Object spread syntax (it's not an operator) allows you to "spread" the values null and undefined. When you do, no properties are added to the object being created. (This is just like Object.assign, which ignores null and undefined in its argument list.) The && operator accepts two operands, evaluates the first, and if that value is falsy, takes that value as its result; it only ... More on stackoverflow.com

It's the syntax of your callback. Change it to a.map(item => ({ ...item, someBool: true})) More on reddit.com

The first thing to understand is that the difference here isn't map vs spread. It's spread vs not spread. Both samples of code call map, and the version with the spread operator works on the result of map, not the array before map is called. Spread also has certain places where it can be used, such as within array (or object) literals and within argument lists while map simply returns and array value. Spread makes it so array values can be "spread" out as though written as individual values within the location they're being used. Beyond that, it depends on the context of where this code is placed. If you're seeing spread used like this: const myArray = [...this.state.courses.map(blabla)]; Then the spread is likely useless (an exception to be mentioned later). Spread in this kind of context can be useful for copying arrays (or other iterables) into a new array, but since map already creates a copy, it's redundant used here and the result of map alone would be fine. It may be necessary if other values are being added to the new array too, though, such as: const myArray = [...this.state.courses.map(blabla), 7, 8, 9]; This creates a new array with the contents of map as well as the values 7, 8, 9. This would be equivalent to saying this.state.courses.map(blabla).concat(7, 8, 9), though the spread syntax also let's you place the map expression anywhere within that new array configuration making it easy to add values both before and after whereas a single concat only allows you to add values after. In the case of a function call spread makes the difference of passing in an array or passing in the values of the array individually. What's needed depends on the signature of the function myFunc(...this.state.courses.map(blabla)) // all map values as individual arguments myFunc(this.state.courses.map(blabla)) // one array argument As I mentioned earlier, there is an exception to the first array literal spread example I used. I seriously doubt this would apply, one because it's uncommon in general, and two because you're code suggests something the use of something like a redux state which would mean a basic tree of plain JS objects and values. Nevertheless, it could be possible that courses is a custom array defined something like this: class EvensArray extends Array { *[Symbol.iterator]() { for (let i = 0; i < this.length; i += 2) { yield this[i]; } } } This is a custom array class which when iterated over (using spread or for..of), only returns the even indexed values. Iteration functions like forEach and map don't use the iterator behavior so they will operate on all values. So, for example, if you wanted to map over an EvensArray and only get the even values in a result array, then you could use this: const myArray = [...this.state.courses.map(blabla)]; The map will iterated over all values, performing blabla them, then the spread will iterate over and return only the even values which get assigned to a new array assigned to myArray. Though in truth, you'd want to optimize this by spreading first so your map would have to go through fewer elements... Anyway the point is, while [...array.map(fn)] is usually unnecessary and the same as array.map(fn), with a some hacking around it can actually mean something completely different. More on reddit.com

Afraid not. Javascript get syntax does not create a property that is enumerable. It can be accessed directly but won't be accessed via an object spread or a for/in loop. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get#using_getters_in_classes More on reddit.com