chainable template tag for joining a bunch of strings over many lines

append-template-tag.js

function append(separator) {
  return typeof separator === "string" ? appender(separator, "") : appender("", "").apply(this, arguments);
}

function appender(separator, s) {
  return function tag(literalParts, ...computedParts) {
    s += literalParts[0];
    for (let i = 1; i < literalParts.length; ++i) {
      s += computedParts[i - 1] + literalParts[i];
    }
    return function(x) { return x == null ? s : appender(separator, s + separator).apply(this, arguments); };
  }
}

test.js

var combined = append
  `aaa`
  `bbb`
  `ccc`();
// "aaabbbccc"

var lines = append("\n")
  `first line`
  `second line`
  `third line`();
// "first line\nsecond line\nthird line"