Can I convert a string representing a boolean value (e.g., 'true', 'false') into a intrinsic type in JavaScript?
I have a hidden form in HTML that is updated based upon a user's selection within a list. This form contains some fields which represent boolean values and are dynamically populated with an intrinsic boolean value. However, once this value is placed into the hidden input field it becomes a string.
The only way I could find to determine the field's boolean value, once it was converted into a string, was to depend upon the literal value of its string representation.
var myValue = document.myForm.IS_TRUE.value;
var isTrueSet = myValue == 'true';
Is there a better way to accomplish this?
Do:
var isTrueSet = (myValue === 'true');
using the identity operator (===), which doesn't make any implicit type conversions when the compared variables have different types.
Don't:
You should probably be cautious about using these two methods for your specific needs:
var myBool = Boolean("false"); // == true
var myBool = !!"false"; // == true
Any string which isn't the empty string will evaluate to true by using them. Although they're the cleanest methods I can think of concerning to boolean conversion, I think they're not what you're looking for.
Community Wiki
Warning
This highly upvoted legacy answer is technically correct but only covers a very specific scenario, when your string value is EXACTLY "true" or "false".
An invalid json string passed into these functions below WILL throw an exception.
Original answer:
How about?
JSON.parse("True".toLowerCase());
or with jQuery
$.parseJSON("TRUE".toLowerCase());
Community Wiki
stringToBoolean: function(string){
switch(string.toLowerCase().trim()){
case "true": case "yes": case "1": return true;
case "false": case "no": case "0": case null: return false;
default: return Boolean(string);
}
}
Community Wiki
I think this is much universal:
if (String(a).toLowerCase() == "true") ...
It goes:
String(true) == "true" //returns true
String(false) == "true" //returns false
String("true") == "true" //returns true
String("false") == "true" //returns false
Community Wiki
Remember to match case:
var isTrueSet = (myValue.toLowerCase() === 'true');
Also, if it's a form element checkbox, you can also detect if the checkbox is checked:
var isTrueSet = document.myForm.IS_TRUE.checked;
Assuming that if it is checked, it is "set" equal to true. This evaluates as true/false.
Community Wiki
You can use regular expressions:
/*
* Converts a string to a bool.
*
* This conversion will:
*
* - match 'true', 'on', or '1' as true.
* - ignore all white-space padding
* - ignore capitalization (case).
*
* ' tRue ','ON', and '1 ' will all evaluate as true.
*
*/
function strToBool(s)
{
// will match one and only one of the string 'true','1', or 'on' rerardless
// of capitalization and regardless off surrounding white-space.
//
regex=/^\s*(true|1|on)\s*$/i
return regex.test(s);
}
If you like extending the String class you can do:
String.prototype.bool = function() {
return strToBool(this);
};
alert("true".bool());
For those (see the comments) that would like to extend the String object to get this but are worried about enumerability and are worried about clashing with other code that extends the String object:
Object.defineProperty(String.prototype, "com_example_bool", {
get : function() {
return (/^(true|1)$/i).test(this);
}
});
alert("true".com_example_bool);
(Won't work in older browsers of course and Firefox shows false while Opera, Chrome, Safari and IE show true. Bug 720760)
Community Wiki
Wood-eye be careful.
After seeing the consequences after applying the top answer with 500+ upvotes, I feel obligated to post something that is actually useful:
Let's start with the shortest, but very strict way:
var str = "true";
var mybool = JSON.parse(str);
And end with a proper, more tolerant way:
var parseBool = function(str)
{
// console.log(typeof str);
// strict: JSON.parse(str)
if(str == null)
return false;
if (typeof str === 'boolean')
{
return (str === true);
}
if(typeof str === 'string')
{
if(str == "")
return false;
str = str.replace(/^\s+|\s+$/g, '');
if(str.toLowerCase() == 'true' || str.toLowerCase() == 'yes')
return true;
str = str.replace(/,/g, '.');
str = str.replace(/^\s*\-\s*/g, '-');
}
// var isNum = string.match(/^[0-9]+$/) != null;
// var isNum = /^\d+$/.test(str);
if(!isNaN(str))
return (parseFloat(str) != 0);
return false;
}
Testing:
var array_1 = new Array(true, 1, "1",-1, "-1", " - 1", "true", "TrUe", " true ", " TrUe", 1/0, "1.5", "1,5", 1.5, 5, -3, -0.1, 0.1, " - 0.1", Infinity, "Infinity", -Infinity, "-Infinity"," - Infinity", " yEs");
var array_2 = new Array(null, "", false, "false", " false ", " f alse", "FaLsE", 0, "00", "1/0", 0.0, "0.0", "0,0", "100a", "1 00", " 0 ", 0.0, "0.0", -0.0, "-0.0", " -1a ", "abc");
for(var i =0; i < array_1.length;++i){ console.log("array_1["+i+"] ("+array_1[i]+"): " + parseBool(array_1[i]));}
for(var i =0; i < array_2.length;++i){ console.log("array_2["+i+"] ("+array_2[i]+"): " + parseBool(array_2[i]));}
for(var i =0; i < array_1.length;++i){ console.log(parseBool(array_1[i]));}
for(var i =0; i < array_2.length;++i){ console.log(parseBool(array_2[i]));}
Community Wiki
I thought that @Steven 's answer was the best one, and took care of a lot more cases than if the incoming value was just a string. I wanted to extend it a bit and offer the following:
function isTrue(value){
if (typeof(value) === 'string'){
value = value.trim().toLowerCase();
}
switch(value){
case true:
case "true":
case 1:
case "1":
case "on":
case "yes":
return true;
default:
return false;
}
}
It's not necessary to cover all the false cases if you already know all of the true cases you'd have to account for. You can pass anything into this method that could pass for a true value (or add others, it's pretty straightforward), and everything else would be considered false
Community Wiki
Universal solution with JSON parse:
function getBool(val) {
return !!JSON.parse(String(val).toLowerCase());
}
getBool("1"); //true
getBool("0"); //false
getBool("true"); //true
getBool("false"); //false
getBool("TRUE"); //true
getBool("FALSE"); //false
UPDATE (without JSON):
function getBool(val){
var num = +val;
return !isNaN(num) ? !!num : !!String(val).toLowerCase().replace(!!0,'');
}
I also created fiddle to test it http://jsfiddle.net/remunda/2GRhG/
Community Wiki
Your solution is fine.
Using === would just be silly in this case, as the field's value will always be a String.
Community Wiki
var falsy = /^(?:f(?:alse)?|no?|0+)$/i;
Boolean.parse = function(val) {
return !falsy.test(val) && !!val;
};
This returns false for every falsy value and true for every truthy value except for 'false', 'f', 'no', 'n', and '0' (case-insensitive).
// False
Boolean.parse(false);
Boolean.parse('false');
Boolean.parse('False');
Boolean.parse('FALSE');
Boolean.parse('f');
Boolean.parse('F');
Boolean.parse('no');
Boolean.parse('No');
Boolean.parse('NO');
Boolean.parse('n');
Boolean.parse('N');
Boolean.parse('0');
Boolean.parse('');
Boolean.parse(0);
Boolean.parse(null);
Boolean.parse(undefined);
Boolean.parse(NaN);
Boolean.parse();
//True
Boolean.parse(true);
Boolean.parse('true');
Boolean.parse('True');
Boolean.parse('t');
Boolean.parse('yes');
Boolean.parse('YES');
Boolean.parse('y');
Boolean.parse('1');
Boolean.parse('foo');
Boolean.parse({});
Boolean.parse(1);
Boolean.parse(-1);
Boolean.parse(new Date());
Community Wiki
The Boolean object doesn't have a 'parse' method. Boolean('false') returns true, so that won't work. !!'false' also returns true, so that won't work also.
If you want string 'true' to return boolean true and string 'false' to return boolean false, then the simplest solution is to use eval(). eval('true') returns true and eval('false') returns false. Keep in mind the performance implications when using eval() though.
Community Wiki
There are a lot of answers and it's hard to pick one. In my case, I prioritise the performance when choosing, so I create this jsPerf that I hope can throw some light here.
Brief of results (the higher the better):
- Conditional statement: 2,826,922
- Switch case on Bool object: 2,825,469
- Casting to JSON: 1,867,774
- !! conversions: 805,322
- Prototype of String: 713,637
They are linked to the related answer where you can find more information (pros and cons) about each one; specially in the comments.
Community Wiki
This has been taken from the accepted answer, but really it has a very weak point, and I am shocked how it got that count of upvotes, the problem with it that you have to consider the case of the string because this is case sensitive
var isTrueSet = (myValue.toLowerCase() === 'true');
Community Wiki
I use the following:
function parseBool(b) {
return !(/^(false|0)$/i).test(b) && !!b;
}
This function performs the usual Boolean coercion with the exception of the strings "false" (case insensitive) and "0".
Community Wiki
There are already so many answers available. But following can be useful in some scenarios.
// One can specify all values against which you consider truthy
var TRUTHY_VALUES = [true, 'true', 1];
function getBoolean(a) {
return TRUTHY_VALUES.some(function(t) {
return t === a;
});
}
This can be useful where one examples with non-boolean values.
getBoolean('aa'); // false
getBoolean(false); //false
getBoolean('false'); //false
getBoolean('true'); // true
getBoolean(true); // true
getBoolean(1); // true
Community Wiki
Boolean.parse = function (str) {
switch (str.toLowerCase ()) {
case "true":
return true;
case "false":
return false;
default:
throw new Error ("Boolean.parse: Cannot convert string to boolean.");
}
};
Community Wiki
The expression you're looking for simply is
/^true$/i.test(myValue)
as in
var isTrueSet = /^true$/i.test(myValue);
This tests myValue against a regular expression , case-insensitive, and doesn't modify the prototype.
Examples:
/^true$/i.test("true"); // true
/^true$/i.test("TRUE"); // true
/^true$/i.test("tRuE"); // true
/^true$/i.test(" tRuE"); // false (notice the space at the beginning)
/^true$/i.test("untrue"); // false (some other solutions here will incorrectly return true
/^true$/i.test("false");// returns false
/^true$/i.test("xyz"); // returns false
Community Wiki
To convert both string("true", "false") and boolean to boolean
('' + flag) === "true"
Where flag can be
var flag = true
var flag = "true"
var flag = false
var flag = "false"
Community Wiki
you can use JSON.parse as follows:
var trueOrFalse='True';
result =JSON.parse(trueOrFalse.toLowerCase());
if(result==true)
alert('this is true');
else
alert('this is false');
in this case .toLowerCase is important
Community Wiki
This function can handle string as well as Boolean true/false.
function stringToBoolean(val){
var a = {
'true':true,
'false':false
};
return a[val];
}
Demonstration below:
function stringToBoolean(val) {
var a = {
'true': true,
'false': false
};
return a[val];
}
console.log(stringToBoolean("true"));
console.log(typeof(stringToBoolean("true")));
console.log(stringToBoolean("false"));
console.log(typeof(stringToBoolean("false")));
console.log(stringToBoolean(true));
console.log(typeof(stringToBoolean(true)));
console.log(stringToBoolean(false));
console.log(typeof(stringToBoolean(false)));
console.log("=============================================");
// what if value was undefined?
console.log("undefined result: " + stringToBoolean(undefined));
console.log("type of undefined result: " + typeof(stringToBoolean(undefined)));
console.log("=============================================");
// what if value was an unrelated string?
console.log("unrelated string result: " + stringToBoolean("hello world"));
console.log("type of unrelated string result: " + typeof(stringToBoolean(undefined)));
Community Wiki
Simplest solution
with ES6+
use the logical NOT twice [ !! ] to get the string converted
Just paste this expression...
const stringToBoolean = (string) => string === 'false' ? false : !!string
And pass your string to it!
stringToBoolean('') // false
stringToBoolean('false') // false
stringToBoolean('true') // true
stringToBoolean('hello my friend!') // true
Bonus!
const betterStringToBoolean = (string) =>
string === 'false' || string === 'undefined' || string === 'null' || string === '0' ?
false : !!string
You can include other strings at will to easily extend the usage of this expression...:
betterStringToBoolean('undefined') // false
betterStringToBoolean('null') // false
betterStringToBoolean('0') // false
betterStringToBoolean('false') // false
betterStringToBoolean('') // false
betterStringToBoolean('true') // true
betterStringToBoolean('anything else') // true
Community Wiki
This is the most easiest way of boolean conversion I came across recently. Thought of adding it.
JSON.parse('true');
let trueResponse = JSON.parse('true');
let falseResponse = JSON.parse('false');
console.log(trueResponse);
console.log(falseResponse);
Community Wiki
I'm using this one
String.prototype.maybeBool = function(){
if ( ["yes", "true", "1", "on"].indexOf( this.toLowerCase() ) !== -1 ) return true;
if ( ["no", "false", "0", "off"].indexOf( this.toLowerCase() ) !== -1 ) return false;
return this;
}
"on".maybeBool(); //returns true;
"off".maybeBool(); //returns false;
"I like js".maybeBool(); //returns "I like js"
Community Wiki
why don't you try something like this
Boolean(JSON.parse((yourString.toString()).toLowerCase()));
It will return an error when some other text is given rather than true or false regardless of the case and it will capture the numbers also as
// 0-> false
// any other number -> true
Community Wiki
You need to separate (in your thinking) the value of your selections and the representation of that value.
Pick a point in the JavaScript logic where they need to transition from string sentinels to native type and do a comparison there, preferably where it only gets done once for each value that needs to be converted. Remember to address what needs to happen if the string sentinel is not one the script knows (i.e. do you default to true or to false?)
In other words, yes, you need to depend on the string's value. :-)
Community Wiki
One Liner
We just need to account for the "false" string since any other string (including "true") is already true.
function b(v){ return v==="false" ? false : !!v; }
Test
b(true) //true
b('true') //true
b(false) //false
b('false') //false
A more exaustive version
function bool(v){ return v==="false" || v==="null" || v==="NaN" || v==="undefined" || v==="0" ? false : !!v; }
Test
bool(true) //true
bool("true") //true
bool(1) //true
bool("1") //true
bool("hello") //true
bool(false) //false
bool("false") //false
bool(0) //false
bool("0") //false
bool(null) //false
bool("null") //false
bool(NaN) //false
bool("NaN") //false
bool(undefined) //false
bool("undefined") //false
bool("") //false
bool([]) //true
bool({}) //true
bool(alert) //true
bool(window) //true
Community Wiki
Hands down the easiest way (assuming you string will be 'true' or 'false') is:
var z = 'true';
var y = 'false';
var b = (z === 'true'); // will evaluate to true
var c = (y === 'true'); // will evaluate to false
Always
Community Wiki
Like @Shadow2531 said, you can't just convert it directly. I'd also suggest that you consider string inputs besides "true" and "false" that are 'truthy' and 'falsey' if your code is going to be reused/used by others. This is what I use:
function parseBoolean(string) {
switch (String(string).toLowerCase()) {
case "true":
case "1":
case "yes":
case "y":
return true;
case "false":
case "0":
case "no":
case "n":
return false;
default:
//you could throw an error, but 'undefined' seems a more logical reply
return undefined;
}
}
Community Wiki
another solution. jsFiddle
var toBoolean = function(value) {
var strValue = String(value).toLowerCase();
strValue = ((!isNaN(strValue) && strValue !== '0') &&
strValue !== '' &&
strValue !== 'null' &&
strValue !== 'undefined') ? '1' : strValue;
return strValue === 'true' || strValue === '1' ? true : false
};
test cases run in node
> toBoolean(true)
true
> toBoolean(false)
false
> toBoolean(undefined)
false
> toBoolean(null)
false
> toBoolean('true')
true
> toBoolean('True')
true
> toBoolean('False')
false
> toBoolean('false')
false
> toBoolean('0')
false
> toBoolean('1')
true
> toBoolean('100')
true
>
Community WikiRetrieved from : http:www.stackoverflow.com/questions/263965/how-can-i-convert-a-string-to-boolean-in-javascript