Can you explain in simple terms whether the JavaScript value of 'undefined' equals zero? Additionally, what distinguishes a value defined in the language as 'null' from one that is 'undeclared'? Is there any relationship between these pre-defined values?

It's quite easy to differentiate between the 'null' and 'undefined' values in JavaScript. While 'null' is explicitly assigned by the programmer, 'undefined' is assigned by JavaScript itself.

That being said, it's technically possible for a programmer to create a variable with a value of 'undefined'. However, doing so is considered poor coding practice and should be avoided whenever possible. A well-written variable declaration would look something like this:

let a;

Alternatively, the variable could be assigned a value of 'null', like so:

let a = null;

Ultimately, the main difference between 'null' and 'undefined' is their source of assignment. Beyond that, there's not much else to know about this topic.

The 'undefined' data type is assigned to a variable as soon as it's declared, and to function arguments that were not passed. It's also returned from functions that have no return value or with a 'return' keyword without a value. The same applies to functions called with the 'new' operator. The value can be set explicitly and is usually encountered when a value is undefined.

To avoid using undefined values, it is good practice to initialize variables immediately or use 'let' and 'const' instead of 'var'. For example:

let foo = 10; // preferred
let bar;
bar = 10; // avoid
var baz = 10; // avoid

In contrast, 'null' represents the absence of any value in an object. It can be created by assigning a null value to a variable or an object property.

The main difference between 'null' and 'undefined' is that 'null' is a certain value of the absence of an object or defined key value, while 'undefined' denotes uncertainty due to a lack of definition or value assignment.

It's important to understand the differences between these two data types and use them appropriately to prevent coding errors and ensure efficient program execution.

The answer to whether 'undefined' is equal to 'null' in JavaScript depends on the type of comparison being used. Specifically, it depends on whether the comparison is strict (===) or not strict (==).

For example:
alert(null === undefined); // false
alert(null == undefined); // true

To avoid uncertainty or unexpected results, it's best to avoid using the comparisons >=, >, <, <= with variables that can potentially be "null" or "undefined". Instead, use strict equality operators (=== and !==) to compare such values and ensure accurate comparisons.

Sure, let's break these down one at a time.

1. **Does 'undefined' equal zero in JavaScript?**

No, 'undefined' is not the same as zero in JavaScript. 'undefined' in JavaScript means a variable has been declared but has not yet been assigned a value. On the other hand, zero is a number, and it's a legitimate value that you can assign to a variable.

In a comparison, 'undefined' is not equal to zero. For example:

```javascript
let test;
console.log(test == 0); // This will log 'false'
```

2. **What distinguishes a value 'null' from one that is 'undeclared'?**

'null' in JavaScript is an assignment value. It means that a variable is not currently pointing to any object or value. It's a intentional value that represents no value or no object.

On the other hand, 'undeclared' in JavaScript means a variable has not been declared at all. The system doesn't know about its existence.

Here's an example:

```javascript
let test = null;
console.log(test); // This will log 'null'

console.log(someRandomVariable); // This will throw 'ReferenceError: someRandomVariable is not defined'
```
In the first case, 'test' is declared and intentionally set to 'null'. In the second case, 'someRandomVariable' is undeclared, the system doesn't know about its existence and will throw an error.

3. **Is there any relationship between these pre-defined values?**

Yes, there is some relationship among 'undefined', 'null', and 'undeclared' variables. Specifically, both 'undefined' and 'null' are special values in JavaScript that represent the absence of a meaningful value.

'undefined' and 'null' are often used interchangeably and they are loosely equal (==), but not strictly equal (===). This is due to their loosely similar meanings of "non-existence" .

```javascript
let test;
console.log(test == null); // This will log 'true'
console.log(test === null); // This will log 'false'
```

In the code snippet above, `test == null` checks whether `test` is either `null` or `undefined` without type coercion. The `test === null` checks not just the value but the type, and since `null` and `undefined` are different types in JavaScript, it returns `false`.

Again, 'undeclared' is different from the other two, if you try to use an undeclared variable, JavaScript will throw an error, because the variable has not been declared at all.


Let's dive a little deeper into the nuances of JavaScript's 'undefined', 'null', and undeclared variables.

**Undefined**

As mentioned, 'undefined' means a variable has been declared, but not assigned a value. It's interesting to note that JavaScript lets you explicitly assign the 'undefined' value to a variable.

Take a look at this code snippet:

```javascript
let x = undefined;
console.log(x); // prints "undefined"
```

Even though 'x' is assigned, its value is 'undefined', which is similar to a situation where a variable is declared without assigning a value.

**Null**

'null' represents the intentional absence of any object value. In practical terms, it's often used when you want to "reset" or "clear" a variable that currently holds an object reference, but it can also be used with other types, not just objects.

In a broader sense, while 'undefined' means a variable has no value or object assigned to it ('this variable is undefined'), 'null' means a variable should have no value or object ('this variable is nullified').

Here's how you can use 'null' in a code snippet:

```javascript
let myObject = { firstKey: 'firstValue', secondKey: 'secondValue' };

console.log(myObject); // logs the object.

myObject = null;

console.log(myObject); // logs 'null'
```

**Undeclared Variables**

A variable is considered to be undeclared if it has been used without being declared with either 'var', 'let', or 'const' keywords in the current scope.

Here's an example:

```javascript
console.log(y); // ReferenceError: y is not defined

y = 10;
console.log(y); // 10
```

In strict mode ('use strict', JavaScript will not allow variables to be used before they've been declared, which helps reduce errors and increases code safety.

let's dive even deeper into JavaScript and its unique properties!

Hoisting

Hoisting is a JavaScript behavior in which variable and function declarations are moved to the top of their containing scope during the compile phase, before any code has been executed. Note that while the declarations (not initializations) are hoisted, the assignments are not.

Code example to understand Hoisting:

console.log(myVar); // undefined

var myVar = 5;

console.log(myVar); // 5
Though it seems like myVar is being used before it's declared, the variable declaration (var myVar) is hoisted to the top of the scope which makes it available anywhere within that scope.

Closure

A Closure in JavaScript is a function that has access to its own scope, the outer function's scope, and the global scope, as well as access to the function parameters and variables.

One use case for closures is data privacy. It's possible to define public APIs while keeping certain data private.

Example of a Closure:

function outer() {
  let count = 0;
 
  function inner() {
    count++;
    return count;
  }
 
  return inner;
}

const addOne = outer();
console.log(addOne()); // 1
console.log(addOne()); // 2
console.log(addOne()); // 3
In the example above, the function 'inner' is a closure that is returned from the function 'outer'. Each time the function 'inner' is called, it increments the count variable, but because 'count' is not accessible outside of 'outer', it's effectively private.

Promises and Async/Await

Promises in JavaScript provide a mechanism to handle the results and errors from asynchronous operations. You can chain promises together for complex async operations.

Async/Await is a syntactic sugar introduced in ES2017 to simplify working with Promises. It makes asynchronous code look and behave a little more like synchronous code.

Below is an example using Promises and Async/Await:

const promise = new Promise((resolve, reject) => {
  setTimeout(function() {
    resolve("Promise Resolved")
  }, 2000);
});

promise
  .then(result => console.log(result)) // "Promise Resolved"
  .catch(error => console.log(error));

async function asyncFunc() {
  try {
    const result = await promise;
    console.log(result); // "Promise Resolved"
  } catch (error) {
    console.log(error);
  }
}

asyncFunc();
In the example above, the Promise is resolved after 2 seconds with the value "Promise Resolved". The Promise's .then() method is used to handle the resolved value. The async function 'asyncFunc' shows how the 'await' keyword is used to wait for the promise to resolve and then log the result.