Category: JavaScript Answers

Sometimes, we may run into the ‘TypeError: can’t access property "x" of "y"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: can’t access property "x" of "y"’ when we’re developing JavaScript apps.

Fix the ‘TypeError: can’t access property "x" of "y"’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t access property "x" of "y"’ when we’re developing JavaScript apps, we should make sure the object we’re trying to access the property for isn’t undefined or null.

On Edge, the error message for this error is TypeError: Unable to get property {x} of undefined or null reference.

On Firefox, the error message for this error is TypeError: can't access property {x} of {y}, TypeError: {y} is undefined, can't access property {x} of it, or TypeError: {y} is null, can't access property {x} of it.

For instance, if we have:

const foo = undefined;
foo.substring(1);

const foo = null;
foo.substring(1);

then we’ll get this error since foo is either undefined or null.

Instead, we check if the variable isn’t undefined or null before accessing its properties:

if (typeof foo !== 'undefined') {
  // ...
}

We check that foo isn’t undefined with the typeof` operator.

Also, we can use the optional chaining operator (?.) to access a property:

foo?.substring(1)

then no error will be thrown if foo is undefined or null.

Conclusion

To fix the ‘TypeError: can’t access property "x" of "y"’ when we’re developing JavaScript apps, we should make sure the object we’re trying to access the property for isn’t undefined or null.

On Edge, the error message for this error is TypeError: Unable to get property {x} of undefined or null reference.

On Firefox, the error message for this error is TypeError: can't access property {x} of {y}, TypeError: {y} is undefined, can't access property {x} of it, or TypeError: {y} is null, can't access property {x} of it.

We can use the optional chaining operator (?.) to access a property.

Sometimes, we may run into the ‘TypeError: can’t access dead object’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: can’t access dead object’ when we’re developing JavaScript apps.

Fix the ‘TypeError: can’t access dead object’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t access dead object’ when we’re developing JavaScript apps, we should make sure we check that the object is still available before accessing it.

To do this, we can use the Component.utils.isDeadWrapper method:

if (Components.utils.isDeadWrapper(window)) {
  // dead
}

We can also try to catch exceptions from trying to accessing unavailable objects:

try {
  String(window);
}
catch (e) {
  console.log("window is dead");
}

Conclusion

To fix the ‘TypeError: can’t access dead object’ when we’re developing JavaScript apps, we should make sure we check that the object is still available before accessing it.

Sometimes, we may run into the ‘TypeError: X.prototype.y called on incompatible type’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: X.prototype.y called on incompatible type’ when we’re developing JavaScript apps.

Fix the ‘TypeError: X.prototype.y called on incompatible type’ When Developing JavaScript Apps

To fix the ‘TypeError: X.prototype.y called on incompatible type’ when we’re developing JavaScript apps, we should make sure that we’re calling Function.prototype.call, Function.prototype.apply, or Function.prototype.bind with a this value that is expected.

For instance, if we have:

const mySet = new Set;
['bar', 'baz'].forEach(mySet.add);

then we’ll get the error since the this value in mySet.add isn’t mySet.

To fix this, we write:

const mySet = new Set;
['bar', 'baz'].forEach(mySet.add.bind(mySet));

to call bind with mySet to set mySet as the value of this in the mySet.add method.

Conclusion

To fix the ‘TTypeError: X.prototype.y called on incompatible type’ when we’re developing JavaScript apps, we should make sure that we’re calling Function.prototype.call, Function.prototype.apply, or Function.prototype.bind with a this value that is expected.

Sometimes, we may run into the ‘TypeError: Reduce of empty array with no initial value’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: Reduce of empty array with no initial value’ when we’re developing JavaScript apps.

Fix the ‘TypeError: Reduce of empty array with no initial value’ When Developing JavaScript Apps

To fix the ‘TypeError: Reduce of empty array with no initial value’ when we’re developing JavaScript apps, we should make sure that we’re calling the Array.prototype.reduce method with an initial value.

For instance, if we have:

const ints = [0, -1, -2, -3, -4, -5];
ints.filter(x => x > 0)
    .reduce((x, y) => x + y)

then we get the error since we didn’t pass in a 2nd argument to reduce.

So the x parameter in the reduce callback is set to undefined and the callback fails to run.

To fix this, we write:

const ints = [0, -1, -2, -3, -4, -5];
ints.filter(x => x > 0)
    .reduce((x, y) => x + y, 0)

to pass in a 2nd argument to reduce.

Then x is set to 0 and that’s returned since filter returned an empty array.

Conclusion

To fix the ‘TypeError: Reduce of empty array with no initial value’ when we’re developing JavaScript apps, we should make sure that we’re calling the Array.prototype.reduce method with an initial value.

Sometimes, we may run into the ‘TypeError: More arguments needed’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: More arguments needed’ when we’re developing JavaScript apps.

Fix the ‘TypeError: More arguments needed’ When Developing JavaScript Apps

To fix the ‘TypeError: More arguments needed’ when we’re developing JavaScript apps, we should make sure that we’re passing in all the required arguments to the Object.create and Object.setPrototypeOf methods.

Different variations of this error includes:

TypeError: argument is not an Object and is not null (Edge)
TypeError: Object.create requires at least 1 argument, but only 0 were passed
TypeError: Object.setPrototypeOf requires at least 2 arguments, but only 0 were passed
TypeError: Object.defineProperties requires at least 1 argument, but only 0 were passed

For instance, if we have:

const obj = Object.create();
const obj = Object.setPrototypeOf({});

In the first line, we didn’t pass in any arguments into Object.create which are expected.

And in the 2nd line, we’re missing the 2nd argument.

To fix this, we write:

const obj = Object.create(null);
const obj = Object.setPrototypeOf({}, null);

Conclusion

To fix the ‘TypeError: More arguments needed’ when we’re developing JavaScript apps, we should make sure that we’re passing in all the required arguments to the Object.create and Object.setPrototypeOf methods.