const imageId = [
            Math.random()
                .toString(36)
                .substr(2, 5)
        ]
            .map(element => (Math.random() > 0.5 ? element : element.toUpperCase()))
            .join('');


VS:

function generateString(base, length)
{
    return Math.random().toString(base).substr(2, lenth);
}

function alternateString(str)
{
    return str.map(element => (Math.random() > 0.5 ? element : element.toUpperCase())).join('');
}

function randomString(base, length)
{
    var generatedString = generateString(base, length);
    alternateString(generatedString);
    return generatedString;
}

// usage:
var numbers = randomString(36, 5);