Variables: Declaration, Definition and Type

Variables are made to receive values directly or through evaluated expressions compounding values and operators or results of function calls: the value is stored internally and the variable references this storage, and takes its value and its type. To fetch the value of a variable, it must have been identified in the lexical phase: which means to be “declared”. Often, JavaScript code starts with declaration instructions. This chapter looks back at the two steps in the interpretation of JavaScript (simplified): lexical‐time and run‐time. It details this double mechanism: declaration of the variables, initialization to undefined at lexical‐time and definition of the variables at run‐time. For a better understanding of the declaration mechanism in JavaScript, we must first learn what the “scope” of a variable is: the function declarations determine the overall logics of the notion of scope.