I want to assign a property to an object, that is part of an array of objects. The value shall be the value of an existing property of the object. How is this done?
const data = [ {id: 111, name: 'AAA'}, ...];
const result= data.map(item => {
item.copyOfId= item.id;
return item;
});
There's no need for map there, you just need to loop through the array, not map the array. So all the usual ways of looping through arrays apply, such as forEach:
const data = [ {id: 111, name: 'AAA'}, ...];
data.forEach(item => {
item.copyOfId = item.id;
});
or for-of:
const data = [ {id: 111, name: 'AAA'}, ...];
for (const item of data) {
item.copyOfId = item.id;
}
As Mörre points out in a comment, in functional programming map is indeed what you would reach for, but usually you'd create new objects to populate the new array, instead of modifying (mutating) the objects:
const data = [ {id: 111, name: 'AAA'}, ...];
const result = data.map(item => {
return {...item, copyOfId: item.id};
});
(That example uses spread properties, which are only a Stage 3 proposal but are supported by up-to-date Chrome and Firefox. You could use Object.assign instead. Either way, it's a shallow copy if that's important.)
map, it would be fine - IF he produced a completely new array (functional programming style, no mutations, etc.). But producing a new array while still mutating the content items - that's a conflicting message. Either all new (use map), or mutate (use forEach). Note that sure, you can use either one, it's just that there are certain associations/assumptions associated with those methods, which come from other languages.
for-of, arrow functions, and const are, forEach isn't. I'd say the question doesn't relate to ES2015 ("ES6") specifically, so probably not.
A slightly different and shorter version (using object destructuring) would look like so:
const data = [ {id: 111, name: 'AAA'}, ...];
const result = data.map(({ id, name }) => ({ id, name, copyOfId: id }));
And using spread syntax:
const result = data.map(item => ({ ...item, copyOfId: item.id }));
Previous solutions work just as well, but I thought you may be interested in some additional ES2015 features.
You can use map and within each iteration, you can use the spread operator to create a duplicate BUT new object.
const data = [{
id: 111,
name: 'AAA'
}];
const result = data.map((item) => {
return { ...item, copyOfId: item.id };
});
console.log(result);
copyOfId?