mod.ts

const indentTag = (strings: TemplateStringsArray, ... expressions: any[]) => {
    let parts: string[] = Array.from(strings);

    // Drop first line feed
    if ('\n' !== strings[0][0]) {
        throw new Error('Expect first character to be a line feed');
    }

    parts[0] = parts[0].slice(1);

    // Remove terminating indentation
    parts[strings.length - 1] = parts[parts.length - 1].replace(/\n[ \t]*$/, '\n');

    // Dedent using indent from first part
    const indent = parts[0].match(/^\n*([ \t]*)/)![1];

    const deindent = new RegExp(`(^|\n)${indent}`, 'g');

    parts = parts.map((string) => string.replace(deindent, '$1'));

    // Ensure expressions are strings
    const values = expressions.map((expression) => String(expression));

    // Treat expression
    for (let i= 0; i < values.length; i++) {
        const prev = parts[i];
        const next = parts[i + 1];

        const match = prev.match(/\n( *)$/);

        // Only treat expressions that are preceded by only indentation and followed by a newline.
        if (!match || !next.startsWith('\n')) {
            continue;
        }

        const indentation = match[1];

        // Fix previous and next parts
        parts[i] = parts[i].slice(0, parts[i].length - indentation.length);
        parts[i + 1] = parts[i + 1].slice(1);

        // Fix value end of line if needed
        if (!values[i].endsWith('\n')) {
            values[i] = `${values[i]}\n`;
        }

        // Indent value
        values[i] = values[i].replace(/(.*)\n/g, `${indentation}$1\n`);
    }

    return parts.flatMap((part, index) => [part, values[index]]).join('');
}

export default indentTag;