Grouping vars in function

You can define a common class for each of the elements where click event is expected using .querySelectorAll() with selector ".myClickFunction", iterate the NodeList using for loop; set Element.dataset of each element to the current index of iteration; attach click handler to the element; again use .querySelectorAll() with selector "[id^myDIV]" to select all elements where id begins with "myDIV"; at click event set each "myDIV" .style to "display:none"; set "myDIV" at index Element.dataset .style to "display:block".

for (var i = 0, el = document.querySelectorAll(".myClickFunction")
  ; i < el.length; i++) {
  el[i].dataset.index = i;
  el[i].onclick = function(event) {
    for (var n = 0, el_ = document.querySelectorAll("[id^=myDIV]")
    ; n < el_.length; n++) {
      el_[n].style.display = "none"
    }
    document.querySelectorAll("[id^=myDIV]")[event.target.dataset.index]
      .style.display = "block"
  }
}

<div id="myDIVA" style="display:none;">blabla 1</div>
<div id="myDIVB" style="display:none;">blabla 2</div>
<div id="myDIVC" style="display:none;">blabla 3</div>
and they are hidden and/or shown by clicking div's:

<div class="myClickFunction">Show only ONE</div>
<div class="myClickFunction">Show only TWO</div>
<div class="myClickFunction">Show only THREE</div>

I assume you don’t want to have a separate function for every div. And you don’t need to. 🙂 You can have a single function that handles the clicks, and works with an array of divs.

First, let’s get an array of those divs:

var divs = Array.prototype.slice.call(document.querySelectorAll("[id^=myDIV]"));

(It would be better if we could give them a common class. That grabs them by the start of their IDs.)

Then, let’s have the clickable divs say which div they relate to:

<div onClick="myClickFunction('myDIVA')">Show only ONE</div>

Then, let’s have a single function that handles clicks:

function myClickFunction(id) {
    divs.forEach(function(div) {
        div.style.display = div.id === id ? "block" : "none";
    });
}

(See compatibility notes at the end.)

However, using onxyz-attribute-style event handlers is generally a bad idea. They require that the functions you use be globals, for one thing.

Instead, let’s hook them up dynamically, and use a data-* attribute to say which target they relate to:

// Scoping function to avoid globals
(function() {
  var divs = Array.prototype.slice.call(document.querySelectorAll(".showable"));
  Array.prototype.forEach.call(document.querySelectorAll("div[data-show]"), function(dshow) {
      dshow.addEventListener("click", myClickFunction, false);
  });

  function myClickFunction() {
    var idToShow = this.getAttribute("data-show");
    divs.forEach(function(div) {
      div.style.display = div.id === idToShow ? "block" : "none";
    });
  }
})();

<div data-show="myDIVA">Show A</div>
<div data-show="myDIVB">Show B</div>
<div data-show="myDIVC">Show C</div>
<div class="showable" id="myDIVA">A</div>
<div class="showable" id="myDIVB" style="display: none">B</div>
<div class="showable" id="myDIVC" style="display: none">C</div>

Now we have no global functions, and the markup defines the relationship between the elements.

Compatibility notes:

If you have to support obsolete browsers that don’t have addEventListener (like IE8 — and sadly many do), this answer has a cross-browser hookEvent function you an use.

Note that if you’re supporting IE8, you’ll also have to use a for loop rather than Array.prototype.forEach, since IE8 doesn’t have it and you can’t reasonably shim it (because IE8 also doesn’t support adding a non-enumerable property to a JavaScript object).

Here’s a solution using addEventListener that removes the need for onclick attributes:

// Select the buttons and items
var buttons = Array.prototype.slice.call(document.querySelectorAll('.showhidebutton'))
var items = Array.prototype.slice.call(document.querySelectorAll('.showhideitem'))

// Add a click eventListener for each button
buttons.forEach(function(button, i) {
  button.addEventListener('click', function() {
    // When a button is clicked, loop through the items
    items.forEach(function(item, j) {
      // If this is the item that should be shown, show it
      if (i === j) item.style.display = 'block'
      // Otherwise, hide it
      else item.style.display = 'none'
    })
  })
})

// This code hides all but item 1 on pageload
// can remove this code if you don't want that
items.forEach(function(item, j) {
  // Arrays are zero-indexed in JS, so the first item is 0
  if (i === 0) return
  // Hide all the other items
  else item.style.display = 'none'
})

<div class="showhideitem">blabla 1</div>
<div class="showhideitem">blabla 2</div>
<div class="showhideitem">blabla 3</div>

<div class="showhidebutton">Show only ONE</div>
<div class="showhidebutton">Show only TWO</div>
<div class="showhidebutton">Show only THREE</div>

Probably a good usecase for an array:

var elemsById = ["myDIVA","myDIVB","myDIVC"]
 .map(id=>[id,document.getElementById(id)]);

So whenever we want to show one of these elements, we iterate over all of them, and hide/show them, depending on their id:

function showThisAndHideRest(showId){
  elemsById.forEach(function([id,el]){
    el.style.display=id===showId?"block":"none";
  });
}

So you can do:

showThisAndHideRest("myDIVA");

Note: the upper uses some cool ES6 features, which are not yet supported by all browsers. And theres actually no need to store id in the array, as its already part of el (el.id), but i just wanted to use some parameter destructuring…

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