (ok) {
            rowFinished();
        } else {
            rowFinished('Bad row: ' + row);
        }
    },
    function (error) {
        if (error) {
            console.log('Error:', error.message);
        } else {
            console.log('done processing');
        }
    }
);

Note: You need to manually close the cursor if you prematurely stop the iteration.

Related commands

• each

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