If you are trying to stretch the <aside> elements to recreate a 3-column layout then this task should be done in CSS only
body {
display: flex;
flex-flow: row nowrap;
}
aside {
flex-basis: 20%;
border: 1px #9bc solid;
}
#aside2 {
order: 3;
}
/* simulate an height for main element */
main {
height: 1234px;
flex: 1;
margin: 0 5px;
border: 1px #ccc solid;
}
<body>
<aside class = "a_left" id="aside1">
<div class="a_left" id="div_aside1">
aside left
</div>
</aside>
<aside class = "a_right" id="aside2">
<div class="a_right" id="div_aside2">
aside right
</div>
</aside>
<main> center </main>
</body>
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