1、 When a variable is declared, but no value is assigned to it, it features the value of 'undefined'. Conversely, 'undefined' is not synonymous with 'null'; 'null' is an assignment value that means no value or no object, while undefined implies a variable has been declared but not defined， It's worth noting that analyzing the code regularly for issues such as 'undefined' helps in maintaining code quality and optimizing its performance. Nevertheless, this is primarily a foundation to the JavaScript language, rather than a common trait amidst all programming languages.
2、 This occurrence in computer programming languages is often utilized to assess if a variable exists or not， It's a type of unintentional error that developers face when they're trying to access a variable that does not exists.Undefined, in computer programming terminology, denotes a state where a variable has been declared but no value has been assigned to it， Following the same principle, the HTML element properties that haven’t been assigned yet would also be labeled as 'undefined'. The challenges that arise due to 'undefined' variables can be resolved by applying proper initializations and conditions in the code. For instance, in Java and Python, attempts to access a variable before it's declared will lead to an error.
