To get the length of a Buffer instance in Node.js, you can use the length property in the Buffer instance.
Let's say we have a buffer from the string Hello World! like this,
// Buffer from string Hello World!
const buff = Buffer.from("Hello World!", "utf-8");
Now let's get its length using the property length in the buff object.
// Buffer from string Hello World!
const buff = Buffer.from("Hello World!", "utf-8");
// get the length of the Buffer
const length = buff.length;
console.log(length); // 12
See this example live in repl.it.