Edit in JSFiddle

var text = "cat, bat, sat, fat";

var res1 = text.replace("bat", "bond"); // cat, bond, sat, fat
var res2 = text.replace(/at/, "ond"); // cond, bat, sat, fat
var res3 = text.replace(/at/g, "ond") // cond, bond, sond, fond

//출력
document.write(text + "<br>");
document.write(res1 + "<br>"); 
document.write(res2 + "<br>");
document.write(res3 + "<br>");