Flexbox: Reverse Direction on Wrap / "Snake Wrap"

Question

I would like to find a way to have elements in a single container wrap to a new line going the opposite direction of the previous line, like a snake curving back on itself. I have not been able to achieve this result with flexbox and any combination of flex-direction and flex-wrap properties.

Image comparison of flexbox result vs desired result:

And here is a snippet showing the flexbox result vs desired result (faked):

body {
  font: 400 14px 'Arial', sans-serif;
}

.title {
  font-size: 1em;
  padding: 20px 0;
}

.title:first-child {
  padding: 0 0 20px;
}

.flex-container, .fake-container, .fake-row {
  width: 500px;
  background: orange;
  display: flex;
}

.flex-container {
  flex-flow: row wrap; /* any combination of row/reverse & wrap/reverse */
}

.item {
  display: inline-block;
  background: purple;
  width: 80px;
  max-width: 80px;
  height: 80px;
  margin: 10px;
  line-height: 80px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}

.fake-container {
  flex-flow: column nowrap;
}

.fake-row {
  flex-flow: row wrap;
  height: 100px;
}

.fake-row:nth-child(2) {
  flex-flow: row-reverse wrap;
}

<div class="title">Flexbox result:</div>
<div class="flex-container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="item">7</div>
</div>

<div class="title">Desired result (faked):</div>
<div class="fake-container">
  <div class="fake-row">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
    <div class="item">4</div>
    <div class="item">5</div>
  </div>
  <div class="fake-row">
    <div class="item">6</div>
    <div class="item">7</div>
  </div>
</div>

As much as I would love to find a pure CSS solution to achieve this "snake wrap", I am starting to doubt its possibility. I'm sure a javascript solution could be employed, but am unsure how to proceed in creating one without making things overly-complicated.

Is there any simple solution out there? Thank you for your time.

Answer 1

A CSS grid solution similar to this one that works with a fixed number of elements per row (aka fixed number of columns).

.flex-container {
  width: 500px;
  background: orange;
  display: grid;
  grid-template-columns:repeat(5,1fr); /*define the number of column*/
  grid-auto-flow:dense; /* this is important to fill all the space*/
  grid-gap:20px;
  padding:10px;
}

.item {
  background: purple;
  height: 80px;
  line-height: 80px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}

.item:nth-child(10n + 6)  {grid-column:5}
.item:nth-child(10n + 7)  {grid-column:4}
.item:nth-child(10n + 8)  {grid-column:3}
.item:nth-child(10n + 9)  {grid-column:2}
/*.item:nth-child(10n + 10) {grid-column:1} not needed*/
/* For N = number of columns 
  .item:nth-child((2xN)n + (N+1)) { grid-column:N; }
  .item:nth-child((2xN)n + (N+2)) { grid-column:(N-1); }
  ....
  .item:nth-child((2xN)n + (2xN)) { grid-column:1; }

*/

.item:before {
  content:counter(num);
  counter-increment:num;
}
body {
  font: 400 14px 'Arial', sans-serif;
  counter-reset:num;
}

<div class="flex-container">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

If you want a generic solution here is one using JS and flexbox. The trick is to set the order property based on the row position of each item. I will rely on this old answer to do most of the calculation in order to find the number of rows/columns:

//total number of element
var elems = document.querySelectorAll('.item');
var n_t = elems.length;
//width of an element
var w = parseInt(document.querySelector('.item').offsetWidth);
//full width of element with margin
var m = document.querySelector('.item').currentStyle || window.getComputedStyle(document.querySelector('.item'));
w = w + parseInt(m.marginLeft) + parseInt(m.marginRight);
//width of container
var w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
//padding of container
var c = document.querySelector('.flex-container').currentStyle || window.getComputedStyle(document.querySelector('.flex-container'));
var p_c = parseInt(c.paddingLeft) + parseInt(c.paddingRight);


var adjust = function(){
   //only the width of container will change
   w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
   //Number of columns
   nb = Math.min(parseInt((w_c - p_c) / w),n_t);
   //Number of rows
   nc = Math.ceil(n_t/nb);
   for(var j = 0;j<nb;j++) {
     for(var i = 0;i<nc;i++) {
       if(j + i*nb >= n_t) /* we exit if we reach the number of elements*/
        break
       if(i%2!=1) 
         elems[j + i*nb].style.order=j + i*nb; /* normal flow */
       else
         elems[j + i*nb].style.order=(nb - j) + i*nb; /* opposite flow */
     }
    }
}

adjust()
window.addEventListener('resize', function(){adjust()})

.flex-container {
  background: orange;
  display: flex;
  flex-wrap:wrap;
}

.item {
  background: purple;
  height: 80px;
  width:80px;
  margin:10px;
  line-height: 80px;
  grid-gap:20px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}


.item:before {
  content:counter(num);
  counter-increment:num;
}
body {
  font: 400 14px 'Arial', sans-serif;
  counter-reset:num;
}

<div class="flex-container">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

There is a little issue with the last line in some cases when it's not full of item. You can rectify this by adding some margin to the last element

//total number of element
var elems = document.querySelectorAll('.item');
var n_t = elems.length;
//width of an element
var w = parseInt(document.querySelector('.item').offsetWidth);
//full width of element with margin
var m = document.querySelector('.item').currentStyle || window.getComputedStyle(document.querySelector('.item'));
w = w + parseInt(m.marginLeft) + parseInt(m.marginRight);
//width of container
var w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
//padding of container
var c = document.querySelector('.flex-container').currentStyle || window.getComputedStyle(document.querySelector('.flex-container'));
var p_c = parseInt(c.paddingLeft) + parseInt(c.paddingRight);


var adjust = function(){
   //only the width of container will change
   w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
   //Number of columns
   nb = Math.min(parseInt((w_c - p_c) / w),n_t);
   //Number of rows
   nc = Math.ceil(n_t/nb);
   for(var j = 0;j<nb;j++) {
     for(var i = 0;i<nc;i++) {
       if(j + i*nb >= n_t) /* we exit if we reach the number of elements*/
        break
       elems[j + i*nb].style.marginLeft='10px'; /*we rest the margin*/
       if(i%2!=1) 
         elems[j + i*nb].style.order=j + i*nb; /* normal flow */
       else {
         elems[j + i*nb].style.order=(nb - j) + i*nb; /* opposite flow */
         /*margin fix*/
         if(i == (nc - 1) && (j + i*nb == (n_t - 1)) && j<(nb-1)) {
            elems[j + i*nb].style.marginLeft=((nb*nc - n_t)*w + 10) + 'px';
         }
       }
     }
    }
}

adjust()
window.addEventListener('resize', function(){adjust()})

.flex-container {
  background: orange;
  display: flex;
  flex-wrap:wrap;
}

.item {
  background: purple;
  height: 80px;
  width:80px;
  margin:10px;
  line-height: 80px;
  grid-gap:20px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}


.item:before {
  content:counter(num);
  counter-increment:num;
}
body {
  font: 400 14px 'Arial', sans-serif;
  counter-reset:num;
}

<div class="flex-container">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

---

You can easily switch to the other direction by simply changing if(i%2!=1) to if(i%2!=0)

//total number of element
var elems = document.querySelectorAll('.item');
var n_t = elems.length;
//width of an element
var w = parseInt(document.querySelector('.item').offsetWidth);
//full width of element with margin
var m = document.querySelector('.item').currentStyle || window.getComputedStyle(document.querySelector('.item'));
w = w + parseInt(m.marginLeft) + parseInt(m.marginRight);
//width of container
var w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
//padding of container
var c = document.querySelector('.flex-container').currentStyle || window.getComputedStyle(document.querySelector('.flex-container'));
var p_c = parseInt(c.paddingLeft) + parseInt(c.paddingRight);


var adjust = function(){
   //only the width of container will change
   w_c = parseInt(document.querySelector('.flex-container').offsetWidth);
   //Number of columns
   nb = Math.min(parseInt((w_c - p_c) / w),n_t);
   //Number of rows
   nc = Math.ceil(n_t/nb);
   for(var j = 0;j<nb;j++) {
     for(var i = 0;i<nc;i++) {
       if(j + i*nb >= n_t) /* we exit if we reach the number of elements*/
        break
       elems[j + i*nb].style.marginLeft='10px'; /*we rest the margin*/
       if(i%2!=0) 
         elems[j + i*nb].style.order=j + i*nb; /* normal flow */
       else {
         elems[j + i*nb].style.order=(nb - j) + i*nb; /* opposite flow */
         /*margin fix*/
         if(i == (nc - 1) && (j + i*nb == (n_t - 1)) && j<(nb-1)) {
            elems[j + i*nb].style.marginLeft=((nb*nc - n_t)*w + 10) + 'px';
         }
       }
     }
    }
}

adjust()
window.addEventListener('resize', function(){adjust()})

.flex-container {
  background: orange;
  display: flex;
  flex-wrap:wrap;
}

.item {
  background: purple;
  height: 80px;
  width:80px;
  margin:10px;
  line-height: 80px;
  grid-gap:20px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}


.item:before {
  content:counter(num);
  counter-increment:num;
}
body {
  font: 400 14px 'Arial', sans-serif;
  counter-reset:num;
}

<div class="flex-container">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>