如何在 Javascript 中追加或连接字符串?

So I'm trying to add to a string and it shows up as empty.

var DNA = "TAG";
var mRNA = "";

m_RNA()

function check(a, b, string) {
  if (string = a) {
    mRNA.concat(b);
  }
}

function m_RNA(){
  //console.log(DNA)
  for (var i = 0; i < DNA.length; i++) {
    console.log("Checking: " + DNA[i]);
    check("T", "A", DNA[i]);
    check("A", "U", DNA[i]);
    check("C", "G", DNA[i]);
    check("G", "C", DNA[i]);
    console.log(mRNA);
  }
}

它应该在控制台中显示 AUC，但它只是空白.顺便说一下，这是通过 Firefox 实现的.