JavaScript: Parsing a string Boolean value? [duplicate]

This question already has answers here: How can I convert a string to boolean in JavaScript? (102 answers) Closed 4 years ago.

JavaScript has parseInt() and parseFloat(), but there's no parseBool or parseBoolean method in the global scope, as far as I'm aware.

I need a method that takes strings with values like "true" or "false" and returns a JavaScript Boolean.

Here's my implementation:

function parseBool(value) {
    return (typeof value === "undefined") ? 
           false : 
           // trim using jQuery.trim()'s source 
           value.replace(/^\s+|\s+$/g, "").toLowerCase() === "true";
}

Is this a goo开发者_如何学Cd function? Please give me your feedback.

Thanks!

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I would be inclined to do a one liner with a ternary if.

var bool_value = value == "true" ? true : false

Edit: Even quicker would be to simply avoid using the a logical statement and instead just use the expression itself:

var bool_value = value == 'true';

This works because value == 'true' is evaluated based on whether the value variable is a string of 'true'. If it is, that whole expression becomes true and if not, it becomes false, then that result gets assigned to bool_value after evaluation.

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You can use JSON.parse for that:

JSON.parse("true"); //returns boolean true

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It depends how you wish the function to work.

If all you wish to do is test for the word 'true' inside the string, and define any string (or nonstring) that doesn't have it as false, the easiest way is probably this:

function parseBoolean(str) {
  return /true/i.test(str);
}

If you wish to assure that the entire string is the word true you could do this:

function parseBoolean(str) {
  return /^true$/i.test(str);
}

---

You can try the following:

function parseBool(val)
{
    if ((typeof val === 'string' && (val.toLowerCase() === 'true' || val.toLowerCase() === 'yes')) || val === 1)
        return true;
    else if ((typeof val === 'string' && (val.toLowerCase() === 'false' || val.toLowerCase() === 'no')) || val === 0)
        return false;

    return null;
}

If it's a valid value, it returns the equivalent bool value otherwise it returns null.

---

You can use JSON.parse or jQuery.parseJSON and see if it returns true using something like this:

function test (input) {
    try {
        return !!$.parseJSON(input.toLowerCase());
    } catch (e) { }
}

---

last but not least, a simple and efficient way to do it with a default value :

ES5

function parseBool(value, defaultValue) {
    return (value == 'true' || value == 'false' || value === true || value === false) && JSON.parse(value) || defaultValue;
}

ES6 , a shorter one liner

const parseBool = (value, defaultValue) => ['true', 'false', true, false].includes(value) && JSON.parse(value) || defaultValue

JSON.parse is efficient to parse booleans

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Personally I think it's not good, that your function "hides" invalid values as false and - depending on your use cases - doesn't return true for "1".

Another problem could be that it barfs on anything that's not a string.

I would use something like this:

function parseBool(value) {
  if (typeof value === "string") {
     value = value.replace(/^\s+|\s+$/g, "").toLowerCase();
     if (value === "true" || value === "false")
       return value === "true";
  }
  return; // returns undefined
}

And depending on the use cases extend it to distinguish between "0" and "1".

(Maybe there is a way to compare only once against "true", but I couldn't think of something right now.)

---

Why not keep it simple?

var parseBool = function(str) {
    if (typeof str === 'string' && str.toLowerCase() == 'true')
            return true;

    return (parseInt(str) > 0);
}

---

You can add this code:

function parseBool(str) {

  if (str.length == null) {
    return str == 1 ? true : false;
  } else {
    return str == "true" ? true : false;
  }

}

Works like this:

parseBool(1) //true
parseBool(0) //false
parseBool("true") //true
parseBool("false") //false

---

Wood-eye be careful. After looking at all this code, I feel obligated to post:

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')
    {
        if(str === true)
            return true;

        return false;
    } 

    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]));}

---

I like the solution provided by RoToRa (try to parse given value, if it has any boolean meaning, otherwise - don't). Nevertheless I'd like to provide small modification, to have it working more or less like Boolean.TryParse in C#, which supports out params. In JavaScript it can be implemented in the following manner:

var BoolHelpers = {
    tryParse: function (value) {
        if (typeof value == 'boolean' || value instanceof Boolean)
            return value;
        if (typeof value == 'string' || value instanceof String) {
            value = value.trim().toLowerCase();
            if (value === 'true' || value === 'false')
                return value === 'true';
        }
        return { error: true, msg: 'Parsing error. Given value has no boolean meaning.' }
    }
}

The usage:

var result = BoolHelpers.tryParse("false");
if (result.error) alert(result.msg);

---

stringjs has a toBoolean() method:

http://stringjs.com/#methods/toboolean-tobool

S('true').toBoolean() //true
S('false').toBoolean() //false
S('hello').toBoolean() //false
S(true).toBoolean() //true
S('on').toBoolean() //true
S('yes').toBoolean() //true
S('TRUE').toBoolean() //true
S('TrUe').toBoolean() //true
S('YES').toBoolean() //true
S('ON').toBoolean() //true
S('').toBoolean() //false
S(undefined).toBoolean() //false
S('undefined').toBoolean() //false
S(null).toBoolean() //false
S(false).toBoolean() //false
S({}).toBoolean() //false
S(1).toBoolean() //true
S(-1).toBoolean() //false
S(0).toBoolean() //false

---

I shamelessly converted Apache Common's toBoolean to JavaScript:

JSFiddle: https://jsfiddle.net/m2efvxLm/1/

Code:

function toBoolean(str) {
  if (str == "true") {
    return true;
  }
  if (!str) {
    return false;
  }
  switch (str.length) {
    case 1: {
      var ch0 = str.charAt(0);
      if (ch0 == 'y' || ch0 == 'Y' ||
          ch0 == 't' || ch0 == 'T' ||
          ch0 == '1') {
        return true;
      }
      if (ch0 == 'n' || ch0 == 'N' ||
          ch0 == 'f' || ch0 == 'F' ||
          ch0 == '0') {
        return false;
      }
      break;
    }
    case 2: {
      var ch0 = str.charAt(0);
      var ch1 = str.charAt(1);
      if ((ch0 == 'o' || ch0 == 'O') &&
          (ch1 == 'n' || ch1 == 'N') ) {
        return true;
      }
      if ((ch0 == 'n' || ch0 == 'N') &&
          (ch1 == 'o' || ch1 == 'O') ) {
        return false;
      }
      break;
    }
    case 3: {
      var ch0 = str.charAt(0);
      var ch1 = str.charAt(1);
      var ch2 = str.charAt(2);
      if ((ch0 == 'y' || ch0 == 'Y') &&
          (ch1 == 'e' || ch1 == 'E') &&
          (ch2 == 's' || ch2 == 'S') ) {
        return true;
      }
      if ((ch0 == 'o' || ch0 == 'O') &&
          (ch1 == 'f' || ch1 == 'F') &&
          (ch2 == 'f' || ch2 == 'F') ) {
        return false;
      }
      break;
    }
    case 4: {
      var ch0 = str.charAt(0);
      var ch1 = str.charAt(1);
      var ch2 = str.charAt(2);
      var ch3 = str.charAt(3);
      if ((ch0 == 't' || ch0 == 'T') &&
          (ch1 == 'r' || ch1 == 'R') &&
          (ch2 == 'u' || ch2 == 'U') &&
          (ch3 == 'e' || ch3 == 'E') ) {
        return true;
      }
      break;
    }
    case 5: {
      var ch0 = str.charAt(0);
      var ch1 = str.charAt(1);
      var ch2 = str.charAt(2);
      var ch3 = str.charAt(3);
      var ch4 = str.charAt(4);
      if ((ch0 == 'f' || ch0 == 'F') &&
          (ch1 == 'a' || ch1 == 'A') &&
          (ch2 == 'l' || ch2 == 'L') &&
          (ch3 == 's' || ch3 == 'S') &&
          (ch4 == 'e' || ch4 == 'E') ) {
        return false;
      }
      break;
    }
    default:
      break;
  }

  return false;
}
console.log(toBoolean("yEs")); // true
console.log(toBoolean("yES")); // true
console.log(toBoolean("no")); // false
console.log(toBoolean("NO")); // false
console.log(toBoolean("on")); // true
console.log(toBoolean("oFf")); // false

Inspect this element, and view the console output.

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Enough to using eval javascript function to convert string to boolean

eval('true')  
eval('false')