diff --git a/1-js/06-advanced-functions/02-rest-parameters-spread-operator/article.md b/1-js/06-advanced-functions/02-rest-parameters-spread-operator/article.md index 0788860e..7a7b95d1 100644 --- a/1-js/06-advanced-functions/02-rest-parameters-spread-operator/article.md +++ b/1-js/06-advanced-functions/02-rest-parameters-spread-operator/article.md @@ -8,7 +8,7 @@ For instance: - `Object.assign(dest, src1, ..., srcN)` -- copies properties from `src1..N` into `dest`. - ...and so on. -In this chapter we'll see how to do the same. And, more important, how to feel comfortable working with such functions and arrays. +In this chapter we'll learn how to do the same. And, more importantly, how to feel comfortable working with such functions and arrays. [cut] @@ -27,7 +27,7 @@ alert( sum(1, 2, 3, 4, 5) ); There will be no error because of "excessive" arguments. But of course in the result only the first two will be counted. -The rest parameters can be mentioned in a function definition with three dots `...`. They literally mean: "gather the remaining parameters into an array". +The rest parameters can be mentioned in a function definition with three dots `...`. They literally mean "gather the remaining parameters into an array". For instance, to gather all arguments into array `args`: @@ -45,9 +45,9 @@ alert( sumAll(1, 2) ); // 3 alert( sumAll(1, 2, 3) ); // 6 ``` -We can choose to get first parameters as variables, and gather only the rest. +We can choose to get the first parameters as variables, and gather only the rest. -Here the first two arguments go into variables and the rest goes to `titles` array: +Here the first two arguments go into variables and the rest go into `titles` array: ```js run function showName(firstName, lastName, ...titles) { @@ -72,7 +72,7 @@ function f(arg1, ...rest, arg2) { // arg2 after ...rest ?! } ``` -The `...rest` must always be the last. +The `...rest` must always be last. ```` ## The "arguments" variable @@ -98,13 +98,13 @@ showName("Julius", "Caesar"); showName("Ilya"); ``` -In old times, rest parameters did not exist in the language, and `arguments` was the only way to get all arguments of the function no matter of their total number. +In old times, rest parameters did not exist in the language, and using `arguments` was the only way to get all arguments of the function, no matter their total number. -And it still works, we can use it. +And it still works, we can use it today. -But the downside is that although `arguments` is both array-like and iterable, it's not an array. It does not support array methods, so we can't say call `arguments.map(...)`. +But the downside is that although `arguments` is both array-like and iterable, it's not an array. It does not support array methods, so we can't call `arguments.map(...)` for example. -Also, it always has all arguments in it, we can't capture them partially, like we did with rest parameters. +Also, it always contains all arguments. We can't capture them partially, like we did with rest parameters. So when we need these features, then rest parameters are preferred. @@ -123,21 +123,19 @@ f(1); // 1 ``` As we remember, arrow functions don't have their own `this`. Now we know they don't have the special `arguments` object either. -```` - ## Spread operator [#spread-operator] We've just seen how to get an array from the list of parameters. But sometimes we need to do exactly the reverse. -For instance, there's a built-in function [Math.max](mdn:js/Math/max) that returns the greatest number from the list: +For instance, there's a built-in function [Math.max](mdn:js/Math/max) that returns the greatest number from a list: ```js run alert( Math.max(3, 5, 1) ); // 5 ``` -Now let's say we have an array `[3, 5, 1]`. How to call `Math.max` with it? +Now let's say we have an array `[3, 5, 1]`. How do we call `Math.max` with it? Passing it "as it" won't work, because `Math.max` expects a list of numeric arguments, not a single array: @@ -149,9 +147,9 @@ alert( Math.max(arr) ); // NaN */!* ``` -...And surely we can't manually list items in the code `Math.max(arg[0], arg[1], arg[2])`, because we may be unsure how much are there. As our script executes, there might be many, or there might be none. Also that would be ugly. +And surely we can't manually list items in the code `Math.max(arg[0], arg[1], arg[2])`, because we may be unsure how many there are. As our script executes, there could be a lot, or there could be none. And that would get ugly. -*Spread operator* to the rescue. It looks similar to rest parameters, also using `...`, but does quite the opposite. +*Spread operator* to the rescue! It looks similar to rest parameters, also using `...`, but does quite the opposite. When `...arr` is used in the function call, it "expands" an iterable object `arr` into the list of arguments. @@ -172,7 +170,7 @@ let arr2 = [8, 3, -8, 1]; alert( Math.max(...arr1, ...arr2) ); // 8 ``` -...And even combine the spread operator with normal values: +We can even combine the spread operator with normal values: ```js run @@ -182,7 +180,7 @@ let arr2 = [8, 3, -8, 1]; alert( Math.max(1, ...arr1, 2, ...arr2, 25) ); // 25 ``` -Also spread operator can be used to merge arrays: +Also, the spread operator can be used to merge arrays: ```js run let arr = [3, 5, 1]; @@ -197,7 +195,7 @@ alert(merged); // 0,3,5,1,2,8,9,15 (0, then arr, then 2, then arr2) In the examples above we used an array to demonstrate the spread operator, but any iterable will do. -For instance, here we use spread operator to turn the string into array of characters: +For instance, here we use the spread operator to turn the string into array of characters: ```js run let str = "Hello"; @@ -225,7 +223,7 @@ But there's a subtle difference between `Array.from(obj)` and `[...obj]`: - `Array.from` operates on both array-likes and iterables. - The spread operator operates only on iterables. -So, for the task of turning something into an array, `Array.from` appears more universal. +So, for the task of turning something into an array, `Array.from` tends to be more universal. ## Summary @@ -234,8 +232,8 @@ When we see `"..."` in the code, it is either rest parameters or the spread oper There's an easy way to distinguish between them: -- When `...` is at the end of function parameters, it's "rest parameters" and gathers the rest of the list into the array. -- When `...` occurs in a function call or alike, it's called a "spread operator" and expands an array into the list. +- When `...` is at the end of function parameters, it's "rest parameters" and gathers the rest of the list of arguments into an array. +- When `...` occurs in a function call or alike, it's called a "spread operator" and expands an array into a list. Use patterns: