TypeError: Cannot match against 'undefined' or 'null'

Question

Code

client.createPet(pet, (err, {name, breed, age}) => {
  if (err) {
    return t.error(err, 'no error')
  }
  t.equal(pet, {name, breed, age}, 'should be equivalent')
})

Error

client.createPet(pet, (err, {name, breed, age}) => {
                        ^

TypeError: Cannot match against 'undefined' or 'null'.

Why am I getting this error? My knowledge of ES6 led me to presume that this error should only arise if the array or object being destructured or its children is undefined or null.

I wasn't aware that function parameters are used as a match. And if they are then why is it only an error if I try to destructure one of them? (that isn't undefined or null).

Answer 1

this error should only arise if the array or object being destructured or its children is undefined or null.

Exactly. In your case, the object being destructured is either undefined or null. For example,

function test(err, {a, b, c}) {
  console.log(err, a, b, c);
}

test(1, {a: 2, b: 3, c: 4});
// 1 2 3 4
test(1, {});
// 1 undefined undefined undefined
test(1);
// TypeError: Cannot match against 'undefined' or 'null'.