Google Sheets: Script to read and write cell value to another cell onSelection(e)

[leighhobson89]

When I select a cell I want to copy the value to another cell before I have chance to edit the value in it. Let's say for example A1 to be copied to B1 when A1 is selected. How would I go about that? I have got lost in the documentation as a non programmer and cannot even find the right commands to get myself started. I presume it will be something along the lines this pseudo code here, but I can't seem to find it in the documentation.
Thanks.

function onSelection(e){
var value = this.sheet.range('A1');
this.sheet.range('B1') = value;
}