How can I easily set a variable to false with a default of true?

Question

I usually set object properties like so,

// Boolean
this.listening = config.listening || true;

But config.listening is either true or false, and in this case this.listening will always be true because if config.listening is false it will equal true.

Is there a better way to set these boolean properties without having to do an if statement?

Is there a if isset function in javascript to check it exists rather than what it equals to?

Ian · Accepted Answer · 2015-07-10T17:46:07.510

16

You could use the ternary (conditional) operator like this:

this.listening = config.listening === false ? false : true;

If config.listening is false, this.listening is set to false. If it's any other value, it's set to true.

If you want to check if it's defined, you could use:

this.listening = typeof config.listening !== "undefined"

References:

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/Conditional_Operator

edited Jul 10 '15 at 17:46

answered May 08 '13 at 13:33

Ian

48,619
13
99
109

ha. Webstorm just simplified your answer to: config.listening !== false , seems to be correct. – Alexander Mills Nov 03 '15 at 00:39
1

@AlexMills You're absolutely right! Condensing is definitely possible, I was just trying to be explicit/descriptive when I wrote this (I'm guessing) – Ian Nov 04 '15 at 19:18
I prefer explicit and you should keep your answer – Alexander Mills Nov 04 '15 at 20:13

score 3 · Answer 2 · edited Sep 08 '16 at 23:47

3

You need to check to make sure it is not undefined, not that it is a "falsey" value.

this.listening = config.listening!==undefined ? config.listening : true;

edited Sep 08 '16 at 23:47

sebnukem

7,673
5
36
48

answered May 08 '13 at 13:34

epascarello

195,511
20
184
225

... or `'listening' in config` possibly – Pointy May 08 '13 at 13:38
1

Would it be better to do `typeof config.listening !== "undefined"` or is that just a readability factor – user2251919 May 08 '13 at 13:43
Yes, it would be better. (Or 'listening' in config). See this question for a discussion on the matter: http://stackoverflow.com/questions/519145/how-can-i-check-whether-a-variable-is-defined-in-javascript – Colin DeClue May 08 '13 at 13:45
typeof would not be "better"unless you are scared that someone in an older browser is going to hijack your undefined. – epascarello May 08 '13 at 13:49
Also the direct test runs faster in modern browsers than typeof – epascarello May 08 '13 at 13:56

How can I easily set a variable to false with a default of true?

2 Answers2