tosource/src/tosource.ts

/* toSource by Marcello Bastea-Forte - zlib license */
export default function toSource(
  object: unknown,
  replacer?: (a: any) => any,
  indent: false | string = '  ',
  startingIndent: string = '',
): string {
  const seen: any[] = [];
  return walk(
    object,
    replacer,
    indent === false ? '' : indent,
    startingIndent,
    seen,
  );

  function walk(
    object: any,
    replacer: ((a: any) => any) | undefined,
    indent: string,
    currentIndent: string,
    seen: any[],
  ): string {
    const nextIndent = currentIndent + indent;
    object = replacer ? replacer(object) : object;

    switch (typeof object) {
      case 'string':
        return JSON.stringify(object);
      case 'number':
        if (Object.is(object, -0)) {
          return '-0';
        }
        return String(object);
      case 'boolean':
      case 'undefined':
        return String(object);
      case 'function':
        return object.toString();
    }

    if (object === null) {
      return 'null';
    }
    if (object instanceof RegExp) {
      return object.toString();
    }
    if (object instanceof Date) {
      return `new Date(${object.getTime()})`;
    }
    if (object instanceof Set) {
      return `new Set(${walk(
        Array.from(object.values()),
        replacer,
        indent,
        nextIndent,
        seen,
      )})`;
    }
    if (object instanceof Map) {
      return `new Map(${walk(
        Array.from(object.entries()),
        replacer,
        indent,
        nextIndent,
        seen,
      )})`;
    }

    if (seen.indexOf(object) >= 0) {
      return '{$circularReference:1}';
    }
    seen.push(object);

    function join(elements: any[]) {
      return (
        indent.slice(1) +
        elements.join(',' + (indent && '\n') + nextIndent) +
        (indent ? ' ' : '')
      );
    }

    if (Array.isArray(object)) {
      return `[${join(
        object.map((element) =>
          walk(element, replacer, indent, nextIndent, seen.slice()),
        ),
      )}]`;
    }
    const keys = Object.keys(object);
    if (keys.length) {
      return `{${join(
        keys.map(
          (key) =>
            (legalKey(key) ? key : JSON.stringify(key)) +
            ':' +
            walk(object[key], replacer, indent, nextIndent, seen.slice()),
        ),
      )}}`;
    }
    return '{}';
  }
}

const KEYWORD_REGEXP =
  /^(abstract|boolean|break|byte|case|catch|char|class|const|continue|debugger|default|delete|do|double|else|enum|export|extends|false|final|finally|float|for|function|goto|if|implements|import|in|instanceof|int|interface|long|native|new|null|package|private|protected|public|return|short|static|super|switch|synchronized|this|throw|throws|transient|true|try|typeof|undefined|var|void|volatile|while|with)$/;

function legalKey(key: string) {
  return (
    /^([a-z_$][0-9a-z_$]*|[0-9]+)$/gi.test(key) && !KEYWORD_REGEXP.test(key)
  );
}