# JS Language

This chapter will not give you a general introduction to JavaScript. There are other books out there for a general introduction to JavaScript, please visit this great side on [Mozilla Developer Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript).

On the surface JavaScript is a very common language and does not differ a lot from other languages:

```js
function countDown() {
  for(var i=0; i<10; i++) {
    console.log('index: ' + i)
  }
}

function countDown2() {
  var i=10;
  while( i>0 ) {
    i--;
  }
}
```

But be warned JS has function scope and not block scope as in C++ (see [Functions and function scope](https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Functions_and_function_scope)).

The statements `if ... else`, `break`, `continue` also work as expected. The switch case can also compare other types and not just integer values:

```js
function getAge(name) {
  // switch over a string
  switch(name) {
  case "father":
    return 58;
  case "mother":
    return 56;
  }
  return unknown;
}
```

JS knows several values which can be false, e.g. `false`, `0`, `""`, `undefined`, `null`). For example, a function returns by default `undefined`. To test for false use the `===` identity operator. The `==` equality operator will do type conversion to test for equality. If possible use the faster and better `===` strict equality operator which will test for identity (see [Comparison operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators).

Under the hood, javascript has its own ways of doing things. For example arrays:

```js
function doIt() {
  var a = [] // empty arrays
  a.push(10) // addend number on arrays
  a.push("Monkey") // append string on arrays
  console.log(a.length) // prints 2
  a[0] // returns 10
  a[1] // returns Monkey
  a[2] // returns undefined
  a[99] = "String" // a valid assignment
  console.log(a.length) // prints 100
  a[98] // contains the value undefined
}
```

Also for people coming from C++ or Java which are used to an OO language JS just works differently. JS is not purely an OO language it is a so-called prototype based language. Each object has a prototype object. An object is created based on his prototype object. Please read more about this in the book [Javascript the Good Parts by Douglas Crockford](http://javascript.crockford.com).

To test some small JS snippets you can use the online [JS Console](http://jsconsole.com) or just build a little piece of QML code:

```qml
import QtQuick 2.5

Item {
  function runJS() {
    console.log("Your JS code goes here");
  }
  Component.onCompleted: {
    runJS();
  }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.novaflowos.com/start/qt6-qml-book/readme/ch16-javascript/js-language.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.