you don't need to pass the entire encoded string to atob method, you need to split the encoded string and pass the required string to atob method
const token= "eyJhbGciOiJIUzUxMiJ9.eyJzdWIiOiJob3NzYW0iLCJUb2tlblR5cGUiOiJCZWFyZXIiLCJyb2xlIjoiQURNSU4iLCJpc0FkbWluIjp0cnVlLCJFbXBsb3llZUlkIjoxLCJleHAiOjE2MTI5NDA2NTksImlhdCI6MTYxMjkzNzA1OX0.8f0EeYbGyxt9hjggYW1vR5hMHFVXL4ZvjTA6XgCCAUnvacx_Dhbu1OGh8v5fCsCxXQnJ8iAIZDIgOAIeE55LUw"
console.log(atob(token.split(".")[1]));
_x000D_