ES6: 10 Use the Rest Operator with Function Parameters


In order to help us create more flexible functions, ES6 introduces the rest operator for function parameters. With the rest operator, you can create functions that take a variable number of arguments. These arguments are stored in an array that can be accessed later from inside the function.

Check out this code:

function howMany(...args) {
return "You have passed " + args.length + " arguments.";
}
console.log(howMany(0, 1, 2)); // You have passed 3 arguments
console.log(howMany("string", null, [1, 2, 3], { })); // You have passed 4 arguments.
The rest operator ...args eliminates the need to check the args array and allows us to apply map(), filter() and reduce() on the parameters array.
const sum = (function() {
"use strict";
return function sum(...args) {
return args.reduce((a, b) => a + b, 0); //return sum of array starting with default value of sum 0
};
})();
console.log(sum(1, 2, 3)); // 6

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