It was really confusing for me to read this syntax in Javascript:
router.route('/:id')
.put((...args) => controller.update(...args))
.get((...args) => controller.findById(...args));What does ...args mean?
54 Answers
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.
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.
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: 6If you know some Python syntaxes, it is exactly the same as *args. Since *args (Python) is tuple object and Javascript has no tuple like Python, ..args is an Array object.
