Invalid Variable Name in JavaScript

Explanation:

In JavaScript, variable names must follow certain rules in order to be valid. These rules are as follows:

1. Start with a letter, underscore (_), or dollar sign ($):
- Variable names must begin with a letter (a-z or A-Z), an underscore (_), or a dollar sign ($).
- They cannot start with a number or any other character.
- This is to ensure that variables are easily distinguishable from other JavaScript syntax elements like keywords or operators.

2. Consist of letters, numbers, underscores, or dollar signs:
- After the first character, variable names can include letters, numbers, underscores, or dollar signs.
- They cannot contain spaces or special characters like punctuation marks.

3. Cannot be a reserved keyword:
- Certain words in JavaScript are reserved keywords that have predefined meanings and cannot be used as variable names.
- Examples of reserved keywords include "if," "else," "function," "for," "while," etc.

Invalid Variable Name:
The given variable name "123variable" is not valid in JavaScript because it violates the first rule mentioned above. It starts with a number, which is not allowed. Variable names must begin with a letter, underscore, or dollar sign.

Summary:
In JavaScript, variable names must follow specific rules to be valid. They must start with a letter, underscore, or dollar sign, and can consist of letters, numbers, underscores, or dollar signs. Variable names cannot be reserved keywords. The variable name "123variable" is not valid because it starts with a number, which is not allowed.