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if var a = true then a == 1 is true but a == 2 is false. Why? [duplicate]

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Javascript: the confuse about comparison "2 == true"
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if var a = true then a == 1 is true but a == 2 is false. Why? I understand that in javascript boolean expressions 0 casts to false and 1 casts to true. But what about the other integers. Why the above behavior?

Why? I understand that in javascript boolean expressions 0 casts to
false and 1 casts to true.
Because Number(true) => 1
As per spec of abstract equality expression evaluation
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
Hence a is first coerced into a Number and then compared with 1.

when you use == or != in js,the operands will be converted to the same type before making the comparison.when converting:
if there is a Boolean, true will be turned to 1 and false turned to 0;
var a = true;
a == 1;// a turned to 1 before compare,so true
a == 2;// a turned to 1 before compare,so false

This is to do with coercion. In the a == 1 scenario, a is coerced into a an integer to compare again 1, true is coerced to 1. In the second example, a is again coerced to 1, so it doesn't equal 2.
Further reading on coercion

You may convert 1 to True and True to 1 but you can't convert other numerics to True.

That's probably inherited from binary world and it makes sense, because in binary 0 is false and 1 is true. Exactly, that is how type conversion been done for boolean types in javascript as well while comparing.
For boolean there are only 2 possibles, true or false. Hence in number/binary system 0 and 1 are apt to represent them.

Well because 0 = false and 1 = true. There are only two values for a boolean: true/false. So what should 2 be?
var has no type, it can be everything. Please consider also the == vs === operator.
var x = 1;
console.log(x === true);
console.log(x == true);
x = 2;
console.log(x === true);
console.log(x == true);
x = 0;
console.log(x === false);
console.log(x == false);
x = -1;
See the example: http://jsfiddle.net/2aqGS/146/

Why === faster than == in JavaScript? [closed]

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See these tests:
== vs ====
== vs ===

Here's what javascript has to do for ===:
If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then
If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.
And here's what it has to do for ==:
If Type(x) is the same as Type(y), then
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then
If x is NaN, return false.
If y is NaN, return false.
If x is the same Number value as y, return true.
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return true.
Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
If Type(x) is String and Type(y) is Number,
return the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If Type(x) is either String or Number and Type(y) is Object,
return the result of the comparison x == ToPrimitive(y).
If Type(x) is Object and Type(y) is either String or Number,
return the result of the comparison ToPrimitive(x) == y.
Return false.
Notice that if Type(x) equals Type(y) then the operators do the same thing. However, if they aren't, then the == might have to do various conversions whereas === just returns false.
For the links you gave, the types that are being compared are actually the same, so the two operators should perform about equally. Differences here would be based on implementation details - since they do different things, they can be optimized for differently. Theoretically, since === does less, one would think it would always be faster, but that doesn't appear to be the case for certain builds of Firefox, at least if those benchmarks are accurate.
However, see the difference if the types are different. When doing "hi" === {} you get ~66 million ops/second, but for "hi" == {} you only have ~4 million ops/second.

JavaScript is a weakly typed language, so it will apply type coercion wherever possible.
Equals Operator
// These are true
new Number(10) == 10; // Number.toString() is converted
// back to a number
10 == '10'; // Strings gets converted to Number
10 == '+10 '; // More string madness
10 == '010'; // And more
isNaN(null) == false; // null converts to 0
// which of course is not NaN
The Strict Equality Operator
It works like the normal equality operator, except that strict equality operator does not perform type coercion between its operands.
"" === "0" // false
0 === "" // false
0 === "0" // false
false === "false" // false
false === "0" // false
false === undefined // false
false === null // false
null === undefined // false
" \t\r\n" === 0 // false
The above results are a lot clearer and allow for early breakage of code. This hardens code to a certain degree and also gives performance improvements in case the operands are of different types.
So === faster than == in Javascript
Here is good Reference

=== compares if the values and the types are the same.
== compares if the values are the same, but it also does type conversions in the comparison. Those type conversions make == slower than ===.

When is (true == x) === !!x false?

JavaScript has different equality comparison operators
Equal ==
Strict equal ===
It also has a logical NOT ! and I've tended to think of using a double logical NOT, !!x, as basically the same as true == x.
However I know this is not always the case, e.g. x = [] because [] is truthy for ! but falsy for ==.
So, for which xs would (true == x) === !!x give false? Alternatively, what is falsy by == but not !! (or vice versa)?

"So, for which xs would (true == x) === !!x give false?"
Any x where its Boolean conversion is not the same as its conversion by the Abstract Equality Comparison Algorithm.
An example is a string with only whitespace:
var x = " ";
Its Boolean conversion is true (as is the case with any non-empty string), but its == comparison is false because a string with only white space will be converted to the number 0, and the true value will be converted to the number 1, and those values are not equal.
x == true; // false
!!x; // true
or to show the ultimate values the == is comparing:
Number(true) == Number(x);
// 1 == 0
1 == 0; // false
and to show the result of !!x, it would be equivalent to this:
Boolean(x); // true
So your original expression could crudely be seen as the following:
var x = " ";
(Number(true) == Number(x)) === Boolean(x);
// ( 1 == 0 ) === true
// ( false ) === true
false === true; // false
I say "crudely" because this certainly doesn't capture all the detail of the algorithm linked above, and won't be the same for all values provided to the operands.
To understand how == treats its operands, you really need to study the algorithm a bit.

Loose equality has nothing to do with truthiness.
The rules for loose equality basically involve comparing the results of the .valueOf() function for each object.
For more details, see the spec.

Is whitespace equals to integer 0 in Javascript?

My app behaves abnormally and figured the code below is going to else statement unexpectedly.
code
if(" " != 0) {
console.log("whitespace is not zero");
}
else {
console.log("bar");
}
Firebug output
bar
I thought whitespace is a String and comparison against integer zero should return false like the case above but I don't know why it is going to else statement.
Could anyone explain why?

In JS, " " == 0 equals true with loose/lenient comparison, you should use strict equality operator ===/!== instead:
" " !== 0
To get to first condition.
Tests:
console.log(" " == 0); // true
console.log(" " === 0); // false
Loose Comparison Chart:
"" == "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
Strict Comparison Chart:
"" === "0" // false
0 === "" // false
0 === "0" // false
false === "false" // false
false === "0" // false
false === undefined // false
false === null // false
null === undefined // false
" \t\r\n" === 0 // false
(Examples by Douglas Crockford)
Good Practice:
Whenever possible, use strict equality operator because with loose equality operator, JS does type coercion which is a performance hit and doesn't always yield expected results as shown in above comparison charts.

The other answers have told you how to solve the issue. My answer will attempt to actually explain why you have the issue in the first place (since that was your actual question!)
The behaviour of the == operator is defined in the spec as the "abstract equality algorithm". It states (among other things) the following:
If Type(x) is String and Type(y) is Number, return the result of the
comparison ToNumber(x) == y.
One of the rules of the ToNumber operation on a string is as follows:
The MV of StringNumericLiteral ::: StrWhiteSpace is 0...
Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space character in the String numeric literal is ‘-’, in which case the rounded value is −0.
So we are left with +0 == 0 which fits another rule of the abstract equality algorithm:
If x is the same Number value as y, return true.
This is the case because 0 is the same as +0. Even if one of the numbers was -0 it would return true:
If x is +0 and y is −0, return true.
If x is −0 and y is +0, return
true.

When you use != rather than !==, JavaScript tries to coerce the values on either side to the same type. In this case, I think it converts both to numbers.
" " happens to be 0 as a number. (Try " " * 1. It evaluates to 0.)
This also works with other operators like > or *. So " " > -1 is true and " " * 100 is 0. This also means you can do neat stuff like "6" * "7" and get 42. Unfortunately, this breaks down with + since it is inexplicably overloaded to do both math and string concatenation.
I personally like all of this behavior except for +. Others' opinions vary.

to compare with the number 0, you must use the strict comparison ===
if(" " !== 0) {
console.log("whitespace is not zero");
}
else {
console.log("bar");
}
a "truth table" found here: Which equals operator (== vs ===) should be used in JavaScript comparisons?

because a string with whitespaces is converted to 0. So
To compare:
if(" " !== 0) {
....

Difference between Boolan(!x) and Boolean(x==0)?

Code-snippet 1:
if ( !x ) { /* do stuff */ }
Code-snippet 2:
if ( x == 0 ) { /* do stuff */ }
For what values of x do these two code-snippets differ?
I am asking because, although I read the chapter on == in the spec, I still find it hard to deal with situations like the above (where it is combined with ToBoolean coercion).
btw, I want to know this just for the sake of knowing it (I want to understand the language), so don't bother telling me about === or asking me what x is.
Update: I corrected the fist snippet. I meant !x.

[] == 0 is true; ![] is false
null == 0 is false; !null is true
NaN == 0 is false; !NaN is true
undefined == 0 is false; !undefined is true
!x will check whether x is "falsy".
x == 0 will check whether x is "equivalent to" 0.
Both of these terms are defined by the Javascript spec.

The following will give you true for the first and false for the second snippet:
NaN
null
undefined
And these will give you false for the first and true for the second snippet:
[]
"0" and any other string that converts to 0 using Number(x) such as "00", "000", "+0", and "-0" (which I will now call "noughty strings")
an array containing a single element that is 0, null, undefined or an empty or noughty string.
For everything else you'll get the same result for both snippets, although there may be one or two more cases I haven't thought of.

Here's an interesting one with regard to a non-empty String that has only space characters:
!!" "; // true
" " == true; // false
This is because when you do a == comparison, and one of the values being compared is a number or a boolean, an attempt is made to convert the other value to a number.
The reason you get the different result is that a string with only space characters converts to the number 0 (or falsey), while a string with only spaces converted to boolean via !! is seen as a non-empty string, and therefore true.
So:
var x = " ";
alert( !x ); // false
alert( x == 0 ); // true
EDIT:
Probably the key thing to remember is that when comparing a number or boolean to a non number type, == uses toNumber conversion if possible, while ! uses toBoolean conversion. They're not always the same.
It is easy to see the result of the toBoolean conversion using !!. As in:
alert( !![] ); // true
But you can't really see the result of the toNumber conversion when using ==.
You can, however, use the unary + to see the result of a toNumber conversion. As in:
alert( +[] ); // 0
I'm pretty sure that what happens in the case of an Array, is that it first gets a toString call. Therefore:
// ---------------------toString result-------toNumber result (from string)
alert( +[] ); // "" 0
alert( +[""] ); // "" 0
alert( +[" "] ); // " " 0
alert( +[0] ); // "0" 0
alert( +["0"] ); // "0" 0
alert( +["3"] ); // "3" 3
alert( +[3,4] ); // "3,4" NaN

Short answer: the two are almost always the same but not 100% the same.
An example would be (!'0') which is false whereas ('0' == 0) is true
Details:
From: http://www.joeyjavas.com/2007/08/04/javascript-true-false-checking-for-boolean-values/
Checking if a value is true or false is simple in JavaScript. All values evaluate to true, except for:
0
-0
null
undefined
NaN
empty string
false
Therefore, (!x) will be true for all of the above values of x and only those.
As for (x == 0), it will be true for any value of x which - when converted according to "==" conversion rules - is converted to 0 when compared to a number (for example, Boolean false value). Other examples that compare true to ==0 are objects which generate 0 from their valueOf() methods, or a string '0', or an empty Array ([])

The first test will succeed when x is non-zero, or evaluates to an object (as opposed to null or undefined), or is a non-empty string. So if x is 0 then the condition fails, but if it is "0" then it succeeds.
The second test will succeed when x is convertible to 0. This means it must not be null, undefined, or an object to pass this test. And it may be "0" or "".
In other words, these conditionals are not opposites. The value "0" will pass both tests, for example.

Code Snippet 1 will execute if x is "falsy" value. In Javascript, this means 0, -0, null, undefined, NaN, "", or false. Code Snippet 2, however, will only execute if x is zero. Unlike the first condition, this does not include other "falsy" values.

The difference between the two is that
if ( x ) { ... }
Tests whether x is "truthy"
Whereas
if ( x == 0 ) { ... }
Does type coercion between x and 0.
I presume you mean something like
if (x == 0) vs if (!x)
The main difference is type coercion of x to a number vs checking if x is falsy.
Clearly NaN itself will never equal 0 since its not a number. undefined will also coerce to NaN so that is not caught by == 0 I can't give a good explanation why null is not caught by 0 since Number(null) == 0

After some lookups, have to change my awnser.
There's no simple logic, implicit equality operations follows an algorithm.
http://interglacial.com/javascript_spec/a-11.html#a-11.9.3
I can't sum it up better then what the algoritm describes, it would just get more confusing.
So it's (!x) is equivalent to (typeof x === false) aka (not true)
And (x == 0) gets compared by algorithm.