JavaScript Tagged strings


A function identified immediately before a template literal is used to interpret it, in what is called a tagged template literal. The tag function can return a string, but it can also return any other type of value.

The first argument to the tag function, strings, is an Array of each constant piece of the literal. The remaining arguments, ...substitutions, contain the evaluated values of each ${} substitution expression.

function settings(strings, ...substitutions) {
  const result = new Map();
  for (let i = 0; i < substitutions.length; i++) {
    result.set(strings[i].trim(), substitutions[i]);
  return result;

const remoteConfiguration = settings`
  label    ${'Content'}
  servers  ${2 * 8 + 1}
  hostname ${location.hostname}
Map {"label" => "Content", "servers" => 17, "hostname" => ""}

The strings Array has a special .raw property referencing a parallel Array of the same constant pieces of the template literal but exactly as they appear in the source code, without any backslash-escapes being replaced.

function example(strings, ...substitutions) {
  console.log('strings:', strings);
  console.log('...substitutions:', substitutions);

example`Hello ${'world'}.\n\nHow are you?`;
strings: ["Hello ", ".\n\nHow are you?", raw: ["Hello ", ".\\n\\nHow are you?"]]
substitutions: ["world"]