Show/hide in javascript when no html access

You could create a function that retrieves several elements by their id, and simply iterate over that collection of elements to hide or show them:

function getElementsByIds(idArray) {
    // initialise an array (over which we'll iterate, later)
    var elements = [];

    // if no arguments have been passed in, we quit here:
    if (!arguments) {
        return false;
    }
    else {
        /* we're running a basic check to see if the first passed-argument
           is an array; if it is, we use it: */
        if (Object.prototype.toString.call(arguments[0]) === '[object Array]') {
            idArray = idArray;
        }
        /* if a string has been passed-in (rather than an array), we
           make an array of those strings: */
        else if ('string' === typeof arguments[0]) {
            idArray = [].slice.call(arguments);
        }

        // here we iterate over the array:
        for (var i = 0, len = idArray.length; i < len; i++) {

            // we test to see if we can retrieve an element by the id:
            if (document.getElementById(idArray[i])) {
                /* if we can, we add that found element to the array
                   we initialised earlier: */
                elements.push(document.getElementById(idArray[i]));
            }
        }
    }
    // returning the elements:
    return elements;
}

// here we toggle the display of the elements (between 'none' and 'block':
function toggle (elems) {
    // iterating over each element in the passed-in array:
    for (var i = 0, len = elems.length; i < len; i++) {
        /* if the current display is (exactly) 'none', we change to 'block'
           otherwise we change it to 'none': */
        elems[i].style.display = elems[i].style.display === 'none' ? 'block' : 'none';
    }
}

function hide (nodes) {
    // iterating over the passed-in array of nodes
    for (var i = 0, len = nodes.length; i < len; i++) {
        // setting each of their display properties to 'none':
        nodes[i].style.display = 'none';
    }
}

// getting the elements:
var relevantElements = getElementsByIds('myid1','myid2','myid3'),
    toggleButton = document.getElementById('buttonThatTogglesVisibilityId');

// binding the click-handling functionality of the button:
toggleButton.onclick = function(){
    toggle (relevantElements);
};

// initially hiding the elements:
hide (relevantElements);

JS Fiddle demo.

References:

• arguments keyword.
• Array.prototype.push().
• Array.protoype.slice().
• document.getElementById().
• Function.prototype.call().
• Object.prototype.toString().
• typeof.

Three options (at least):

1) Wrap them all in a parent container if this is an option. Then you can just target that, rather than the individual elements.

document.getElementById('#my_container').style.display = 'block';

2) Use a library like jQuery to target multiple IDs:

$('#myid1, #myid2, #myid3').show();

3) Use some ECMA5 magic, but it won't work in old browsers.

[].forEach.call(document.querySelectorAll('#myid1, #myid2, #myid3'), function(el) {
    el.style.display = 'block'; //or whatever
});

If id="myid< increment-number >" then you can select these elements very easily.

Below code will select all elements that START WITH "myid".

$("div[id^='myid']").each(function (i, el) {
    //do what ever you want here
}

See jquery doc http://api.jquery.com/attribute-contains-selector/