Material Flipside Button with animation

HTML
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2
<div class="btn">
<div class="btn-back">
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SCSS
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$transition-duration: 0.8s;
$transition-easing: cubic-bezier(0
    .230, 1.000, 0.320, 1.000);
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JAVASCRIPT
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var btn = document.querySelector( '
    .btn' );
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