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JSON.parse()

Parses a JSON string, constructing the JavaScript value or object described by the string.

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JSON.parse(text, [reviver])
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What it does

Overview

Parses a JSON string, constructing the JavaScript value or object described by the string.

The `JSON.parse()` method is a static function that takes a JSON formatted string and converts it into a JavaScript value or object. This is a fundamental operation for working with data received from web servers (e.g., via AJAX requests), local storage, or file systems, as these sources often transmit data as JSON strings. The method can parse JSON representations of objects, arrays, strings, numbers, booleans, and `null`. If the input `text` is not a valid JSON string, `JSON.parse()` will throw a `SyntaxError`. The optional `reviver` argument is a function that can transform the parsed value before it is returned. It's called for each key-value pair in the parsed object (or array), allowing for custom deserialization logic, such as converting date strings back into `Date` objects. The time complexity is generally O(n) where n is the length of the JSON string, as it needs to iterate through the string to build the object.

Quick reference

Syntax

JSON.parse(text, [reviver])

Inputs

Parameters

textString · The JSON string to parse.
reviver (optional)Function · An optional function that transforms the results.

See it in practice

Examples

1

Parsing a simple JSON object

const jsonString = '{"name": "Alice", "age": 30}';
const obj = JSON.parse(jsonString);
console.log(obj.name);
console.log(obj.age);
Output:
Alice 30

The JSON string is converted into a JavaScript object, and its properties can be accessed.

2

Parsing a JSON array

const jsonArrayString = '[10, "hello", true]';
const arr = JSON.parse(jsonArrayString);
console.log(arr[0]);
console.log(arr[1]);
Output:
10 hello

The JSON string representing an array is converted into a JavaScript array.

3

Using a reviver function

const jsonDateString = '{"event": "meeting", "date": "2023-10-26T10:00:00.000Z"}';
const parsedWithReviver = JSON.parse(jsonDateString, (key, value) => {
  if (key === 'date') {
    return new Date(value);
  }
  return value;
});

console.log(parsedWithReviver.date instanceof Date);
console.log(parsedWithReviver.date.getFullYear());
Output:
true 2023

The `reviver` function intercepts the 'date' property and converts its string value into a `Date` object.

Debug faster

Common Errors

1

SyntaxError for invalid JSON

Cause: Attempting to parse a string that is not valid JSON (e.g., unquoted keys, trailing commas, single quotes instead of double quotes for strings).

Fix: Ensure the input string strictly adheres to the JSON specification. Use a JSON validator if unsure.

const invalidJson = "{name: 'Bob'}"; // Keys must be double-quoted, strings must be double-quoted
try {
  JSON.parse(invalidJson);
} catch (e) {
  console.error(e.name + ": " + e.message);
}

Runtime support

Compatibility

Node.jsAll modernES5

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Parses a JSON string, constructing the JavaScript value or object described by the string.

text: The JSON string to parse. reviver: An optional function that transforms the results.

SyntaxError for invalid JSON: Ensure the input string strictly adheres to the JSON specification. Use a JSON validator if unsure.