javascript - Why does map mutate array of objects?

Question

Might be a stupid question, but why does map mutate array of objects.

var obj = {
  items: [{
    value: 1,
    selected: true
  }, {
    value: 2,
    selected: false
  }]
};

var items = obj.items.map(i => {
  if (i.value === 2) i.selected = true;
  return i;
});

console.log(obj);

Answer 1

If you want a quick solution for an unmutable version of .map over an array of objects you can use the spread operator:

myArrayOfObjects.map(({...obj}) => { });

Example:

const foo = [];

for(let i = 0; i < 5; i++) {
    foo.push({label: "foo"});
}

const bar = foo.map(({...val}) => {
    val.id = Math.random();
  return val;
});

console.log(foo);
console.log(bar);