getDecodedJWTComponents.js

function getDecodedJWTComponents( token ) {

    const periodSplitToken = token.split( '.' );

    const endcodedJWTHeader = periodSplitToken[0];
    
    const header = getDecodedJWTComponent( endcodedJWTHeader );

    const endcodedJWTPayload = periodSplitToken[1];

    const payload = getDecodedJWTComponent( endcodedJWTPayload );

    return {

        header,
        payload
    };
}

// helper function
function getDecodedJWTComponent( encodedTokenComponent ) {

    const tokenComponentAsBuffer = new Buffer( encodedTokenComponent, 'base64' );

    const tokenComponentAsString = tokenComponentAsBuffer.toString( 'ascii' );

    const decodedTokenComponent = JSON.parse( tokenComponentAsString );

    return decodedTokenComponent;
}

Related to this article about JWT.