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minor fixes

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Ilya Kantor 2020-07-12 16:20:39 +03:00
parent 9e59ec101f
commit 1f0fcb7de6
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1-js/04-object-basics/07-optional-chaining/article.md
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@ -40,8 +40,7 @@ AND'ing the whole path to the property ensures that all components exist, but is
The optional chaining `?.` stops the evaluation and returns `undefined` if the part before `?.` is `undefined` or `null`. The optional chaining `?.` stops the evaluation and returns `undefined` if the part before `?.` is `undefined` or `null`.
Further in this article, for brevity, we'll be saying that something "exists" if it's not `null` and not `undefined`. **Further in this article, for brevity, we'll be saying that something "exists" if it's not `null` and not `undefined`.**
Here's the safe way to access `user.address.street`: Here's the safe way to access `user.address.street`:
@ -57,14 +56,14 @@ Reading the address with `user?.address` works even if `user` object doesn't exi
let user = null; let user = null;
alert( user?.address ); // undefined alert( user?.address ); // undefined
alert( user?.address.street ); // undefined alert( user?.address.street ); // undefined
alert( user?.address.street.anything ); // undefined
``` ```
Please note: the `?.` syntax works exactly where it's placed, not any further. Please note: the `?.` syntax makes optional the value before it, but not any further.
In the last two lines the evaluation stops immediately after `user?.`, never accessing further properties. But if the `user` actually exists, then the further intermediate properties, such as `user.address` must exist. In the example above, `user?.` allows only `user` to be `null/undefined`.
On the other hand, if `user` does exist, then it must have `user.address` property, otherwise `user?.address.street` gives an error at the second dot.
```warn header="Don't overuse the optional chaining" ```warn header="Don't overuse the optional chaining"
We should use `?.` only where it's ok that something doesn't exist. We should use `?.` only where it's ok that something doesn't exist.
@ -75,13 +74,13 @@ So, if `user` happens to be undefined due to a mistake, we'll know about it and
``` ```
````warn header="The variable before `?.` must be declared" ````warn header="The variable before `?.` must be declared"
If there's no variable `user`, then `user?.anything` triggers an error: If there's no variable `user` at all, then `user?.anything` triggers an error:
```js run ```js run
// ReferenceError: user is not defined // ReferenceError: user is not defined
user?.address; user?.address;
``` ```
The optional chaining only tests for `null/undefined`, doesn't interfere with any other language mechanics. There must be `let/const/var user`. The optional chaining works only for declared variables.
```` ````
## Short-circuiting ## Short-circuiting