JavaScript and Open Data

1Variables: 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.

There are two kinds of values:

• – primitive values: numbers or strings in literal notation, “built-in” values such as true, false, NaN, infinity, null, undefined, etc.;
• – objects, including functions, and arrays, which are “containers”, and, as such, their value is the address of the container.

NOTE.– The content of a container can be modified, while the address remains unchanged (this is important for understanding the “const” declaration below).

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 such as:

var tableauCandidats = [];
var n = 0; // … …

We will show why it is highly preferable to write:

const tableauCandidats = [];
let n = 0; // … …

Let us look back at the two steps in the interpretation of JavaScript (simplified):

• – Lexical-time: It deals with the lexical analysis of the code. The names of the declared functions and variables are recorded in a tree structure (lexical tree). Declarations are moved up to the beginning of their block of code: this is named the “hoisting”. Functions are hoisted first, for they define the nodes of the structure, then variables ...