Expand a div to fill the remaining width

Question

I want a two-column div layout, where each one can have variable width e.g.

Answer 1

flex-grow - This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.

If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least). See more here

.parent {
  display: flex;
}

.child {
  flex-grow: 1; // It accepts a unitless value that serves as a proportion
}

.left {
  background: red;
}

.right {
  background: green;
}

 
  

      Left 50%
  
   

      Right 50%