How to Decode JWT Tokens in JavaScript Without Using a Library?

Decoding JWT Tokens in JavaScript Without a Library

Decoding the payload of a JWT token without relying on external libraries is a common task in frontend development. To achieve this, you can follow these steps:

Step 1: Extract Token Segments

The JWT token consists of three segments separated by periods: header, payload, and signature. Extract the second segment, which is the payload.

Step 2: Decode Base64 Payload

The payload is encoded in Base64. Decode it using the atob() function in the browser or Buffer.from().toString() in Node.js.

Step 3: URL Decode Payload

The atob() function uses base64, which needs to be URL decoded to produce the actual JSON payload.

Step 4: Parse JSON payload

Convert the decoded payload back to a JavaScript object using JSON.parse().

Example

Consider this example JWT token:

xxxxxxxx.XXXXXXXX.xxxxxxxx

Decoding the payload using the steps above would result in the following JSON object:

{
  "exp": 10012016,
  "name": "john doe",
  "scope": ["admin"]
}

Note: This method does not validate the authenticity of the token, relying only on the signature of the token provider.

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