```Python
>>> r1 = {'height': 10, 'width': 20}
>>> r1['area'] = r1['height']*r1['width']
>>> print(r1['area'])
200
>>> r1
{'height': 10, 'width': 20, 'area': 200}
```

================================================================

```JS
> r1 = {'height': 10, 'width': 20};
{ height: 10, width: 20 }
> r1['area'] = r1['height']*r1['width'];
200
> console.log(r1['area']);
200
undefined
> r1
{ height: 10, width: 20, area: 200 }
```

================================================================

```JS
> r2 = {'height': 10, 'width': 20};
{ height: 10, width: 20 }
> r2.area = r2.height*r2.width;
200
> console.log(r2['area']);
200
undefined
> r2
{ height: 10, width: 20, area: 200 }
```

================================================================

Here's the Python in action:

```Python
>>> p = 'width'
>>> r1[p] = 2 * r1[p]
>>> print(r1)
{'height': 10, 'width': 40, 'area': 200}
```

================================================================

Let's see the JS in action:

```JS
> p = 'width';
'width'
> r1[p] = 2 * r1[p];
40
> console.log(r1);
{ height: 10, width: 40, area: 200 }
```

================================================================


Here's a variant example:

```JS
var a = b + 'c';
```

That's almost the same, but this time it's string concatenation rather
than addition. The two strings are "whatever is in the variable `b`" and
the string 'c'. Again, the second is a literal. Contrast that with;

```JS
var a = b + c;
```

Now both values are in varibles and we have no idea what either is. So the
quotation marks around `c` in the earlier example means literally "c".