如何从 forEach 循环中实现扁平化的 promise 数组？

Question

I am attempting to utilize a chain of promises (via Q & the node child-process-promise library) to pull drive letters, free space, and total size utilizing the Windows WMIC command like so:

var stdout = '';
var exec = require('child-process-promise').exec;

exec('wmic logicaldisk where drivetype=3 get caption', { capture: ['stdout', 'stderr'] })
    .then(function(result) {
        var data = result.stdout.split('\r\n');
        data = data.splice(1, data.length - 3);
        data = data.map(Function.prototype.call, String.prototype.trim);
        data.forEach(function(drive) {
            // This is where I am getting hung up
        });
    })
    .then(function(result) {
        console.log(result);
    })
    .fail(function(err) {
        console.error(err);
    });

As you can in the code above, I am returning an array of drive letters from the initial execution of WMIC, though after this I will need to run WMIC several more times depending on the count of drives returned by the first query. For instance, if my data array looks like so:

['C:','D:']

I'll need to run:

wmic logicaldisk where caption="C:" get freespace
wmic logicaldisk where caption="C:" get size
wmic logicaldisk where caption="D:" get freespace
wmic logicaldisk where caption="D:" get freespace

Though I would like to do this from a flat promise chain to aggregate the results, I cannot seem to wrap my head around how to do this. I've seen several other questions where utilizing a "reduce" is suggested, but can't seem to figure how to apply this to my current situation.

Answer 1

Create a collection of Promises by iterating over the data and storing the result of each call to exec in an array. Then wait for the resolution of all the Promises by passing the array to Q#all .

Using nested forEach :

var promises = [];

data.forEach(function (drive) {
    var commands = [
        'wmic logicaldisk where caption="' + drive + '" get freespace',
        'wmic logicaldisk where caption="' + drive + '" get size'
    ];

    commands.forEach(function (cmd) {
        promises.push(exec(cmd, {capture: ['stdout', 'stderr']}));
    });
});

return Q.all(promises);

Or using reduce and map :

var promises = data.reduce(function (result, drive) {
    var commands = [
        'wmic logicaldisk where caption="' + drive + '" get freespace',
        'wmic logicaldisk where caption="' + drive + '" get size'
    ].forEach(function (cmd) {
        result.push(exec(cmd, {capture: ['stdout', 'stderr']}));
    });

    return result;
}, []);

return Q.all(promises);