From the spec: Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Therefore, the correct answer would be [1,2,3]. Firefox has fixed the problem in version 43 (bug 889158).
The length is a property of strings, and can be destructured and returned in the map. It’s mostly the same as: materials.map((str) => { const {length} = str; return length; });
You can write an inline type literal for the call signatures the function supports: class TestOverloads2 { private status = “blah”; public doStuff: { (selector: JQuery): void; (selector: string): void; } = (selector: any) => { alert(this.status); } } That’s sort of hideous, so you might want to extract it into an interface instead: interface … Read more
The pair of braces forms a block, containing a list of statements. You need to use a return statement explicitly to make the function return something: (one) => { return oneTodo(one, action); // ^^^^^^ } If you omit the braces, the arrow function has a concise body, which consists solely of a single expression whose … Read more
When you use the function body version of an arrow function (with {}), there is no implied return. You have to specify it. When you use the concise body (no {}), the result of the body expression is implicitly returned by the function. So you would write that either with an explicit return: const f … Read more
CoffeeScript links both this and @ to the outer context, therefore you cannot access the context that jQuery provided (aka the desired “this”). Use event.target instead: class PostForm constructor: -> $(‘ul.tabs li’).on ‘click’, (event) => tab = $(event.target) @highlight_tab(tab)
Believe it or not… Testing for presence of “=>” in string representation of a function is likely the most reliable way (but not 100%). Obviously we can’t test against either of 2 conditions you mentioned — lack of prototype property and lack of [[Construct]] as that might give false positives with either host objects or … Read more
No. Those parentheses produce an object literal. Arrow functions have many syntaxes, one of which is: ( … ) => expression This will implicitly return an expression, for example: () => 1 + 1 This function will implicitly return 1 + 1, which is 2. Another one is this: ( … ) => { … … Read more
The React FAQs provide an explanation to it Are Hooks slow because of creating functions in render? No. In modern browsers, the raw performance of closures compared to classes doesn’t differ significantly except in extreme scenarios. In addition, consider that the design of Hooks is more efficient in a couple ways: Hooks avoid a lot … Read more