When I am adding elements in my template string inside a \"for\" loop, it returns the word \"undefined\" at the beginning of the string.
Despite the fact that I initiali
It's not problem of your loop.
You didn't specify any value in variable declaration let templateString so it's value is undefined. It works with let templateString = '';.
// Hey JavaScript, it's string!
let templateString = '';
let objectStock={
  A:"A", 
  B:"B",
  C:"C",
  D:"D",
}
for(let objectItem in objectStock){
  templateString+= `
${objectItem}: ${ objectStock[objectItem] || "empty field."}
`
}
console.log("templateString: ", templateString)