JSON.parse() with reviver

ES5+

Uses a reviver function to transform values during JSON parsing.

Syntax

JSON.parse(text, reviver)

Parameters

text string

The string to parse as JSON

reviver Function

A function that receives key and value, and returns the transformed value

Return Value

any

The transformed JavaScript value

Examples

JavaScript

const json = '{"created":"2024-01-15","count":"42"}';
const obj = JSON.parse(json, (key, value) => {
  if (key === 'created') return new Date(value);
  if (key === 'count') return parseInt(value, 10);
  return value;
});
console.log(obj.created instanceof Date);
console.log(typeof obj.count);

Output:

// true number

📌 When to Use

Use a reviver function when parsing JSON that contains date strings, numeric strings that should be numbers, or any values that need type conversion.

⚠️ Common Mistakes

Not returning the value from the reviver, which replaces it with undefined.

Modifying nested objects before their children are processed - reviver processes bottom-up.

✅ Best Practices

Use ISO 8601 date format detection: /^\d{4}-\d{2}-\d{2}T/.test(value) for reliable date parsing.

⚡ Performance Notes

The reviver is called for every value, so keep pattern matching efficient. For large datasets, consider transforming after parsing.

🌍 Real World Example

Type-Safe JSON Parser

Parse JSON with automatic type conversion for dates and numeric strings.

function typedParse(jsonString) {
  const isoDatePattern = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/;

  return JSON.parse(jsonString, (key, value) => {
    // Convert ISO date strings to Date objects
    if (typeof value === 'string' && isoDatePattern.test(value)) {
      const date = new Date(value);
      return isNaN(date.getTime()) ? value : date;
    }

    // Convert numeric strings (from certain APIs)
    if (key === 'count' || key === 'total' || key === 'amount') {
      const num = Number(value);
      return isNaN(num) ? value : num;
    }

    return value;
  });
}

const json = '{"created":"2024-01-15T10:30:00Z","count":"42","name":"Test"}';
const parsed = typedParse(json);
console.log(parsed.created instanceof Date); // true
console.log(typeof parsed.count); // "number"

Related Methods

JSON.parse() JSON.stringify()