Change animation origin on mouse off event - css

Question

I've got some css/html code. I wanna improve my "unhover" state by doing following: when I hover over a button there is a before element sliding from left to right. I can easily change it from right to left. However when I do the "unhover" action, before element slides in the opposite direction - from right to left. What I want to achieve is animating it's width from 100% to 0% but from left to right. What should I do to get the result?
https://codepen.io/trueFalse24/pen/YzKNgYm
a{
background: #7f8c8d;
padding: 20px 30px;
text-transform: uppercase;
color: #fff;
position: relative;
&:before{
content: '';
position: absolute;
top: 0;
left: 0;
width: 0%;
height: 100%;
background: rgba(236,240,241, 0.3);
transition: all 0.3s ease;
}
&:hover{
&:before{
width: 100%;
}
}
}

Answer 1

I've modified your pen to get the effect by changing a few usages of the left and right properties. My edits are marked by comments below.
.container{
padding:0;
margin: 0;
box-sizing: border-box;
background: #3498db;
display: flex;
justify-content: center;
align-items: center;
height: 100vh;
a{
background: #7f8c8d;
padding: 20px 30px;
text-transform: uppercase;
color: #fff;
position: relative;
&:before{
content: '';
position: absolute;
top: 0;
right: 0; /* Replaced left */
width: 0%;
height: 100%;
background: rgba(236,240,241, 0.3);
transition: all 0.3s ease;
}
&:hover{
&:before{
width: 100%;
right: auto; /* Added */
left: 0; /* Added */
}
}
}
}