A variable is a "named storage" for data. We can use variables to store goodies, visitors and other data. To create a variable in JavaScript, we need to use the let keyword.
The statement below creates (in other words: declares or defines) a variable with the name "message":
var message;
Now we can put some data into it by using the assignment operator =:
var message;
console.log(message); //undefined
message = 'Hello'; // store the string
console.log(message); // 'Hello'
let num = 10;
console.log(num); // 10The var keyword is almost the same as let. It also declares a variable, but in a slightly different, "old-school" fashion. There are subtle differences between let and var, but they do not matter for us yet. We'll learn in detail later, in the coming articles.
Normally, we need to define a variable before using it. But in the old times, it was technically possible to create a variable by a mere assignment of the value, without let.
This still works now if we don't put use strict, the behavior is kept for compatibility with old scripts.
// note: no "use strict" in this example
num = 5; // the variable "num" is created if didn't exist
console.log(num); // 5That's a bad practice, it gives an error in the strict mode:
"use strict";
num = 5; // error: num is not defined