使用Javascript中的三元运算符将字符串转换为布尔值？

Question

I have a result coming from PHP side and json_encode send all the values as string (not as it's original type). The string coming at quote_tax is a 1|0 and of course this is true|false for example:

{
  "country_id": "3",
  "country_name": "Italy",
  "contract_tax": "1",
  "quote_tax": "0",
  "contract_inflation": "0",
  "quote_inflation": "0"
}

When I want to perform some operation based on such values and because they are coming as string I need to do something like this:

 data.quote_tax == '1' 
                ? $('#contract_tax').val(1).attr('checked', 'checked') 
                : $('#contract_tax').val(0).removeAttr('checked');

I did know about .parseInt() to convert them into a integer but I believe then I would need to have the same comparison but this case comparing with INT :

 data.quote_tax == 1 
                ? $('#contract_tax').val(1).attr('checked', 'checked') 
                : $('#contract_tax').val(0).removeAttr('checked');

I have tried this way:

 Boolean(data.quote_tax) == true 
                ? $('#contract_tax').val(1).attr('checked', 'checked') 
                : $('#contract_tax').val(0).removeAttr('checked');

And it doesn't work since Boolean(data.quote_tax) always evaluate as true even if data.quote_tax = '0' .

I have check this posts already but I couldn't found a proper solution:

• How can I convert a string to boolean in JavaScript?
• Convert string to Boolean in javascript

Any ideas what's wrong here or how to do this?

Answer 1

Values in Javascript are either "truthy" or "falsy", meaning they'll be interpreted as true or false in a boolean context. In the case of numbers, 0 is false and all others are true . If you convert your value to a number, you don't have to perform any other conversion to treat it as a boolean.

 var x = parseInt('1'); x ? console.log(true) : console.log(false); x = parseInt('0'); x ? console.log(true) : console.log(false); 

See here for more information.

Answer 2

You can combine the operators - parse the string to a number, and then convert to Boolean:

 console.log(Boolean(parseInt('0', 10))); console.log(!!parseInt('0', 10)); console.log(!!+'0'); 

Answer 3

Your last approach fails because any non-empty string evaluates to true if converted to a boolean, including "0" .

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The integer 0 evaluates to false, while non-zero integers evaluate to true . For this reason, parseInt(data.quote_tax) == 1 would be equivalent to parseInt(data.quote_tax) in your case.

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However, I would stick with the first approach in this situation:

data.quote_tax == '1'

This requires no type conversions, the intent is immediately obvious and is also doesn't require the developer to know the things I listed above to be understood. Don't make your code more complicated than it has to be.

Answer 4

You can do something like this:

!!parseInt('1'); //true
!!parseInt('0'); //false

Answer 5

Personally I always try and stay away from coercing types . The problem is that if you do as others have suggested, if (x) this works great for 0 & 1 . What if for some reason you get a "false" , 2 , null or undefined . What would you want your code to do here?

 undefined ? console.log(true) : console.log(false); null ? console.log(true) : console.log(false); NaN ? console.log(true) : console.log(false); 2 ? console.log(true) : console.log(false); "false" ? console.log(true) : console.log(false); "true" ? console.log(true) : console.log(false); 

If you use === (as opposed to == ) it prevents Javascript coercion . You can then test for the string value "1"

 "1" === "1" ? console.log(true) : console.log(false); 1 === "1" ? console.log(true) : console.log(false); undefined === "1" ? console.log(true) : console.log(false); null === "1" ? console.log(true) : console.log(false); NaN === "1" ? console.log(true) : console.log(false); 2 === "1" ? console.log(true) : console.log(false); "false" === "1" ? console.log(true) : console.log(false); "true" === "1" ? console.log(true) : console.log(false); 

It now works a lot more consistently. May or may not be an issue in your case but I prefer this method for the just in case factor.