4 ms·

In Javascript - value =!!value

8 points by amazedsaint 13y ago

milliams 13y ago
Does JavaScript not have something like "bool(value)"?
- benjoffe 13y ago
  You can use Boolean(value) but it will return an 'object' of a 'boolean' type, rather than a primitive boolean (it's one of JS's flaws so is generally avoided). Note: their example of a similar piece of code is not identical, as it won't cast values like null to false. I think the best example of a clear equivalent would be: value = value ? true : false;
  - gsnedders 13y ago
    `Boolean(value)` returns a boolean primitive; `new Boolean(value)` returns a boolean object (which you never want). Compare: > typeof Boolean(1) 'boolean' > typeof new Boolean(1) 'object'
  - Guillaume86 13y ago
    In chrome console I get an object for new Boolean(false) but I think it is the primitive for Boolean(false), am I right or I missed something?
- deleted 13y ago
  [deleted]
- onestone 13y ago
  The !! mentioned in the article does exactly that.
ars 13y ago
The edited title is very confusing because "- value = !!value" is not correct. Assuming you mean a dash and not a negative, it's still not correct. I'm sure the mods will change the title to the one from the actual post: "What the Heck is !! in JavaScript?" which is a much better title.
ajanuary 13y ago
While I get the arguments for not using progressive enhancement in lots of modern use cases, I'm not sure it's wise to require javascript just so the page content can spin into view.
kibwen 13y ago
This is true in a whole slew of languages. Here's Ruby: irb(main):001:0> 3 => 3 irb(main):002:0> !3 => false irb(main):003:0> !!3 => true And here's C: #include<stdio.h> main() { printf("%d %d %d", 3, !3, !!3); // 3 0 1 } Even the venerable and (relatively) strongly-typed Python falls prey to this: >>> 3 3 >>> not 3 False >>> not not 3 True Basically any language where the ! operator coerces to boolean (or in C's case, the nearest approximation to boolean). Though personally I tend to agree that implicit coercions are undesirable, don't be too quick to cry JAVASCRIPT WUT, as it has quite a lot of precedence. :)
xhrpost 13y ago
Recently wanted to convert a string of '0' or '1' to a native boolean, shortcut value = !!~~value;
- bpicolo 13y ago
  jsPerf shows !!parseInt(str) and parseInt(str)? true : false as being decently faster...assuming I used jsperf properly. http://jsperf.com/test-bool-cast http://jsperf.com/test-bool-cast
  - mmastrac 13y ago
    Interestingly enough, it's probably faster to test != '0': http://jsperf.com/test-bool-cast/2 http://jsperf.com/test-bool-cast/2
  - xhrpost 13y ago
    Yup, good call.

8 points by amazedsaint 13y ago

milliams 13y ago

Does JavaScript not have something like "bool(value)"?

benjoffe 13y ago
You can use Boolean(value) but it will return an 'object' of a 'boolean' type, rather than a primitive boolean (it's one of JS's flaws so is generally avoided). Note: their example of a similar piece of code is not identical, as it won't cast values like null to false. I think the best example of a clear equivalent would be: value = value ? true : false;
- gsnedders 13y ago
  `Boolean(value)` returns a boolean primitive; `new Boolean(value)` returns a boolean object (which you never want). Compare: > typeof Boolean(1) 'boolean' > typeof new Boolean(1) 'object'
- Guillaume86 13y ago
  In chrome console I get an object for new Boolean(false) but I think it is the primitive for Boolean(false), am I right or I missed something?
deleted 13y ago
[deleted]
onestone 13y ago
The !! mentioned in the article does exactly that.

ars 13y ago

The edited title is very confusing because "- value = !!value" is not correct. Assuming you mean a dash and not a negative, it's still not correct. I'm sure the mods will change the title to the one from the actual post: "What the Heck is !! in JavaScript?" which is a much better title.

ajanuary 13y ago

While I get the arguments for not using progressive enhancement in lots of modern use cases, I'm not sure it's wise to require javascript just so the page content can spin into view.

kibwen 13y ago

This is true in a whole slew of languages. Here's Ruby: irb(main):001:0> 3 => 3 irb(main):002:0> !3 => false irb(main):003:0> !!3 => true And here's C: #include<stdio.h> main() { printf("%d %d %d", 3, !3, !!3); // 3 0 1 } Even the venerable and (relatively) strongly-typed Python falls prey to this: >>> 3 3 >>> not 3 False >>> not not 3 True Basically any language where the ! operator coerces to boolean (or in C's case, the nearest approximation to boolean). Though personally I tend to agree that implicit coercions are undesirable, don't be too quick to cry JAVASCRIPT WUT, as it has quite a lot of precedence. :)

xhrpost 13y ago

Recently wanted to convert a string of '0' or '1' to a native boolean, shortcut value = !!~~value;

bpicolo 13y ago
jsPerf shows !!parseInt(str) and parseInt(str)? true : false as being decently faster...assuming I used jsperf properly. http://jsperf.com/test-bool-cast http://jsperf.com/test-bool-cast
- mmastrac 13y ago
  Interestingly enough, it's probably faster to test != '0': http://jsperf.com/test-bool-cast/2 http://jsperf.com/test-bool-cast/2
- xhrpost 13y ago
  Yup, good call.