JavaScript: Function's “arguments” Object

arguments is a builtin variable, available in function body defined using function. (it is not in Arrow Function)

• The arguments object is a Array-Like Object.
• Each slot is the value of argument, when a function is called.

function ff() {
  return arguments;
}

console.log(ff("a", "b"));
// { "0": "a", "1": "b" }

console.log(Array.isArray(ff()) === false);

TIP: you should never use the argument object. Always declare function parameters. It makes your code readable. If needed, use JavaScript: Function Rest Parameters .