What is the reason behind the TypeError thrown when attempting to assign to `NaN` or `undefined` in JavaScript

Question

What is the reason behind the TypeError thrown when attempting to assign to `NaN` or `undefined` in JavaScript

A.S.: The question pertains to the type of error rather than the phenomenon itself

"use strict" results in a TypeError when system variables like NaN and undefined are modified.
But why is it categorized as a TypeError instead of a SyntaxError?

Edit: In reality, this is not a SyntaxError because there are no syntax errors in the code snippet.
The actual issue lies in the fact that certain protected objects cannot be altered manually; therefore, it could be considered an AccessError although such designation does not officially exist.
So, why do access-errors manifest similarly to type-errors?

javascript typeerror

Answer 1

Answer №1

Within ES5, there exist 6 distinct categories of native errors:

EvalError

The presence of this exception is not currently utilized in this specification. It continues to be included to maintain compatibility with earlier versions of the specification.
RangeError

Signifies that a numeric value has surpassed the allowable range.
ReferenceError

Demonstrates that an invalid reference value has been identified.
SyntaxError

Indicates the occurrence of a parsing error.
TypeError

Specifies that the actual type of an operand differs from the expected type.
URIError

Shows that one of the global URI handling functions was used in a manner conflicting with its definition.

In your situation, the error emerges when attempting to allocate a value to window.NaN or window.undefined, both non-writable properties.

Prior to assignment, the internal [[Put]] method verifies [[CanPut]], which returns false due to the property being non-enumerable. Consequently, [[Put]] will trigger an exception.

The core issue lies in the writability of the assigned reference (left operand in the assignment expression) differing from expectations. In light of the 6 aforementioned error types, TypeError appears most suitable, despite writability not aligning precisely with a type.

Answer 2

Within ES5, there exist 6 distinct categories of native errors:

EvalError

The presence of this exception is not currently utilized in this specification. It continues to be included to maintain compatibility with earlier versions of the specification.
RangeError

Signifies that a numeric value has surpassed the allowable range.
ReferenceError

Demonstrates that an invalid reference value has been identified.
SyntaxError

Indicates the occurrence of a parsing error.
TypeError

Specifies that the actual type of an operand differs from the expected type.
URIError

Shows that one of the global URI handling functions was used in a manner conflicting with its definition.

In your situation, the error emerges when attempting to allocate a value to window.NaN or window.undefined, both non-writable properties.

Prior to assignment, the internal [[Put]] method verifies [[CanPut]], which returns false due to the property being non-enumerable. Consequently, [[Put]] will trigger an exception.

The core issue lies in the writability of the assigned reference (left operand in the assignment expression) differing from expectations. In light of the 6 aforementioned error types, TypeError appears most suitable, despite writability not aligning precisely with a type.

Answer 3

Answer №2

The reason why you cannot assign values to NaN and undefined is because they are considered as read-only properties of the window object. These are not operators (such as > or +) or keywords (like var or class).

If you try to assign a value to NaN, Chrome will throw the following error:

TypeError: Cannot assign to read only property 'NaN' of object '#<Window>'

Answer 4

The reason why you cannot assign values to NaN and undefined is because they are considered as read-only properties of the window object. These are not operators (such as > or +) or keywords (like var or class).

If you try to assign a value to NaN, Chrome will throw the following error:

TypeError: Cannot assign to read only property 'NaN' of object '#<Window>'

Answer 5

Answer №3

It presents a valid argument that lacks coherence, albeit on the semantic level rather than the syntactic.

Consider this analogy:

1+1=42

While clearly incorrect, this equation is flawed in a different manner compared to:

42 1 1 + =

The former can be interpreted as an arithmetic statement (one plus one equals forty-two), whereas the latter is incomprehensible.

In a similar vein, the code snippet NaN=300; seems plausible in Javascript, equating `NaN` to 300. However, upon execution, the engine rejects it with a resounding "Nope, cannot perform this operation".

Answer 6

It presents a valid argument that lacks coherence, albeit on the semantic level rather than the syntactic.

Consider this analogy:

1+1=42

While clearly incorrect, this equation is flawed in a different manner compared to:

42 1 1 + =

The former can be interpreted as an arithmetic statement (one plus one equals forty-two), whereas the latter is incomprehensible.

In a similar vein, the code snippet NaN=300; seems plausible in Javascript, equating `NaN` to 300. However, upon execution, the engine rejects it with a resounding "Nope, cannot perform this operation".

Answer 7

Answer №4

Oops, something went wrong
When you see a syntax error in your code, it usually means you've misplaced an operator or used it incorrectly.
To learn more about "syntax errors", check out this article: https://en.wikipedia.org/wiki/Syntax_error

Mismatched data types
Type errors occur when a specific data type does not match the requirements of the program.
For detailed information on "data types", refer to this article: https://en.wikipedia.org/wiki/Data_type

Therefore, the type error is triggered because NaN and undefined are global constants representing data types with fixed values, and "strict mode" ensures their immutability to avoid potential issues.

Answer 8

Oops, something went wrong
When you see a syntax error in your code, it usually means you've misplaced an operator or used it incorrectly.
To learn more about "syntax errors", check out this article: https://en.wikipedia.org/wiki/Syntax_error

Mismatched data types
Type errors occur when a specific data type does not match the requirements of the program.
For detailed information on "data types", refer to this article: https://en.wikipedia.org/wiki/Data_type

Therefore, the type error is triggered because NaN and undefined are global constants representing data types with fixed values, and "strict mode" ensures their immutability to avoid potential issues.

What is the reason behind the TypeError thrown when attempting to assign to `NaN` or `undefined` in JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

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