I am working with the following array of objects and I am trying to run a function with forEach. However, I don't want to destroy the original tbl1. Hence, I made a copy of tbl1 in tbl2 and I am running forEach on tbl2.
What surprises me, if I run a forEach on tbl2 so that I can preserve the tbl1 structure, it also takes place in tbl1 which I don't want.
const tbl1 = [{ "Month": 1, "Value": 100 }, { "Month": 2, "Value": 200 }, { "Month": 3, "Value": 300 }]
const tbl2 = tbl1;
tbl2.forEach(
(d)=>{d.newCol=d.Month*152;});
console.log(tbl1,tbl2);Is it possible to run forEach on a copy table without destroying the source table?
That is because when you write const tbl2 = tbl1;, it doesn't clone the array, rather takes a reference to the original. So when you modify the the reference, the original gets modified as well.
you can use map
const tbl1 = [{ "Month": 1, "Value": 100 }, { "Month": 2, "Value": 200 }, { "Month": 3, "Value": 300 }]
const tbl2 = tbl1.map(d => {
return {
...d,
newCol: d.Month*152
}
})
console.log(tbl1,tbl2);JSON.parse and JSON.stringify
const tbl1 = [{ "Month": 1, "Value": 100 }, { "Month": 2, "Value": 200 }, { "Month": 3, "Value": 300 }]
const tbl2 = JSON.parse(JSON.stringify(tbl1));
tbl2.forEach(
(d)=>{d.newCol=d.Month*152;});
console.log(tbl1,tbl2);