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问题 Please note this is not duplicate of existing var vs let scope. I'm aware of the scope of var and let declarations and differences. But below scenario I could not justify with the understanding I had with let and var difference. In the below code, function foo accepts argument by name 'x' which has implicit let scope - as I cannot redeclare same variable name using let inside that function (uncommenting last line in function foo will throw JS error) "use strict"; function foo(x) { console.log

问题 Please note this is not duplicate of existing var vs let scope. I'm aware of the scope of var and let declarations and differences. But below scenario I could not justify with the understanding I had with let and var difference. In the below code, function foo accepts argument by name 'x' which has implicit let scope - as I cannot redeclare same variable name using let inside that function (uncommenting last line in function foo will throw JS error) "use strict"; function foo(x) { console.log

问题 Please note this is not duplicate of existing var vs let scope. I'm aware of the scope of var and let declarations and differences. But below scenario I could not justify with the understanding I had with let and var difference. In the below code, function foo accepts argument by name 'x' which has implicit let scope - as I cannot redeclare same variable name using let inside that function (uncommenting last line in function foo will throw JS error) "use strict"; function foo(x) { console.log

问题 This question already has answers here : Why let and var bindings behave differently using setTimeout function? [duplicate] (2 answers) Closed 2 years ago . I understand that let has block scope and var has functional scope. But I do not understand in this case, how using let will solve the problem const arr = [1,2,3,4]; for (var i = 0; i < arr.length; i++) { setTimeout(function() { console.log(arr[i]) }, 1000); } // Prints undefined 5 times const arr = [1,2,3,4]; for (let i = 0; i < arr

问题 I do not understand the difference between the three syntaxes: where a = f (b) do a <- f (b) do let a = f (b) I do understand somehow though that a <- f(b) is different from the other two, in most cases where I tried all three worked. Also I read somewhere on the net that per block you should try to get along with one let binding only in order to be "idiomatic". But I never seem to manage. How do I decide what to use? 回答1: let foo = bar in ... simply defines foo to be the exact same thing as

问题 I'm already aware of how private(set) works. But the below code is give compile-time error, class Person { private(set) let name: String //Error. private(set) let age: Int //Error. init(name: String, age: Int){ self.name = name self.age = age } } Error: 'private(set)' modifier cannot be applied to read-only properties Since name and age are not read-only properties, it shouldn't give such an error. If I use let instead of var , it is working fine. Just trying to know why? 回答1: private(set)

问题 I recently came across this code: for (const temp of [1,2]) { // do something } I thought that it'd be better to use let declaration for temp because this way the variable would be declared only once. However, I also ran this example as well as the version with let through babel and this is what I see: for (const p of [1,2]) { } for (let s of [1,2]) { } became: for (var _i = 0, _arr = [1, 2]; _i < _arr.length; _i++) { var p = _arr[_i]; } for (var _i2 = 0, _arr2 = [1, 2]; _i2 < _arr2.length;

问题 I recently came across this code: for (const temp of [1,2]) { // do something } I thought that it'd be better to use let declaration for temp because this way the variable would be declared only once. However, I also ran this example as well as the version with let through babel and this is what I see: for (const p of [1,2]) { } for (let s of [1,2]) { } became: for (var _i = 0, _arr = [1, 2]; _i < _arr.length; _i++) { var p = _arr[_i]; } for (var _i2 = 0, _arr2 = [1, 2]; _i2 < _arr2.length;

问题 it's known that to declare multiple variables, one uses a format like: let k = 0, j = 5 /*etc....*/ It's also known that to execute multiple statements in one line (which is useful for arrow functions, making it not necessary to write the return keyword), the comma "," operator is also used, like so: let r = "hello there world, how are you?" .split("") .map(x => (x+=5000, x.split("").map( y => y+ + 8 ).join(""))) .join("") console.log(r) not the most elegant example, but the point is you can

问题 Consider the following implementation of take: const take = (n, [x, ...xs]) => n === 0 || x === undefined ? [] : [x, ...take(n - 1, xs)]; console.log(take(7, [1, 2, 3, 4, 5])); // [1, 2, 3, 4, 5] console.log(take(3, [1, 2, 3, 4, 5])); // [1, 2, 3] console.log(take(1, [undefined, 1])); // [] As you can see, it doesn't work for arrays with undefined because x === undefined is not the best way to test whether an array is empty. The following code fixes this problem: const take = (n, xs) => n ===