Non-array argument to function with destructured Array default parameters gives a TypeError

Question

I've defined the function prod which makes use of ES6's default arguments and destructuring:

function prod([a, b, c] = [1, 2, 3]) {
  console.log(a * b * c);
}

When called without arguments, it logs 6 to the console as expected.

prod() // 6

When called with an array of arguments, it logs the correct product:

prod([2, 3, 4]) // 24

When called with a number of arguments, it throws an error:

prod(2, 3, 4) // Uncaught TypeError: undefined is not a function(…)

Why does it throw an undefined is not a function error?

Edit

I understand why it throws an error. What I don't understand is why it throws that particular error.

I did and it worked as expected. I want to know why the error is undefined is not a function. I'll update the question. — gnerkus
– gnerkus, Commented Jan 30, 2016 at 20:46

winhowes · Accepted Answer · 2016-01-30 20:53:26Z

2

The issue is that it wants an array for arguments, not 3 separate arguments. So doing this: prod([1, 2, 3]) will give you back your expected answer.

When I run prod(1, 2, 3) in Firefox I get this error: TypeError: [Symbol.iterator] is not a function.

According to the mdn docs (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/iterator#Non-well-formed_iterables)

The function is expecting an interable object to be returned with the method @@iterator. Because that method doesn't exist (and therefore isn't a function) you're getting the is not a function error

edited Jan 30, 2016 at 20:53

answered Jan 30, 2016 at 20:46

winhowes

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2 Comments

gnerkus Over a year ago

Thank you. I understand why it throws the error. I want to know why it throws that particular error.

gnerkus Over a year ago

Thank you so much for this. I'll do to test code in Firefox's Dev tools as well. The error in Chrome's dev tools provided much less information. The stack trace wasn't helpful either.

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