Hoisting in JavaScript is a behavior where variable and function declarations are moved, or “hoisted,” to the top of their containing scope during the compilation phase, before the code is executed. It's important to note that only the declarations are hoisted, not the initializations.

The variable declaration using var is hoisted, but the initialization is not. So, if you reference a variable before it's initialized, it will return undefined.
Example:

console.log(a); // undefined
var a = 5;
console.log(a); // 5

What's happening behind the scenes is something like this:

var a;
console.log(a); // undefined
a = 5;
console.log(a); // 5

Variables that are declared without any keyword (such as var, let, or const) are not hoisted. Rather, they become global variables (if not already within a global scope), and their declaration and initialization happen at the line where they are assigned.

Example:

console.log(myVar); // ReferenceError: myVar is not defined
myVar = "abc";
console.log(myVar); // "abc"

In this case, myVar is not declared or initialized before the first console.log, so attempting to access it results in a ReferenceError.

If you were to declare the variable without initializing it within a function, it would be hoisted as a property of the global object (such as window in a browser environment):

function myFunction() {
  myVar = "abc";
}
myFunction();
console.log(window.myVar); // "abc" in a browser environment

This behavior can lead to unexpected results and make the code harder to understand and maintain.

Function declarations are also hoisted, and this includes both the name and the code of the function.
Example:

console.log(myFunction()); // "I'm hoisted!"
 
function myFunction() {
  return "I'm hoisted!";
}

In this case, the entire function declaration is hoisted, so you can call it even before the point in the code where you define it.

console.log(myFunc); // undefined
console.log(myFunc()); // TypeError: myFunc is not a function
 
var myFunc = function() {
  return "Hello!";
};
 
console.log(myFunc()); // "Hello!"