such a navigation 
jumps through koarouter, but when you jump like this, give him a style or class name
router.get("/contantus", async ctx => {
console.log(ctx.response)
await ctx.render("contantus", {title: ""})
})
router.get("/products", async ctx => {
await ctx.render("products", {title: ""})
})
HMTL
<ul class="nav-list">
<li></li>
<li class={{active}}><a href="/products"></a> </li>
<li></li>
<li><a href="/contantus"></a> </li>
</ul>
koa-nunjucks-2 active