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:
falsenull undefined''0 NaNEverything else is treated as true.
'false' is none of the above, so it's true.
The string 'false' evaluates to the boolean true
true unless they are empty ('').
String 'false'. He's getting the value of the element, not the element itself. Look, he's using .value but he should be using .val() instead.
.value'false'. If I was more careful I would have told him to use .val() but regardless, the point of the question is that a non-empty String evaluates to the boolean trueThis 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 == '' // true0 == '0' // true
false == 'false' // false
false == '0' // truefalse == undefined // false
false == null // falsenull == 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/>
'false' is considered like a true value in boolean contexts. whatever && false should always be false regardless of whatever brings to the table, and yes, the effective value of ""||false may be "", but I would understand this as "false" in a boolean context.
document.getElementById("someBoolean");return'false'?document.getElementById("someBoolean");returns a DOM element (ornull). To get the value you'd have to dodocument.getElementById("someBoolean").value;.