svelte/src/utils/getCodeFrame.ts

import repeat from './repeat';

function tabsToSpaces(str: string) {
	return str.replace(/^\t+/, match => match.split('\t').join('  '));
}

export default function getCodeFrame(
	source: string,
	line: number,
	column: number
) {
	const lines = source.split('\n');

	const frameStart = Math.max(0, line - 2);
	const frameEnd = Math.min(line + 3, lines.length);

	const digits = String(frameEnd + 1).length;

	return lines
		.slice(frameStart, frameEnd)
		.map((str, i) => {
			const isErrorLine = frameStart + i === line;

			let lineNum = String(i + frameStart + 1);
			while (lineNum.length < digits) lineNum = ` ${lineNum}`;

			if (isErrorLine) {
				const indicator =
					repeat(' ', digits + 2 + tabsToSpaces(str.slice(0, column)).length) + '^';
				return `${lineNum}: ${tabsToSpaces(str)}\n${indicator}`;
			}

			return `${lineNum}: ${tabsToSpaces(str)}`;
		})
		.join('\n');
}