Code for refreshing AWS Cognito user pool tokens using refresh token from browser.

aws-cognito-refresh-tokens-from-brower.js

fetch("https://cognito-idp.<cognito-user-pool-region>.amazonaws.com/", {
    headers: {
        "X-Amz-Target": "AWSCognitoIdentityProviderService.InitiateAuth",
        "Content-Type": "application/x-amz-json-1.1",
    },
    mode: 'cors',
    cache: 'no-cache',
    method: 'POST',
    body: JSON.stringify({
        ClientId: "<cognito-user-pool-client-id>",
        AuthFlow: 'REFRESH_TOKEN_AUTH',
        AuthParameters: {
            REFRESH_TOKEN: "<cognito-refresh-toke>",
            //SECRET_HASH: "your_secret", // In case you have configured client secret
        }
    }),
}).then((res) => {
    return res.json(); // this will give jwt id and access tokens
});