Flex grid with odd numbers of items and fixed margin between - html

Question

Ok this is giving me quite an headache...
this is my code/example
https://codepen.io/anon/pen/xXpjYa
.flex {
display: flex;
flex-wrap: wrap;
}
.imageContainer {
flex: 1 0 30%;
height: 200px;
border: 5px solid black;
background-color: deeppink;
margin:15px;
}
.imageContainer:empty {
height: 0;
border: none;
};
<div class="flex">
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer"></div>
<div class="imageContainer"></div>
</div>
the problem is that I want to achieve/fix two things:
the far left and far right column should be touching the edge of the viewport
the fixed gap should be 30px and only the pink boxes stretch responsively
notice the last item (5) is slightly wider than the others...why??
please help! thanks!

Answer 1

You set the margin to be 15px, on .imageContainer this applies to all sides, including the sides between these imageContainers and the viewport. So basically your first ask and your second ask are fighting against each other.
You can set a margin on .imageContainer and a negative margin on the parent .flex to accomplish both.
The last visible item is longer because the two divs following it are collapsing and not showing, which affects the layout. If you add the css attribute box-sizing: border-box to your .imageContainer rule, it will fix this issue. You can also assigned a fixed height to .imageContainer, or add content to all of the .imageContainer divs to prevent this from happening.
See this codepen for the modified code.

Answer 2

Instead of margin use justify-content: space-between and set flex: 0 0 30% instead of flex: 1 0 30%. Last item is bigger because there is no border on other two.
body {
margin: 0;
}
.flex {
display: flex;
flex-wrap: wrap;
justify-content: space-between;
}
.imageContainer {
flex: 0 0 30%;
height: 200px;
border: 5px solid black;
background-color: deeppink;
margin: 15px 0;
}
.imageContainer:empty {
height: 0;
border: none;
}
<div class="flex">
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer">a</div>
<div class="imageContainer"></div>
<div class="imageContainer"></div>
</div>