This is my code:

<img src={"/upload/images/"+Currency.getCurrent()+".svg"} />

And this my function:

static getCurrent() {
    var currency = "usd"
    const cookies = parseCookies()
    if (cookies.currency) {
        currency = cookies.currency
    }
    return currency
}

I get this error:

next-dev.js?3515:32 Warning: Prop `src` did not match. Server: "/upload/images/usd.svg" Client: "/upload/images/btc.svg"

So, usd.svg and btc.svg are exists.

For usd I dont have any error, but for btc I get error.

Note: This code return true url:

console.log(Currency.getCurrent()) // /upload/images/btc.svg