Which of the following is a valid way to check if a variable is defined and not null in JavaScript?
The valid way to check if a variable is defined and not null is if(variable !== undefined && variable !== null);. This explicitly checks that the variable is neither undefined nor null. While if(variable) would work for many cases, it would consider other falsy values (like 0 or an empty string) as equivalent to being undefined or null, which might not be what you want. In modern JavaScript, you can also use the nullish coalescing operator to check for nullish values: variable ?? defaultValue will return defaultValue only if variable is null or undefined, not for other falsy values like 0 or ''.