Checking two arrays for matches

Question

Is there a better way to do this?

    var duplicate = false;
    for (var i = 0; i < acceptedFiles.length; i++) {
        for (var j = 0; j < attachments.attachments.length; j++) {
            if (attachments.attachments[j].name === acceptedFiles[i].name) {
                duplicate = true;
            }
        }
    }

Answer 1

Presumably, once a duplicate is found you don't need to continue looping:

if (attachments.attachments[j].name === acceptedFiles[i].name) {
    duplicate = true;
}

With JavaScript, the best thing would be to wrap up your for loops in a function:

function containsDuplicate(acceptedFiles, attachments) {
    for (var i = 0; i < acceptedFiles.length; i++) {
        for (var j = 0; j < attachments.attachments.length; j++) {
            if (attachments.attachments[j].name === acceptedFiles[i].name) {
                return true;
            }
        }
    }
    return false;
}

As this is a problem that applies to multiple languages, look at the break keyword.

Answer 2

Using ES5 you can generally avoid having to do tedious for loops to iterate through the values. In this case you can use the some method on arrays in order avoid using a flag and having i and j iterators to get the array values. You also get the added bonus of some short circuiting and stopping when the first true is found so as far as big O notation it is more efficient.

function hasDuplicate() {
    return acceptedFiles.some(function(acceptedValue) {
        return attachments.attachments.some(function(attachmentValue) {
            return acceptedValue.name === attachmentValue.name;
        });
    });
}

If your browser doesn't support ES5 you can use a shim

Answer 3

If using external libraries is an option, then LoDash would allow for something more like:

var duplicate = _.intersection(_.pluck(acceptedFiles, 'name'),
    _.pluck(attachments, 'name')).length > 0;

or:

function areDuplicates(acceptedFiles, attachments) {
    var acceptedFileNames = _.pluck(acceptedFiles, 'name');
    var attachmentNames = _.pluck(attachments, 'name');
    return _.intersection(acceptedFileNames, attachmentNames).length > 0;
}

Answer 4

You can return as soon as a match is found.

for (var i = 0; i < acceptedFiles.length; i++) {
    for (var j = 0; j < attachments.attachments.length; j++) {
        if (attachments.attachments[j].name === acceptedFiles[i].name) {
            return true;
        }
    }
}