declare a boolean in Javascript

let isValid = true;

This code sets a boolean value of true to the variable isValid. A boolean value is a type of data type in programming with only two values, true or false. In this case, the value true is being assigned to the variable isValid. This value can be used to check if certain conditions are valid or not within a program. For example, you could use this variable to check if a user input is valid or not, and then depending on the value, perform an action.