@thefourtheye is correct in saying that these variables cannot be accessed before they are declared. However, it’s a bit more complicated than that. Are variables declared with let or const not hoisted? What is really going on here? All declarations (var, let, const, function, function*, class) are “hoisted” in JavaScript. This means that if a … Read more
Note: As of ES2020, even older operations like for-in and Object.keys are required to follow property order. That doesn’t change the fact that using property order for fundamental program logic probably isn’t a good idea, since the order for non-integer-index properties depends on when the properties were created. Answer for ES2015-ES2019: For for-in, Object.keys, and … Read more
Your state needs some time to mutate, and since console.log(this.state.boardAddModalShow) executes before the state mutates, you get the previous value as output. So you need to write the console in the callback to the setState function openAddBoardModal() { this.setState({ boardAddModalShow: true }, function () { console.log(this.state.boardAddModalShow); }); } setState is asynchronous. It means you can’t … Read more
You can use Promise.all (spec, MDN) for that: It accepts a bunch of individual promises and gives you back a single promise that is resolved when all of the ones you gave it are resolved, or rejected when any of them is rejected. So if you make doSomeAsyncStuff return a promise, then: const promises = … Read more
Jackson has partially answered this in a similar question: Implicit return, but only if there is no block. This will result in errors when a one-liner expands to multiple lines and the programmer forgets to add a return. Implicit return is syntactically ambiguous. (name) => {id: name}returns the object {id: name}… right? Wrong. It returns … Read more
Arrow functions are not designed to be used in every situation merely as a shorter version of old-fashioned functions. They are not intended to replace function syntax using the function keyword. The most common use case for arrow functions is as short “lambdas” which do not redefine this, often used when passing a function as … Read more
What It Is This is an arrow function. Arrow functions are a short syntax, introduced by ECMAscript 6, that can be used similarly to the way you would use function expressions. In other words, you can often use them in place of expressions like function (foo) {…}. But they have some important differences. For example, … Read more
tl;dr: No! Arrow functions and function declarations / expressions are not equivalent and cannot be replaced blindly. If the function you want to replace does not use this, arguments and is not called with new, then yes. As so often: it depends. Arrow functions have different behavior than function declarations / expressions, so let’s have … Read more
If you take advantage of how == works, you could simply create an object with a custom toString (or valueOf) function that changes what it returns each time it is used such that it satisfies all three conditions. const a = { i: 1, toString: function () { return a.i++; } } if(a == 1 … Read more
What’s the difference between using “let” and “var”?