Function Declarations:

function functionName(parameters) {
  code to be executed
}

EXAMPLE:

<!DOCTYPE html>
<html>
<body>

<p>This example calls a function which performs a calculation, and returns the result:</p>

<p id="demo"></p>

<script>
function myFunction(a, b) {
    return a * b;
}
document.getElementById("demo").innerHTML = myFunction(4, 3);
</script>

</body>
</html>

the result: 12

The Function() Constructor:

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>
var myFunction = function (a, b) {return a * b}
document.getElementById("demo").innerHTML = myFunction(4, 3);
</script>

</body>
</html>

the result: 12

Self-Invoking Functions:

<!DOCTYPE html>
<html>
<body>

<p>Functions can be invoked automatically without being called:</p>

<p id="demo"></p>

<script>
(function () {
    document.getElementById("demo").innerHTML = "Hello! I called myself";
})();
</script>

</body>
</html>

 

the result: Hello! I called myself

Functions are Objects!!!

 

JavaScript Closures:

<!DOCTYPE html>
<html>
<body>

<p>Counting with a local variable.</p>

<button type="button" onclick="myFunction()">Count!</button>

<p id="demo">0</p>

<script>
var add = (function () {
    var counter = 0;
    return function () {return counter += 1;}
})();

function myFunction(){
    document.getElementById("demo").innerHTML = add();
}
</script>

</body>
</html>

every click the botton,the number will add one.