svelte/src/utils/deindent.ts

const start = /\n(\t+)/;

export default function deindent(strings: string[], ...values: any[]) {
	const indentation = start.exec(strings[0])[1];
	const pattern = new RegExp(`^${indentation}`, 'gm');

	let result = strings[0].replace(start, '').replace(pattern, '');

	let trailingIndentation = getTrailingIndentation(result);

	for (let i = 1; i < strings.length; i += 1) {
		let expression = values[i - 1];
		const string = strings[i].replace(pattern, '');

		if (Array.isArray(expression)) {
			expression = expression.length ? expression.join('\n') : null;
		}

		if (expression || expression === '') {
			const value = String(expression).replace(
				/\n/g,
				`\n${trailingIndentation}`
			);
			result += value + string;
		} else {
			let c = result.length;
			while (/\s/.test(result[c - 1])) c -= 1;
			result = result.slice(0, c) + string;
		}

		trailingIndentation = getTrailingIndentation(result);
	}

	return result.trim().replace(/\t+$/gm, '');
}

function getTrailingIndentation(str: string) {
	let i = str.length;
	while (str[i - 1] === ' ' || str[i - 1] === '\t') i -= 1;
	return str.slice(i, str.length);
}