I want following output ["a","b"] out of loop but isnot getting. How can I do it? instead of getting [].

post_with_medias(text, medias) {
    var listOfIds = []
    medias.forEach((element) => {
      this.upload_media(element).then((res) => {
        // we can assign alt text to the media, for use by screen readers and
        // other text-based presentations and interpreters
        var a = res.data
   
        listOfIds.push(a) // returns ["a","b"]
      })
      
      console.log(a); // this doesnot give the data appended returns []

    })