The difference between flex:1 and flex-grow:1

Question

In mdn

• flex:1

means the same as

• flex-grow:1.

However, in fact it has different show in browser.

You can try it in this jsFiddle by changing the comment in css.

When I use flex: 1 the element who's classname is test-container will be height:100% but it won't happen when you use flex-grow: 1.

I don't understand why. Looking for some help. Thanks very much.

.flex-1 {
  display: flex;
  flex-direction: column;
  height: 500px;
  background: red;
}

.child-1 {
  height: 50px;
  background: blue;
}

.child-2 {
  flex-grow: 1;
  /* flex:1; */
  background: green;
}

.test-container {
  display: flex;
  flex-direction: row;
  background: white;
  height: 100%;
}

<div class="flex-1">
  <div class="child-1"></div>
  <div class="child-2">
    <div class="test-container"></div>
  </div>
</div>

Answer 1

flex

The flex property is a shorthand for setting:

• flex-grow
• flex-shrink
• flex-basis

The flex: 1 rule is supposed to compute to this:

• flex-grow: 1
• flex-shrink: 1
• flex-basis: 0

These values are defined in the spec. See section 7.1.1. Basic Values of flex

I say "supposed to compute" because, in IE11 and possibly other browsers, a unit of measure, such as px or %, is appended to the 0 value in flex-basis. This can make a difference (example).

---

flex-grow

The flex-grow property (which distributes free space in the container among flex items), when declared by itself, leaves flex-shrink and flex-basis at their initial values.

So when you set flex-grow: 1, the browser renders this:

• flex-grow: 1 (overrides the default value, which is 0)
• flex-shrink: 1 (this is the default value)
• flex-basis: auto (this is the default value)

---

The difference between flex: 1 and flex-grow: 1

Ultimately, the difference between flex: 1 and flex-grow: 1 is that the former sets flex-basis: 0, and the latter keeps the default flex-basis: auto.

For a complete explanation of the difference between flex-basis: 0 and flex-basis: auto see this post:

• Make flex-grow expand items based on their original size

---

Your code example

The reason you're seeing a difference in your code is that flex-basis controls height in a column-direction container.

In Chrome, with flex-basis: auto, the height of the element is 450px (500px parent - 50px header). In other words, flex-grow is free to distribute the free space.

With flex-basis: 0, the height of the element is 0, and flex-grow has no free space to distribute.

The height: 100% on the child of the flex item is simply ignored because it isn't being applied properly, as explained in these posts:

• Working with the CSS height property and percentage values
• Chrome / Safari not filling 100% height of flex parent

In reading the posts above you'll also understand why your code renders differently in Firefox, Safari, Edge and IE.