diff --git a/1-js/04-object-basics/07-optional-chaining/article.md b/1-js/04-object-basics/07-optional-chaining/article.md index 4b078327..5bcaba59 100644 --- a/1-js/04-object-basics/07-optional-chaining/article.md +++ b/1-js/04-object-basics/07-optional-chaining/article.md @@ -32,7 +32,7 @@ In many practical cases we'd prefer to get `undefined` instead of an error here let html = document.querySelector('.elem').innerHTML; // error if it's null ``` -Once again, if the element doesn't exist, we'll get an error accessing `.innerHTML` of `null`. And in some cases, when the absence of the element is normal, we'd like to avoid the error and just accept `html = null` as the result. +Once again, if the element doesn't exist, we'll get an error accessing `.innerHTML` property of `null`. And in some cases, when the absence of the element is normal, we'd like to avoid the error and just accept `html = null` as the result. How can we do this? @@ -44,11 +44,19 @@ let user = {}; alert(user.address ? user.address.street : undefined); ``` -It works, there's no error... But it's quite inelegant. As you can see, the `"user.address"` appears twice in the code. For more deeply nested properties, that becomes a problem as more repetitions are required. +It works, there's no error... But it's quite inelegant. As you can see, the `"user.address"` appears twice in the code. -E.g. let's try getting `user.address.street.name`. +Here's how the same would look for `document.querySelector`: -We need to check both `user.address` and `user.address.street`: +```js run +let html = document.querySelector('.elem') ? document.querySelector('.elem').innerHTML : null; +``` + +We can see that the element search `document.querySelector('.elem')` is actually called twice here. Not good. + +For more deeply nested properties, it becomes even uglier, as more repetitions are required. + +E.g. let's get `user.address.street.name` in a similar fashion. ```js let user = {}; // user has no address @@ -58,7 +66,7 @@ alert(user.address ? user.address.street ? user.address.street.name : null : nul That's just awful, one may even have problems understanding such code. -Don't even care to, as there's a better way to write it, using the `&&` operator: +There's a little better way to write it, using the `&&` operator: ```js run let user = {}; // user has no address @@ -92,6 +100,12 @@ alert( user?.address?.street ); // undefined (no error) The code is short and clean, there's no duplication at all. +Here's an example with `document.querySelector`: + +```js run +let html = document.querySelector('.elem')?.innerHTML; // will be null, if there's no element +``` + Reading the address with `user?.address` works even if `user` object doesn't exist: ```js run @@ -162,11 +176,11 @@ userAdmin.admin?.(); // I am admin */!* *!* -userGuest.admin?.(); // nothing (no such method) +userGuest.admin?.(); // nothing happens (no such method) */!* ``` -Here, in both lines we first use the dot (`userAdmin.admin`) to get `admin` property, because we assume that the user object exists, so it's safe read from it. +Here, in both lines we first use the dot (`userAdmin.admin`) to get `admin` property, because we assume that the `user` object exists, so it's safe read from it. Then `?.()` checks the left part: if the admin function exists, then it runs (that's so for `userAdmin`). Otherwise (for `userGuest`) the evaluation stops without errors. @@ -199,10 +213,9 @@ For example: let user = null; user?.name = "John"; // Error, doesn't work -// because it evaluates to undefined = "John" +// because it evaluates to: undefined = "John" ``` -It's just not that smart. ```` ## Summary @@ -217,4 +230,4 @@ As we can see, all of them are straightforward and simple to use. The `?.` check A chain of `?.` allows to safely access nested properties. -Still, we should apply `?.` carefully, only where it's acceptable that the left part doesn't exist. So that it won't hide programming errors from us, if they occur. +Still, we should apply `?.` carefully, only where it's acceptable, according to our code logic, that the left part doesn't exist. So that it won't hide programming errors from us, if they occur.