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Javascript Optimal way of checking false statement except zero

In javascript we use shortand notation if(input){} to check for empty or null input. We will get false for Null, Undefined, false, 0, NAN, "". My requirements is that I want to get true for any number including zero so I have created a method as follows
function checkFalseExceptZero(value){
if( value !== undefined && value !== null && value !== false && value !== "" && value.length !== 0 ) return true;
else return false;
}
I have added all possible checks in the above method to get the desired result. I am curious if there is any quick, sweet, short or more robust approach to achieve the same?

A simple to understand answer. The three equal signs in JS will not convert type to check if the values equal unlike the two equal signs.
0 == false //true
0 === false //false, because 0 is a number and false is a boolean
Therefore, the answer, if you want to put it inside a function:
JS:
function isNumber(v) {
return typeof v === "number"
}
The function will check the type of the variable. So it is not actually comparing the value to determine the result. The function will only return true if the type of the variable is called number.
Test runs:
isNumber(0) //true
isNumber(false) //false
isNumber("0") //false
isNumber(undefined) //false
In case you want to know, typeof variable will return a string.

My requirements is that I want to get true for any number including
zero so I have created a method as follows
function checkFalseExceptZero(value){
if ( variable.constructor === Array) return !!value.length;
return value === 0 || !!value;
}
This a shorter version of your function. This returns true only when value is 0 or a trully value.
So :
checkFalseExceptZero(null) ==> false;
checkFalseExceptZero(undefined) ==> false;
checkFalseExceptZero(false) ==> false;
checkFalseExceptZero("") ==> false;
checkFalseExceptZero(0) ==> true;
checkFalseExceptZero([]) ==> false;
checkFalseExcpetZero([1]) ==> true;

For any valid number, including 0, JavaScript already exposes the isFinite function. This returns false for all non-numbers, as well as for infinite and NaN
Examples (excerpt from the linked mdn page):
isFinite(Infinity); // false
isFinite(NaN); // false
isFinite(-Infinity); // false
isFinite(0); // true
isFinite(2e64); // true
isFinite("0"); // true, would've been false with the
// more robust Number.isFinite("0")

javascript if conditional: object exists and has a value appended

Getting an 'Invalid left-hand side in assignment' error in my console. Am I missing syntax here?
if ($images[next] && images[next].loaded = false){
//stuff
}
Each condition passes on it's own, but fail when combined.

Using a single = means you're assigning a value to a variable. For comparisons, use == or === for strict equality.

Use double equals for comparison. Alternatively, you could just write:
var item = $images[next];
if (item && !item.loaded){
}

it should be
if ($images[next] && $images[next].loaded == false){
//stuff
}
Or Better
var images = $images[next];
if (images && images .loaded === false){
//stuff
}

As everybody already told you, you should be using == (or ===) for comparison. What caused the error is that the assignment operator = has a lower precedence than the boolean && operator, unlike == or any other comparison operator. So your expression was evaluated as:
($images[next] && images[next].loaded) = false
The left-hand part, in parentheses, will be either true or false. So that becomes either
true = false
or
false = false
Neither is allowed, because you can't assign anything to a boolean value (only to a variable or object property). Hence an Invalid left-hand side in assignment error is thrown.
When you use a proper comparison operator, the precedence rules cause it to be evaluated like this:
$images[next] && (images[next].loaded == false)
That becomes true && true, or true && false, or false && false, or false && true, then eventually just true or false, which is what an if statement expects.

Maybe a "==" instead of a "=" in the equality check would help
if ($images[next] && images[next].loaded == false)
{
//stuff
}
For a list of the javascript comparison operators, have a look here

Everyone'll prly say the same thing: ... .loaded = false should be ... .loaded == false. = is an assignment operator, == is a comparative operator..
Sometimes you'll see if (x = someFunction()) which will run the code block as long as someFunction() doesn't return false or throw an error (as it does in your case).
Let me know if you have any questions :)
https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Expressions_and_Operators

JS if/else trouble

I have a simple code:
<script type="text/javascript">
jQuery(document).ready(function() {
jQuery("#continue").click(function() {
var value = jQuery('#continue').attr('value')
alert (value);
if (value='next'){
jQuery('#msg_title').html('blablabla');
jQuery('#msg').html('blablabla'');
jQuery('#continue').attr('value','removeConfig');
value = jQuery('#continue').attr('value');
}
else if (value='removeConfig') {
jQuery('#msg_title').html('It Works!');
}
else {
alert ('Something wrong')
}
return false;
});
});
</script>
It works well in firs if phase, changes button's value (as I see from alert), but doesn't make else if and else statements.

Your comparison operator is wrong: if (value='next') should be if (value == 'next'){ or if (value === 'next'){.
Note the extra = signs.

You need ==
(value='next'){
should be:
(value == 'next'){
You are testing if ('next') { which is always true. Only string the evaluates to false is empty string, ""

Use ==, not =. A single = is an assignment operator, which means it's assigning "next" to value, then testing if value is false, not if value equals next

You need double equals signs as singles will return true in this case:
if (value =="next")
Single = is an assignment operator and double == is a comparison operator. Since you were using an assignment operator and setting the variable to a non falsy value the first if statement resolved to true.

Javascript if condition on boolean

Can you explain why the if condition doesn't work without the eval function:
var myBoolean= document.getElementById("someBoolean").value; //This is a 'false'
if(myBoolean)
{
alert(Your boolean is True); //This condition always getting executed even though myBoolean is false;
}
if(eval(myBoolean))
{
alert("You will never see this alert bcoz boolean is false");
}

In Javascript the following values are treated as false for conditionals:
false
null
undefined
The empty string ''
The number 0
The number NaN
Everything else is treated as true.
'false' is none of the above, so it's true.

The string 'false' evaluates to the boolean true

This is because it's not actually a boolean, it's a the string 'false'. When you convert a string to a boolean, '' is false and anything else is true.
You check if it's equal to the string 'false' (or 'true') or not.
var myBoolean = 'false'; // (string)
myBoolean = myBoolean !== 'false'; //false (boolean)

'false' == true, crazily enough because of JavaScript's implicit type coercion. Check out these other examples from Crockford's The Elements of JavaScript Style.
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined
// true
' \t\r\n ' == 0 // true
You could solve this particular problem by changing your code to something like
var myBoolean = document.getElementById("someBoolean").value === "true"
Also, it is almost always better to use !! and === rather than ! and ==

document.getElementById("someBoolean") does not return a boolean true/false it returns an element or undefined / null
you could reverse your logic and get the expected result:
if(!myBoolean)
{
alert('This element does not exist');
}
if(!eval(myBoolean))
{
alert("Do not know why you would ever want to do this");
// you could do typeof()
}

Try:
var myBoolean= document.getElementById("someBoolean").value; //This is a 'false'
if(myBoolean != "false")
{
alert(Your boolean is True); //This condition always getting executed even though myBoolean is false;
}
Like others have said, a string isn't a boolean value, so using it as though it was will give you a logical error.

A string IS a boolean truth value according to ECMA
var a = ""<br/>
a&&false //-> false<br/>
a||false //-> "" (truthy)<br/>

Convert string to Boolean in javascript [duplicate]

This question already has answers here:
How can I convert a string to boolean in JavaScript?
(102 answers)
Closed 5 years ago.
How to convert a string to Boolean ?
I tried using the constructor Boolean("false"), but it's always true.

I would use a simple string comparison here, as far as I know there is no built in function for what you want to do (unless you want to resort to eval... which you don't).
var myBool = myString == "true";

I would like to answer this to improve upon the accepted answer.
To improve performance, and in real world cases where form inputs might be passing values like 'true', or 'false', this method will produce the best results.
function stringToBool(val) {
return (val + '').toLowerCase() === 'true';
}
JSPerf

you can also use JSON.parse() function
JSON.parse("true") returns true (Boolean)
JSON.parse("false") return false (Boolean)

Actually you don't get the meaning of Boolean method.It always return true if the variable is not null or empty.
var variable = some value;
Boolean(variable);
If my variable have some value then it will
return true
else
return false
You can't use Boolean as you think.

trick string to boolean conversion in javascript. e.g.
var bool= "true";
console.log(bool==true) //false
var bool_con = JSON.parse(bool);
console.log(bool_con==true) //true

I am still amazed how people vote blindly for solutions that won't work, like:
var myBool = myString == "true";
The above is so BUGGY!!!
Not convinced? Just try myString = true (I mean the boolean true). What is the evaluation now? Opps: false!
Alternative
var myString=X; // X={true|false|"true"|"false"|"whatever"}
myString=String(myString)=='true';
console.log(myString); // plug any value into X and check me!
will always evaluate right!

Depends on what you see as false in a string.
Empty string, the word false, 0, should all those be false or is only empty false or only the word false.
You probably need to buid your own method to test the string and return true or false to be 100 % sure that it does what you need.

I believe the following code will do the work.
function isBoolean(foo) {
if((foo + "") == 'true' || (foo + "") == 'false') {
foo = (foo + "") == 'true';
} else {
console.log("The variable does not have a boolean value.");
return;
}
return foo;
}
Explaining the code:
foo + ""
converts the variable 'foo' to a string so if it is already boolean the function will not return an invalid result.
(foo + "") == 'true'
This comparison will return true only if 'foo' is equal to 'true' or true (string or boolean). Note that it is case-sensitive so 'True' or any other variation will result in false.
(foo + "") == 'true' || (foo + "") == 'false'
Similarly, the sentence above will result in true only if the variable 'foo' is equal to 'true', true, 'false' or false. So any other value like 'test' will return false and then it will not run the code inside the 'if' statement. This makes sure that only boolean values (string or not) will be considered.
In the 3rd line, the value of 'foo' is finally "converted" to boolean.

These lines give the following output:
Boolean(1).toString(); // true
Boolean(0).toString(); // false

Unfortunately, I didn't find function something like Boolean.ParseBool('true') which returns true as Boolean type like in C#.
So workaround is
var setActive = 'true';
setActive = setActive == "true";
if(setActive)
// statements
else
// statements.

javascript:var string="false";alert(Boolean(string)?'FAIL':'WIN')
will not work because any non-empty string is true
javascript:var string="false";alert(string!=false.toString()?'FAIL':'WIN')
works because compared with string represenation

See this question for reference:
How can I convert a string to boolean in JavaScript?
There are a few ways:
// Watch case sensitivity!
var boolVal = (string == "true");
or
var boolVal = Boolean("false");
or
String.prototype.bool = function() {
return (/^true$/i).test(this);
};
alert("true".bool());

You can try this:
var myBoolean = Boolean.parse(boolString);