Dedenting Template Strings

dedent-template.js

function dedent(callSite, ...args) {

    function format(str) {

        let size = -1;

        return str.replace(/\n(\s+)/g, (m, m1) => {

            if (size < 0)
                size = m1.replace(/\t/g, "    ").length;

            return "\n" + m1.slice(Math.min(m1.length, size));
        });
    }

    if (typeof callSite === "string")
        return format(callSite);

    if (typeof callSite === "function")
        return (...args) => format(callSite(...args));

    let output = callSite
        .slice(0, args.length + 1)
        .map((text, i) => (i === 0 ? "" : args[i - 1]) + text)
        .join("");

    return format(output);
}

let output;

console.log("=== As a string function ===");

output = dedent(`
    this
    is
  the ${ "end" }
      my only
      friend
    the end
`);

console.log(output);

console.log("=== As a template tag ===");

output = dedent`
    this
    is
  the ${ "end" }
      my only
      friend
    the end
`;

console.log(output);

console.log("=== As a higher-order template tag ===");

output = dedent(String.raw)`
    this
    is
  the ${ "end" }
      my only
      friend
    the \end
`;

console.log(output);