What is the difference between the flex and height properties? - html

Question

Given the following example, both will fill out the center to consume the remaining space in the page, given the page is using flex. I am leaning towards using the css property flex vs height in the body. Is there a difference that needs to be considered when applying one over the other?
CSS
.page {
display: flex;
flex-direction: column;
}
.header {
height: 100px;
}
.body {
flex: 1; // vs height: 100%;
}
.footer {
height: 40px;
}
HTML
<div class="page">
<div class="header">Sample Header</div>
<div class="body">Sample Body</div>
<div class="footer">Sample Footer</div>
</div>

Answer 1

When you set an element to flex: 1, that breaks down to:
flex-grow: 1
flex-shrink: 1
flex-basis: 0
In a column-direction container (like you have), the flex properties above apply vertically. This means that flex-basis and height are equivalent properties.
flex-basis = height (in a column-direction container)
There is an obvious difference between flex-basis: 0 and height: 100%. It's the same difference as height: 0 and height: 100%.
In your situation, where there is a .header and a .footer consuming 140px of vertical space, setting the middle item (.body) to height: 100% would normally cause an overflow.
But since an initial value of a flex container is flex-shrink: 1, flex items are permitted to shrink, and this wouldn't happen. However, it's still sloppy and imprecise coding, in my view.
By setting .body to flex: 1, you're setting the height to 0, but also allowing it to consume free height with flex-grow: 1. I would say, in this case, that this solution is more efficient.
More details:
What are the differences between flex-basis and width?
§ 7.1.1. Basic Values of flex

Answer 2

There is a huge difference between flex and height.
First to answer your question.
Height 100% doesn't use the remaining space. It will use all the spaces of parent, in your case if page dom is height 200px; then body will also be height: 200px;.
Flex will be correct solution here to fill up the space (flex: 1).
Flex is more than filling the space, its more of a layout and it has influences on its child, how they position and align.

Answer 3

Try below code
.page {
display: flex;
flex-direction: column;
height: 100%;
}
.header {
height: 100px;
display: flex;
align-items: center;
justify-content: center;
}
.body {
display: flex;
flex-direction: column;
height: 80vh;
align-items: center;
justify-content: center;
}
.footer {
height: 40px;
display: flex;
align-items: center;
justify-content: center;
}
<div class="page">
<div class="header">Sample Header</div>
<div class="body">Sample Body</div>
<div class="footer">Sample Footer</div>
</div>