Whether a markdown is existed in dataviewjs block

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What I’m trying to do

use dataviewjs to find out whether a markdown note abcd.md is existed in folder A

if the note abcd.md is existed, var thelink = [[abcd.md]], i.e., return a link to abcd.md

if abcd.md does not exist in folder A, var thelink = [[newabce.md]]

Things I have tried

let thelink
var thelink = dv.func.link(“abcd”)
if (dv.page(thelink) == null ) {
thelink = “[[new”+thelink+“]]”
} else {
thelink = “[[”+thelink+“]]”

However, it seems do not work.

Also, I have no idea how to specify the folder A

Just began to learn coding in order to better use this magic app.

Thanks for helps