makeArrowString(paramType, indent, key, valueDump) {
    const startWith = `${indent}${this.spaces}`;
    const valuePart = `${valueDump},\n`;

    let keyPart;
    if (Number.isInteger(parseInt(key)) || (paramType === 'array' && typeof key !== 'string')) {
      keyPart = `[${key}]`;
    } else {
      keyPart = `'${key}'`;
    }

    return `${startWith + keyPart} => ${valuePart}`;
  }
Make the arrow string.

@param {string} paramType
@param {string} indent
@param {string|number} key
@param {*} valueDump
@return {string}