function f(x) {

  var y = 0;

  y = x;

}

y = 3;

f(6)

document.write("<h1>" + y + "</h1>");

y
6
3
<h1>y</h1>
nothing, an error will be reported

function f(x) {

  y = 0;

  y = x;

}

y = 3;

f(6)

document.write("<h1>" + y + "</h1>"); 

y
6
3
<h1>y</h1>
nothing, an error will be reported

function vector() {

  this.length = 0;

}

function addTo(vect, elem) {

  vect[vect.length + 1] = elem;

  vect.length += 1;

}

function s(t) {

  var v = 0;

  for (u in t) {

    v += t[u];

  }

  document.writeln(v); 

}

v = new vector();

addTo(v, 1);

addTo(v, 2);

addTo(v, 3);

s(v);

3
6
7
9
length 1 2 3

function Globe() {

  this.x = 3;

  this.y = 3;

}

g = new Globe();

g["x"] += g.y; 

document.write(g.x);

NaN
undefined
3
6
error:: JavaScript interpreter reports incorrect syntax

function a(x, y) {
  return x + y;
}
function b(x, y) {
  return x * y;
}
// a = b;
document.write(a(a(3, 4), b(3, 4))); 

an error
no output
19
84
144
343

function a(x, y) {
  return x + y;
}
function b(x, y) {
  return x * y;
}
a = b;
document.write(a(a(3, 4), b(3, 4))); 

an error
no output
19
84
144
343

<form onSubmit="return false">

<input type="submit">

</form>

the form does not show and the button either
the button is there but nothing happens
error: the form has no method or action attributes
error: there is JavaScript error in the event handler