函数体中的 JavaScript 逗号分隔值

Question

I recently read some source code that contained a function similar to the one below:

function foo(someArray) {
  const arrayPrime = someArray.map(x => x * 2,);
  return arrayPrime;
}

To my surprise, the code above is valid - no syntax errors.

Playing with this code more, we can add any number of values separated by commas without generating a syntax error:

function foo(someArray) {
  const arrayPrime = someArray.map(x => x * 2, 3, 4, 5, 6);
  return arrayPrime;
}

Invoking both versions of foo with foo([11,2,3]) yields 22,4,6 .

Can someone explain why this is legal?

Answer 1

Those are additional arguments to the map() method.

map() takes 1 optional argument, the value to be passed as the this context to the callback function. Since arrow functions can't have their this value altered, so it's ignored in this case.

The remaining arguments are ignored completely. JavaScript doesn't report errors when a function is called with more arguments than it needs. (It also doesn't report errors for missing arguments, they're just set to undefined ).