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fixes ~n

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This commit is contained in:
Ilya Kantor 2019-07-01 13:20:41 +03:00
parent d303295945
commit 3c7952f275
1 changed files with 8 additions and 4 deletions
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1-js/05-data-types/03-string/article.md
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@ -305,7 +305,8 @@ if (str.indexOf("Widget") != -1) {
} }
``` ```
````smart header="The bitwise NOT trick" #### The bitwise NOT trick
One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators#Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation. One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators#Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation.
For 32-bit integers the call `~n` means exactly the same as `-(n+1)` (due to IEEE-754 format). For 32-bit integers the call `~n` means exactly the same as `-(n+1)` (due to IEEE-754 format).
@ -321,9 +322,9 @@ alert( ~-1 ); // 0, the same as -(-1+1)
*/!* */!*
``` ```
As we can see, `~n` is zero only if `n == -1`. As we can see, for 32-bit integers `~n` is zero only if `n == -1`.
So, the test `if ( ~str.indexOf("...") )` is truthy that the result of `indexOf` is not `-1`. In other words, when there is a match. So, the test `if ( ~str.indexOf("...") )` is truthy only if the result of `indexOf` is not `-1`. In other words, when there is a match.
People use it to shorten `indexOf` checks: People use it to shorten `indexOf` checks:
@ -338,7 +339,10 @@ if (~str.indexOf("Widget")) {
It is usually not recommended to use language features in a non-obvious way, but this particular trick is widely used in old code, so we should understand it. It is usually not recommended to use language features in a non-obvious way, but this particular trick is widely used in old code, so we should understand it.
Just remember: `if (~str.indexOf(...))` reads as "if found". Just remember: `if (~str.indexOf(...))` reads as "if found".
````
Technically speaking, numbers are truncated to 32 bits by `~` operator, so there exist other big numbers that give `0`, the smallest is `~4294967295=0`. That makes such check is correct only if a string is not that long.
Right now we can see it only in the old code, as modern JavaScript provides `.includes` method (see below).
### includes, startsWith, endsWith ### includes, startsWith, endsWith