What is the meaning of "...args" (three dots) in a function definition?

The meaning of “…args” (three dots) is Javascript spread operator.

function sum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

console.log(sum(...numbers));
// expected output: 6

With respect to (...args) =>, ...args is a rest parameter. It always has to be the last entry in the parameter list and it will be assigned an array that contains all arguments that haven't been assigned to previous parameters.

It's basically the replacement for the arguments object. Instead of writing

function max() {
  var values = Array.prototype.slice.call(arguments, 0);
  // ...
}
max(1,2,3);

you can write

function max(...value) {
  // ...
}
max(1,2,3);

Also, since arrow functions don't have an arguments object, this is the only way to create variadic (arrow) functions.

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As controller.update(...args), see What is the meaning of "foo(...arg)" (three dots in a function call)? .

Essentially, what's being done is this:

.put((a, b, c) => controller.update(a, b, c))

Of course, what if we want 4 parameters, or 5, or 6? We don't want to write a new version of the function for all possible quantities of parameters.

The spread operator (...) allows us to accept a variable number of arguments and store them in an array. We then use the spread operator again to pass them to the update function:

.put((...args) => controller.update(...args))

This is transparent to the update function, who receives them as normal arguments.