Complete a flat or nested JSON (without arrays). Default to "VALUE" for a missing key value.

parse-json.ts

const partialJson = `{"a1":"v1","a2":{"a3":"v3","a4":{"a5":"v5","a6"`;
// const partialJson = `{"a1":"v1","a2":{"a3":"v3","a4":{"a5`;
// const partialJson = `{"a1":"v1","a2":{"a3":"v3","a4":{"a5":"v5"}`;

const removeQuote = (str: string) => str.replace(/"/g, '');

const findSubstringAtIndex = (json: string) => (index: number) =>
  json.substring(index + 1);

const insertClosingBrackets = (json: string) => {
  const openBracketsCount = json.match(/{/g)?.length || 0;
  const closeBracketsCount = json.match(/}/g)?.length || 0;
  const closingBrackets = Array(openBracketsCount - closeBracketsCount)
    .fill('}')
    .join('');
  return `${json}${closingBrackets}`;
};

const completePartialJson = (partialJson: string) => {
  const lastIndexOfComma = partialJson.lastIndexOf(',');
  const lastIndexOfColon = partialJson.lastIndexOf(':');
  const isLastCharClosingBracket = partialJson[partialJson.length - 1] === '}';
  const findLastItem = findSubstringAtIndex(partialJson);

  // check if the last character is a closing bracket, only add matching brackets in that case
  if (isLastCharClosingBracket) {
    return insertClosingBrackets(partialJson);
  }

  if (lastIndexOfComma > lastIndexOfColon) {
    const lastItem = findLastItem(lastIndexOfComma);
    const formattedJson = partialJson.replace(
      lastItem,
      `"${removeQuote(lastItem)}":"VALUE"`
    );
    return insertClosingBrackets(formattedJson);
  }

  let updatedJson = partialJson;
  const lastIndexOfOpeningBracket = partialJson.lastIndexOf('{');

  if (lastIndexOfOpeningBracket > lastIndexOfColon) {
    const lastItem = findLastItem(lastIndexOfOpeningBracket);
    updatedJson = partialJson.replace(
      lastItem,
      `"${removeQuote(lastItem)}":"VALUE"`
    );
  } else {
    const lastItem = findLastItem(lastIndexOfColon);
    updatedJson = partialJson.replace(lastItem, `"${removeQuote(lastItem)}"`);
  }

  return insertClosingBrackets(updatedJson);
};

console.log(completePartialJson(partialJson));