Category: JavaScript Answers

Sometimes, we may run into the ‘TypeError: invalid ‘instanceof’ operand "x"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: invalid ‘instanceof’ operand "x"’ when we’re developing JavaScript apps.

Fix the ‘TypeError: invalid ‘instanceof’ operand "x"’ When Developing JavaScript Apps

To fix the ‘TypeError: invalid ‘instanceof’ operand "x"’ when we’re developing JavaScript apps, we should make sure we’re using the instanceof operator on a constructor or class.

On Firefox, the error message for this error is TypeError: invalid 'instanceof' operand "x" or TypeError: "x" is not a function.

And on Chrome, the error message for this error is TypeError: Right-hand side of 'instanceof' is not an object or TypeError: Right-hand side of 'instanceof' is not callable.

For instance, the following code will all throw this error since the right operand of instanceof aren’t a constructor or class:

"test" instanceof "";
42 instanceof 0;     

function Foo() {}
const f = Foo();       
const x = new Foo();

x instanceof f;      
x instanceof x;

To fix this, we write:

typeof "test" == "string"; 
typeof 42 == "number"      

function Foo() {}
const f = Foo;               
const x = new Foo();

x instanceof f;            
x instanceof Foo;  

We use the typeof operator to test for primitive types.

And we use instanceof with Foo without calling to check if x is an instance of Foo.

Conclusion

To fix the ‘TypeError: invalid ‘instanceof’ operand "x"’ when we’re developing JavaScript apps, we should make sure we’re using the instanceof operator on a constructor or class.

On Firefox, the error message for this error is TypeError: invalid 'instanceof' operand "x" or TypeError: "x" is not a function.

And on Chrome, the error message for this error is TypeError: Right-hand side of 'instanceof' is not an object or TypeError: Right-hand side of 'instanceof' is not callable.

Sometimes, we may run into the ‘TypeError: can’t redefine non-configurable property "x"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: can’t redefine non-configurable property "x"’ when we’re developing JavaScript apps.

Fix the ‘TypeError: can’t redefine non-configurable property "x"’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t redefine non-configurable property "x"’ when we’re developing JavaScript apps, we should make sure we aren’t trying to redefine a non-configurable property with Object.defineProperty.

For instance, we’ll get the error if we have:

const obj = Object.create({});
Object.defineProperty(obj, "foo", {value: "bar"});
Object.defineProperty(obj, "foo", {value: "baz"});

where we tried to redefine the non-configurable foo property on obj.

To fix this, we should make foo configurable:

const obj = Object.create({});
Object.defineProperty(obj, "foo", {value: "bar", configurable: true});
Object.defineProperty(obj, "foo", {value: "baz", configurable: true}

Conclusion

To fix the ‘TypeError: can’t redefine non-configurable property "x"’ when we’re developing JavaScript apps, we should make sure we aren’t trying to redefine a non-configurable property with Object.defineProperty.

Sometimes, we may run into the ‘TypeError: can’t delete non-configurable array element’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: can’t delete non-configurable array element’ when we’re developing JavaScript apps.

Fix the ‘TypeError: can’t delete non-configurable array element’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t delete non-configurable array element’ when we’re developing JavaScript apps, we should make sure we aren’t trying to shorten an array with entries that aren’t configurable.

For instance, we’ll get the error if we write:

"use strict";
const arr = [];
Object.defineProperty(arr, 0, {value: 0});
Object.defineProperty(arr, 1, {value: "1"});

arr.length = 1;

since both entries are non-configurable.

We’ll also get the error if we write:

"use strict";
const arr = [1,2,3];
Object.seal(arr);

arr.length = 1;

to seal the array with Object.seal.

Instead, we write:

"use strict";
var arr = [];
Object.defineProperty(arr, 0, {value: 0, configurable: true});
Object.defineProperty(arr, 1, {value: "1", configurable: true});

arr.length = 1;

to make the entries configurable.

Or make a copy of the sealed array and then modify it:

"use strict";
const arr = [1,2,3];
Object.seal(arr);

const copy = [...arr];
copy.length = 1;

Conclusion

To fix the ‘TypeError: can’t delete non-configurable array element’ when we’re developing JavaScript apps, we should make sure we aren’t trying to shorten an array with entries that aren’t configurable.

Sometimes, we may run into the ‘TypeError: can’t define property "x": "obj" is not extensible’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: can’t define property "x": "obj" is not extensible’ when we’re developing JavaScript apps.

Fix the ‘TypeError: can’t define property "x": "obj" is not extensible’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t define property "x": "obj" is not extensible’ when we’re developing JavaScript apps, we should make sure we’re trying to add properties to an object that’s not extensible.

An object isn’t extensible after we called Object.preventExtensions on it.

On Edge, the error message for this error is TypeError: Cannot create property for a non-extensible object.

On Firefox, the error message for this error is TypeError: can't define property "x": "obj" is not extensible.

And on Chrome, the error message for this error is TypeError: Cannot define property: "x", object is not extensible.

For instance, we’ll get this error if we write:

'use strict';

const obj = {};
Object.preventExtensions(obj);

obj.x = 'foo';

Since we called Object.preventExtensions on obj before we tried to all the x property to obj.

Instead, we write:

'use strict';

const obj = {};

obj.x = 'foo';
Object.preventExtensions(obj);

to add the x property before we call Object.preventExtensions.

Conclusion

To fix the ‘TypeError: can’t define property "x": "obj" is not extensible’ when we’re developing JavaScript apps, we should make sure we’re trying to add properties to an object that’s not extensible.

An object isn’t extensible after we called Object.preventExtensions on it.

On Edge, the error message for this error is TypeError: Cannot create property for a non-extensible object.

On Firefox, the error message for this error is TypeError: can't define property "x": "obj" is not extensible.

And on Chrome, the error message for this error is TypeError: Cannot define property: "x", object is not extensible.

Sometimes, we may run into the ‘TypeError: can’t assign to property "x" on "y": not an object’ when we’re developing JavaScript apps.

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

Fix the ‘TypeError: can’t assign to property "x" on "y": not an object’ When Developing JavaScript Apps

To fix the ‘TypeError: can’t assign to property "x" on "y": not an object’ when we’re developing JavaScript apps, we should make sure that we’re creating a property on an object instead of a primitive value.

On Firefox, the error message for this error is TypeError: can't assign to property "x" on {y}: not an object.

On Chrome, the error message for this error is TypeError: Cannot create property 'x' on {y}.

For instance, if we have:

'use strict';

const foo = "string";
foo.bar = {};

then we’ll get the error since foo is a string, so we can’t add new properties to it.

To fix this, we should add properties to an object:

'use strict';

const foo = {};
foo.bar = {};

Conclusion

To fix the ‘TypeError: can’t assign to property "x" on "y": not an object’ when we’re developing JavaScript apps, we should make sure that we’re creating a property on an object instead of a primitive value.

On Firefox, the error message for this error is TypeError: can't assign to property "x" on {y}: not an object.

On Chrome, the error message for this error is TypeError: Cannot create property 'x' on {y}.