Declare Variables

• You can also declare multiple variables in one line:

let user = 'John', 
	age = 25, 
	message = 'Hello';

• In older scripts, you may also find another keyword: var instead of let.
• When the name contains multiple words, camelCase is commonly used.
  • Number cannot be used as the first character.
  • The dollar sign '$' and the underscore '_' can also be used in names, even as the first character.
• Normally, we need to define a variable before using it.
  • But traditionally, it was technically possible to create a variable by a mere assignment of the value without using let.
  • This still works now if we don’t put use strict in our scripts to maintain compatibility with old scripts.

// Note: no "use strict" in this example
num = 5; // the variable "num" is created if it didn't exist
alert(num); // 5

• This is a bad practice and would cause an error in strict mode:

"use strict";
num = 5; // error: num is not defined

var Differences

• var has no block scope and therefore, var variables are either function-wide or global, they are visible through blocks.
• var basically ignores the code blocks, so we get a global variable.

if (true) {
  var test = true; // use "var" instead of "let"
}
alert(test); // true

for (var i = 0; i < 10; i++) {
  // ...
}
alert(i); // 10, "i" is visible after loop, it's a global variable

• If a code block is inside a function, then var becomes a function-level variable:

function sayHi() {
  if (true) {
    var phrase = "Hello";
  }

  alert(phrase); // works
}

sayHi();
alert(phrase); // Error: phrase is not defined

Hoisting

• var variables are defined from the beginning of the function, no matter where the definition is (assuming that the definition is not in the nested function).
• So this code:

function sayHi() {
  phrase = "Hello";
  alert(phrase);
  var phrase;
}

• Is technically parses this way: