Well, no you couldn't do this on the position property:
position = [0,0];
Or you could, but the first line wouldn't do anything, then the second line is like saying "apply the position property's value to the position property", so the result is the same as without the expression.
In an expression you can create a new variable and assign a value to it, but you can't directly assign a value to the property. You can of course read the value of a property and you do this by using the name of the property, but it's essentially read only so it's not a good idea to try using it as a variable.
Then, the resulting value from the last line of the expression is what becomes the property's value.
Hope that makes sense.