Spread element or syntax (note that it’s not an operator) works only with objects that are iterable (i.e. implement the @@iterator method). Array.from() works also on array-like objects which are not iterable (i.e. objects that have indexed elements and a length property). See this example: const arrayLikeObject = { 0: ‘a’, 1: ‘b’, length: 2 … Read more
The properties are added in order, so if you want to override existing properties, you need to put them at the end instead of at the beginning: return { value: { …initialState, …newObject } } You don’t need newObject (unless you already have it lying around), though: return { value: { …initialState, isAvailable: newValue } … Read more
As noted in the comments, and summarized by @ftor from #687, object spread is equivalent1 to Object.assign() (issues #687, #45), whereas spread in array literal context is iterable spread. Quoting Ecma-262 6.0, Object.assign() is defined as: 19.1.2.1 Object.assign ( target, …sources ) The assign function is used to copy the values of all of the … Read more
Why can’t spread operator be used multiple times? … is not an operator. (…arr) is not valid JavaScript. … is only allowed inside array literals and in arguments lists, but those are special forms of the syntax (notice the … in the production rules below). ArrayLiteral ArrayLiteral : [ Elision_opt ] [ ElementList ] [ … Read more
Circular type aliases are not really supported except in certain cases. (UPDATE TS 4.1, these are more supported now, but I’m still inclined to represent flow() as operating on AsChain that verifies a particular array of functions instead of trying to come up with a Chain that matches all valid arrays of functions) Instead of … Read more
Use JSON for deep copy var newObject = JSON.parse(JSON.stringify(oldObject)) var oldObject = { name: ‘A’, address: { street: ‘Station Road’, city: ‘Pune’ } } var newObject = JSON.parse(JSON.stringify(oldObject)); newObject.address.city = ‘Delhi’; console.log(‘newObject’); console.log(newObject); console.log(‘oldObject’); console.log(oldObject);
In your example given, there is essentially no difference between the two .concat is significantly more efficient: http://jsperf.com/spread-into-array-vs-concat because … (spread) is merely syntax sugar on top of more fundamental underlying syntax that explicitly iterates over indexes to expand the array. Spread allows sugared syntax on top of more clunky direct array manipulation To expand … Read more
When using spread, you are expanding a single variable into more: var abc = [‘a’, ‘b’, ‘c’]; var def = [‘d’, ‘e’, ‘f’]; var alpha = [ …abc, …def ]; console.log(alpha)// alpha == [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’]; When using rest arguments, you are collapsing all remaining arguments of a function into one array: … Read more
That’s property spread notation. It was added in ES2018 (spread for arrays/iterables was earlier, ES2015), but it’s been supported in React projects for a long time via transpilation (as “JSX spread attributes” even though you could do it elsewhere, too, not just attributes). {…this.props} spreads out the “own” enumerable properties in props as discrete properties … Read more
I know this is a bit of an old issue but the easiest solution in ES2015/ES6 I could come up with was actually quite simple, using Object.assign(), Hopefully this helps: /** * Simple object check. * @param item * @returns {boolean} */ export function isObject(item) { return (item && typeof item === ‘object’ && !Array.isArray(item)); … Read more