We Keep Coding

can a string be equal to a number [duplicate]

This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over ==?

The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal.
Reference: Javascript Tutorial: Comparison Operators
The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '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
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.
Update:
A good point was brought up by #Casebash in the comments and in #Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.
Reference
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion, meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion, and thus does not convert the values when comparing.

Here's an interesting visualisation of the equality comparison between == and ===.
Source: https://github.com/dorey/JavaScript-Equality-Table (demo, unified demo)
var1 === var2
When using === for JavaScript equality testing, everything is as is.
Nothing gets converted before being evaluated.
var1 == var2
When using == for JavaScript equality testing, some funky conversions take place.
Summary of equality in Javascript
Conclusion:
Always use ===, unless you fully understand the funky conversions that take place with ==.

In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type, but that's not really true. It actually means that both operands reference the same object, or in case of value types, have the same value.
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object, while b is of type string. Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.

Let me add this counsel:
If in doubt, read the specification!
ECMA-262 is the specification for a scripting language of which JavaScript is a dialect. Of course in practice it matters more how the most important browsers behave than an esoteric definition of how something is supposed to be handled. But it is helpful to understand why new String("a") !== "a".
Please let me explain how to read the specification to clarify this question. I see that in this very old topic nobody had an answer for the very strange effect. So, if you can read a specification, this will help you in your profession tremendously. It is an acquired skill. So, let's continue.
Searching the PDF file for === brings me to page 56 of the specification: 11.9.4. The Strict Equals Operator ( === ), and after wading through the specificationalese I find:
11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
  1. If Type(x) is different from Type(y), return false.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is not Number, go to step 11.
  5. If x is NaN, return false.
  6. If y is NaN, return false.
  7. If x is the same number value as y, return true.
  8. If x is +0 and y is −0, return true.
  9. If x is −0 and y is +0, return true.
  10. Return false.
  11. 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.
  12. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
Interesting is step 11. Yes, strings are treated as value types. But this does not explain why new String("a") !== "a". Do we have a browser not conforming to ECMA-262?
Not so fast!
Let's check the types of the operands. Try it out for yourself by wrapping them in typeof(). I find that new String("a") is an object, and step 1 is used: return false if the types are different.
If you wonder why new String("a") does not return a string, how about some exercise reading a specification? Have fun!
Aidiakapi wrote this in a comment below:
From the specification
11.2.2 The new Operator:
If Type(constructor) is not Object, throw a TypeError exception.
With other words, if String wouldn't be of type Object it couldn't be used with the new operator.
new always returns an Object, even for String constructors, too. And alas! The value semantics for strings (see step 11) is lost.
And this finally means: new String("a") !== "a".

I tested this in Firefox with Firebug using code like this:
console.time("testEquality");
var n = 0;
while (true) {
n++;
if (n == 100000)
break;
}
console.timeEnd("testEquality");
and
console.time("testTypeEquality");
var n = 0;
while (true) {
n++;
if (n === 100000)
break;
}
console.timeEnd("testTypeEquality");
My results (tested five times each and averaged):
==: 115.2
===: 114.4
So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.

In PHP and JavaScript, it is a strict equality operator. Which means, it will compare both type and values.

In JavaScript it means of the same value and type.
For example,
4 == "4" // will return true
but
4 === "4" // will return false

Why == is so unpredictable?
What do you get when you compare an empty string "" with the number zero 0?
true
Yep, that's right according to == an empty string and the number zero are the same time.
And it doesn't end there, here's another one:
'0' == false // true
Things get really weird with arrays.
[1] == true // true
[] == false // true
[[]] == false // true
[0] == false // true
Then weirder with strings
[1,2,3] == '1,2,3' // true - REALLY?!
'\r\n\t' == 0 // true - Come on!
It get's worse:
When is equal not equal?
let A = '' // empty string
let B = 0 // zero
let C = '0' // zero string
A == B // true - ok...
B == C // true - so far so good...
A == C // **FALSE** - Plot twist!
Let me say that again:
(A == B) && (B == C) // true
(A == C) // **FALSE**
And this is just the crazy stuff you get with primitives.
It's a whole new level of crazy when you use == with objects.
At this point your probably wondering...
Why does this happen?
Well it's because unlike "triple equals" (===) which just checks if two values are the same.
== does a whole bunch of other stuff.
It has special handling for functions, special handling for nulls, undefined, strings, you name it.
It get's pretty wacky.
In fact, if you tried to write a function that does what == does it would look something like this:
function isEqual(x, y) { // if `==` were a function
if(typeof y === typeof x) return y === x;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof y === "function" || typeof x === "function") {
// if either value is a string
// convert the function into a string and compare
if(typeof x === "string") {
return x === y.toString();
} else if(typeof y === "string") {
return x.toString() === y;
}
return false;
}
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
// actually the real `==` is even more complicated than this, especially in ES6
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
So what does this mean?
It means == is complicated.
Because it's complicated it's hard to know what's going to happen when you use it.
Which means you could end up with bugs.
So the moral of the story is...
Make your life less complicated.
Use === instead of ==.
The End.

The === operator is called a strict comparison operator, it does differ from the == operator.
Lets take 2 vars a and b.
For "a == b" to evaluate to true a and b need to be the same value.
In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.
Take the following example
var a = 1;
var b = "1";
if (a == b) //evaluates to true as a and b are both 1
{
alert("a == b");
}
if (a === b) //evaluates to false as a is not the same type as b
{
alert("a === b");
}
In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.
In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.

=== checks same sides are equal in type as well as value.
Example:
'1' === 1 // will return "false" because `string` is not a `number`
Common example:
0 == '' // will be "true", but it's very common to want this check to be "false"
Another common example:
null == undefined // returns "true", but in most cases a distinction is necessary
Many times an untyped check would be handy because you do not care if the value is either undefined, null, 0 or ""

Javascript execution flow diagram for strict equality / Comparison '==='
Javascript execution flow diagram for non strict equality / comparison '=='

JavaScript === vs == .
0==false // true
0===false // false, because they are of a different type
1=="1" // true, auto type coercion
1==="1" // false, because they are of a different type

It means equality without type coercion
type coercion means JavaScript do not automatically convert any other data types to string data types
0==false // true,although they are different types
0===false // false,as they are different types
2=='2' //true,different types,one is string and another is integer but
javaScript convert 2 to string by using == operator
2==='2' //false because by using === operator ,javaScript do not convert
integer to string
2===2 //true because both have same value and same types

In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.
But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.

The equal comparison operator == is confusing and should be avoided.
If you HAVE TO live with it, then remember the following 3 things:
It is not transitive: (a == b) and (b == c) does not lead to (a == c)
It's mutually exclusive to its negation: (a == b) and (a != b) always hold opposite Boolean values, with all a and b.
In case of doubt, learn by heart the following truth table:
EQUAL OPERATOR TRUTH TABLE IN JAVASCRIPT
Each row in the table is a set of 3 mutually "equal" values, meaning that any 2 values among them are equal using the equal == sign*
** STRANGE: note that any two values on the first column are not equal in that sense.**
'' == 0 == false // Any two values among these 3 ones are equal with the == operator
'0' == 0 == false // Also a set of 3 equal values, note that only 0 and false are repeated
'\t' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\r' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\t\r\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
null == undefined // These two "default" values are not-equal to any of the listed values above
NaN // NaN is not equal to any thing, even to itself.

There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.

Simply
== means comparison between operands with type coercion
and
=== means comparison between operands without type coercion.
Type coercion in JavaScript means automatically converting data types to other data types.
For example:
123 == "123" // Returns true, because JS coerces string "123" to number 123
// and then goes on to compare `123 == 123`.
123 === "123" // Returns false, because JS does not coerce values of different types here.

Yes! It does matter.
=== operator in javascript checks value as well as type where as == operator just checks the value (does type conversion if required).
You can easily test it. Paste following code in an HTML file and open it in browser
<script>
function onPageLoad()
{
var x = "5";
var y = 5;
alert(x === 5);
};
</script>
</head>
<body onload='onPageLoad();'>
You will get 'false' in alert. Now modify the onPageLoad() method to alert(x == 5); you will get true.

As a rule of thumb, I would generally use === instead of == (and !== instead of !=).
Reasons are explained in in the answers above and also Douglas Crockford is pretty clear about it (JavaScript: The Good Parts).
However there is one single exception:
== null is an efficient way to check for 'is null or undefined':
if( value == null ){
// value is either null or undefined
}
For example jQuery 1.9.1 uses this pattern 43 times, and the JSHint syntax checker even provides the eqnull relaxing option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;

It's a strict check test.
It's a good thing especially if you're checking between 0 and false and null.
For example, if you have:
$a = 0;
Then:
$a==0;
$a==NULL;
$a==false;
All returns true and you may not want this. Let's suppose you have a function that can return the 0th index of an array or false on failure. If you check with "==" false, you can get a confusing result.
So with the same thing as above, but a strict test:
$a = 0;
$a===0; // returns true
$a===NULL; // returns false
$a===false; // returns false

=== operator checks the values as well as the types of the variables for equality.
== operator just checks the value of the variables for equality.

JSLint sometimes gives you unrealistic reasons to modify stuff. === has exactly the same performance as == if the types are already the same.
It is faster only when the types are not the same, in which case it does not try to convert types but directly returns a false.
So, IMHO, JSLint maybe used to write new code, but useless over-optimizing should be avoided at all costs.
Meaning, there is no reason to change == to === in a check like if (a == 'test') when you know it for a fact that a can only be a String.
Modifying a lot of code that way wastes developers' and reviewers' time and achieves nothing.

A simple example is
2 == '2' -> true, values are SAME because of type conversion.
2 === '2' -> false, values are NOT SAME because of no type conversion.

The top 2 answers both mentioned == means equality and === means identity. Unfortunately, this statement is incorrect.
If both operands of == are objects, then they are compared to see if they are the same object. If both operands point to the same object, then the equal operator returns true. Otherwise,
the two are not equal.
var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b) // false
console.log(a === b) // false
In the code above, both == and === get false because a and b are not the same objects.
That's to say: if both operands of == are objects, == behaves same as ===, which also means identity. The essential difference of this two operators is about type conversion. == has conversion before it checks equality, but === does not.

The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...
var x = 0;
var isTrue = x == null;
var isFalse = x === null;
Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...
CString x;
delete x;
Which obviously during runtime does very undefined things...
Google for implicit conversions in C++ and STL to get some of the arguments against it...

From the core javascript reference
=== Returns true if the operands are strictly equal (see above)
with no type conversion.

Equality comparison:
Operator ==
Returns true, when both operands are equal. The operands are converted to the same type before being compared.
>>> 1 == 1
true
>>> 1 == 2
false
>>> 1 == '1'
true
Equality and type comparison:
Operator ===
Returns true if both operands are equal and of the same type. It's generally
better and safer if you compare this way, because there's no behind-the-scenes type conversions.
>>> 1 === '1'
false
>>> 1 === 1
true

Here is a handy comparison table that shows the conversions that happen and the differences between == and ===.
As the conclusion states:
"Use three equals unless you fully understand the conversions that take
place for two-equals."
http://dorey.github.io/JavaScript-Equality-Table/

null and undefined are nothingness, that is,
var a;
var b = null;
Here a and b do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.

In Javascript why, console.log(0 && 0 === 0); returns 0 instead of true? [duplicate]

Why do these logical operators return an object and not a boolean?
var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );
var _ = obj && obj._;
I want to understand why it returns result of obj.fn() (if it is defined) OR obj._ but not boolean result.

In JavaScript, both || and && are logical short-circuit operators that return the first fully-determined “logical value” when evaluated from left to right.
In expression X || Y, X is first evaluated, and interpreted as a boolean value. If this boolean value is “true”, then it is returned. And Y is not evaluated. (Because it doesn’t matter whether Y is true or Y is false, X || Y has been fully determined.) That is the short-circuit part.
If this boolean value is “false”, then we still don’t know if X || Y is true or false until we evaluate Y, and interpret it as a boolean value as well. So then Y gets returned.
And && does the same, except it stops evaluating if the first argument is false.
The first tricky part is that when an expression is evaluated as “true”, then the expression itself is returned. Which counts as "true" in logical expressions, but you can also use it. So this is why you are seeing actual values being returned.
The second tricky part is that when an expression is evaluated as “false”, then in JS 1.0 and 1.1 the system would return a boolean value of “false”; whereas in JS 1.2 on it returns the actual value of the expression.
In JS false, 0, -0, "", null, undefined, NaN and document.all all count as false.
Here I am of course quoting logical values for discussion’s sake. Of course, the literal string "false" is not the same as the value false, and is therefore true.

In the simplest terms:
The || operator returns the first truthy value, and if none are truthy, it returns the last value (which is a falsy value).
The && operator returns the first falsy value, and if none are falsy, it return the last value (which is a truthy value).
It's really that simple. Experiment in your console to see for yourself.
console.log("" && "Dog"); // ""
console.log("Cat" && "Dog"); // "Dog"
console.log("" || "Dog"); // "Dog"
console.log("Cat" || "Dog"); // "Cat"

var _ = ((obj.fn && obj.fn() ) || obj._ || ( obj._ == {/* something */}))? true: false
will return boolean.
UPDATE
Note that this is based on my testing. I am not to be fully relied upon.
It is an expression that does not assign true or false value. Rather it assigns the calculated value.
Let's have a look at this expression.
An example expression:
var a = 1 || 2;
// a = 1
// it's because a will take the value (which is not null) from left
var a = 0 || 2;
// so for this a=2; //its because the closest is 2 (which is not null)
var a = 0 || 2 || 1; //here also a = 2;
Your expression:
var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );
// _ = closest of the expression which is not null
// in your case it must be (obj.fn && obj.fn())
// so you are gettig this
Another expression:
var a = 1 && 2;
// a = 2
var a = 1 && 2 && 3;
// a = 3 //for && operator it will take the fartest value
// as long as every expression is true
var a = 0 && 2 && 3;
// a = 0
Another expression:
var _ = obj && obj._;
// _ = obj._

In most programming languages, the && and || operators returns boolean. In JavaScript it's different.
OR Operator:
It returns the value of the first operand that validates as true (if any), otherwise it returns the value of the last operand (even if it validates as false).
Example 1:
var a = 0 || 1 || 2 || 3;
^ ^ ^ ^
f t t t
^
first operand that validates as true
so, a = 1
Example 2:
var a = 0 || false || null || '';
^ ^ ^ ^
f f f f
^
no operand validates as true,
so, a = ''
AND Operator:
It returns the value of the last operand that validates as true (if all conditions validates as true), otherwise it returns the value of the first operand that validates as false.
Example 1:
var a = 1 && 2 && 3 && 4;
^ ^ ^ ^
t t t t
^
last operand that validates as true
so, a = 4
Example 2:
var a = 2 && '' && 3 && null;
^ ^ ^ ^
t f t f
^
return first operand that validates as false,
so, a = ''
Conclusion:
If you want JavaScript to act the same way how other programming languages work, use Boolean() function, like this:
var a = Boolean(1 || 2 || 3);// a = true

You should think of the short-circuit operators as conditionals rather than logical operators.
x || y roughly corresponds to:
if ( x ) { return x; } else { return y; }
and x && y roughly corresponds to:
if ( x ) { return y; } else { return x; }
Given this, the result is perfectly understandable.
From MDN documentation:
Logical operators are typically used with Boolean (logical) values. When they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they will return a non-Boolean value.
And here's the table with the returned values of all logical operators.

I think you have basic JavaScript methodology question here.
Now, JavaScript is a loosely typed language. As such, the way and manner in which it treats logical operations differs from that of other standard languages like Java and C++. JavaScript uses a concept known as "type coercion" to determine the value of a logical operation and always returns the value of the first true type. For instance, take a look at the code below:
var x = mystuff || document;
// after execution of the line above, x = document
This is because mystuff is an a priori undefined entity which will always evaluate to false when tested and as such, JavaScript skips this and tests the next entity for a true value. Since the document object is known to JavaScript, it returns a true value and JavaScript returns this object.
If you wanted a boolean value returned to you, you would have to pass your logical condition statement to a function like so:
var condition1 = mystuff || document;
function returnBool(cond){
if(typeof(cond) != 'boolean'){ //the condition type will return 'object' in this case
return new Boolean(cond).valueOf();
}else{ return; }
}
// Then we test...
var condition2 = returnBool(condition1);
window.console.log(typeof(condition2)); // outputs 'boolean'

We can refer to the spec(11.11) of JS here of:
Semantics
The production LogicalANDExpression :LogicalANDExpression &&BitwiseORExpression is evaluated as follows:
Evaluate LogicalANDExpression.
2.Call GetValue(Result(1)).
3.Call ToBoolean(Result(2)).
4.If Result(3) is false, return Result(2).
5.Evaluate BitwiseORExpression.
6.Call GetValue(Result(5)).
7.Return Result(6).
see here for the spec

First, it has to be true to return, so if you are testing for truthfulness then it makes no difference
Second, it lets you do assignments along the lines of:
function bar(foo) {
foo = foo || "default value";

Compare:
var prop;
if (obj.value) {prop=obj.value;}
else prop=0;
with:
var prop=obj.value||0;
Returning a truthy expression - rather than just true or false - usually makes your code shorter and still readable. This is very common for ||, not so much for &&.

How does Javascript equality operator works? [duplicate]

This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with ===?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over ==?

The strict equality operator (===) behaves identically to the abstract equality operator (==) except no type conversion is done, and the types must be the same to be considered equal.
Reference: Javascript Tutorial: Comparison Operators
The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then === produces true and !== produces false. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '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
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use === and !==. All of the comparisons just shown produce false with the === operator.
Update:
A good point was brought up by #Casebash in the comments and in #Phillipe Laybaert's answer concerning objects. For objects, == and === act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString or valueOf method. For example, consider the comparison of a string primitive with a string object created using the String constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the == operator is checking the values of the two objects and returning true, but the === is seeing that they're not the same type and returning false. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String constructor to create string objects from string literals.
Reference
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion, meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion, and thus does not convert the values when comparing.

Here's an interesting visualisation of the equality comparison between == and ===.
Source: https://github.com/dorey/JavaScript-Equality-Table (demo, unified demo)
var1 === var2
When using === for JavaScript equality testing, everything is as is.
Nothing gets converted before being evaluated.
var1 == var2
When using == for JavaScript equality testing, some funky conversions take place.
Summary of equality in Javascript
Conclusion:
Always use ===, unless you fully understand the funky conversions that take place with ==.

In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type, but that's not really true. It actually means that both operands reference the same object, or in case of value types, have the same value.
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object, while b is of type string. Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.

Let me add this counsel:
If in doubt, read the specification!
ECMA-262 is the specification for a scripting language of which JavaScript is a dialect. Of course in practice it matters more how the most important browsers behave than an esoteric definition of how something is supposed to be handled. But it is helpful to understand why new String("a") !== "a".
Please let me explain how to read the specification to clarify this question. I see that in this very old topic nobody had an answer for the very strange effect. So, if you can read a specification, this will help you in your profession tremendously. It is an acquired skill. So, let's continue.
Searching the PDF file for === brings me to page 56 of the specification: 11.9.4. The Strict Equals Operator ( === ), and after wading through the specificationalese I find:
11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
  1. If Type(x) is different from Type(y), return false.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is not Number, go to step 11.
  5. If x is NaN, return false.
  6. If y is NaN, return false.
  7. If x is the same number value as y, return true.
  8. If x is +0 and y is −0, return true.
  9. If x is −0 and y is +0, return true.
  10. Return false.
  11. 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.
  12. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
  13. Return true if x and y refer to the same object or if they refer to objects joined to each other (see 13.1.2). Otherwise, return false.
Interesting is step 11. Yes, strings are treated as value types. But this does not explain why new String("a") !== "a". Do we have a browser not conforming to ECMA-262?
Not so fast!
Let's check the types of the operands. Try it out for yourself by wrapping them in typeof(). I find that new String("a") is an object, and step 1 is used: return false if the types are different.
If you wonder why new String("a") does not return a string, how about some exercise reading a specification? Have fun!
Aidiakapi wrote this in a comment below:
From the specification
11.2.2 The new Operator:
If Type(constructor) is not Object, throw a TypeError exception.
With other words, if String wouldn't be of type Object it couldn't be used with the new operator.
new always returns an Object, even for String constructors, too. And alas! The value semantics for strings (see step 11) is lost.
And this finally means: new String("a") !== "a".

I tested this in Firefox with Firebug using code like this:
console.time("testEquality");
var n = 0;
while (true) {
n++;
if (n == 100000)
break;
}
console.timeEnd("testEquality");
and
console.time("testTypeEquality");
var n = 0;
while (true) {
n++;
if (n === 100000)
break;
}
console.timeEnd("testTypeEquality");
My results (tested five times each and averaged):
==: 115.2
===: 114.4
So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.

In PHP and JavaScript, it is a strict equality operator. Which means, it will compare both type and values.

In JavaScript it means of the same value and type.
For example,
4 == "4" // will return true
but
4 === "4" // will return false

Why == is so unpredictable?
What do you get when you compare an empty string "" with the number zero 0?
true
Yep, that's right according to == an empty string and the number zero are the same time.
And it doesn't end there, here's another one:
'0' == false // true
Things get really weird with arrays.
[1] == true // true
[] == false // true
[[]] == false // true
[0] == false // true
Then weirder with strings
[1,2,3] == '1,2,3' // true - REALLY?!
'\r\n\t' == 0 // true - Come on!
It get's worse:
When is equal not equal?
let A = '' // empty string
let B = 0 // zero
let C = '0' // zero string
A == B // true - ok...
B == C // true - so far so good...
A == C // **FALSE** - Plot twist!
Let me say that again:
(A == B) && (B == C) // true
(A == C) // **FALSE**
And this is just the crazy stuff you get with primitives.
It's a whole new level of crazy when you use == with objects.
At this point your probably wondering...
Why does this happen?
Well it's because unlike "triple equals" (===) which just checks if two values are the same.
== does a whole bunch of other stuff.
It has special handling for functions, special handling for nulls, undefined, strings, you name it.
It get's pretty wacky.
In fact, if you tried to write a function that does what == does it would look something like this:
function isEqual(x, y) { // if `==` were a function
if(typeof y === typeof x) return y === x;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof y === "function" || typeof x === "function") {
// if either value is a string
// convert the function into a string and compare
if(typeof x === "string") {
return x === y.toString();
} else if(typeof y === "string") {
return x.toString() === y;
}
return false;
}
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
// actually the real `==` is even more complicated than this, especially in ES6
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
So what does this mean?
It means == is complicated.
Because it's complicated it's hard to know what's going to happen when you use it.
Which means you could end up with bugs.
So the moral of the story is...
Make your life less complicated.
Use === instead of ==.
The End.

The === operator is called a strict comparison operator, it does differ from the == operator.
Lets take 2 vars a and b.
For "a == b" to evaluate to true a and b need to be the same value.
In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.
Take the following example
var a = 1;
var b = "1";
if (a == b) //evaluates to true as a and b are both 1
{
alert("a == b");
}
if (a === b) //evaluates to false as a is not the same type as b
{
alert("a === b");
}
In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.
In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.

=== checks same sides are equal in type as well as value.
Example:
'1' === 1 // will return "false" because `string` is not a `number`
Common example:
0 == '' // will be "true", but it's very common to want this check to be "false"
Another common example:
null == undefined // returns "true", but in most cases a distinction is necessary
Many times an untyped check would be handy because you do not care if the value is either undefined, null, 0 or ""

Javascript execution flow diagram for strict equality / Comparison '==='
Javascript execution flow diagram for non strict equality / comparison '=='

JavaScript === vs == .
0==false // true
0===false // false, because they are of a different type
1=="1" // true, auto type coercion
1==="1" // false, because they are of a different type

It means equality without type coercion
type coercion means JavaScript do not automatically convert any other data types to string data types
0==false // true,although they are different types
0===false // false,as they are different types
2=='2' //true,different types,one is string and another is integer but
javaScript convert 2 to string by using == operator
2==='2' //false because by using === operator ,javaScript do not convert
integer to string
2===2 //true because both have same value and same types

In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.
But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.

The equal comparison operator == is confusing and should be avoided.
If you HAVE TO live with it, then remember the following 3 things:
It is not transitive: (a == b) and (b == c) does not lead to (a == c)
It's mutually exclusive to its negation: (a == b) and (a != b) always hold opposite Boolean values, with all a and b.
In case of doubt, learn by heart the following truth table:
EQUAL OPERATOR TRUTH TABLE IN JAVASCRIPT
Each row in the table is a set of 3 mutually "equal" values, meaning that any 2 values among them are equal using the equal == sign*
** STRANGE: note that any two values on the first column are not equal in that sense.**
'' == 0 == false // Any two values among these 3 ones are equal with the == operator
'0' == 0 == false // Also a set of 3 equal values, note that only 0 and false are repeated
'\t' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\r' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
'\t\r\n' == 0 == false // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
null == undefined // These two "default" values are not-equal to any of the listed values above
NaN // NaN is not equal to any thing, even to itself.

There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.

Simply
== means comparison between operands with type coercion
and
=== means comparison between operands without type coercion.
Type coercion in JavaScript means automatically converting data types to other data types.
For example:
123 == "123" // Returns true, because JS coerces string "123" to number 123
// and then goes on to compare `123 == 123`.
123 === "123" // Returns false, because JS does not coerce values of different types here.

Yes! It does matter.
=== operator in javascript checks value as well as type where as == operator just checks the value (does type conversion if required).
You can easily test it. Paste following code in an HTML file and open it in browser
<script>
function onPageLoad()
{
var x = "5";
var y = 5;
alert(x === 5);
};
</script>
</head>
<body onload='onPageLoad();'>
You will get 'false' in alert. Now modify the onPageLoad() method to alert(x == 5); you will get true.

As a rule of thumb, I would generally use === instead of == (and !== instead of !=).
Reasons are explained in in the answers above and also Douglas Crockford is pretty clear about it (JavaScript: The Good Parts).
However there is one single exception:
== null is an efficient way to check for 'is null or undefined':
if( value == null ){
// value is either null or undefined
}
For example jQuery 1.9.1 uses this pattern 43 times, and the JSHint syntax checker even provides the eqnull relaxing option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;

It's a strict check test.
It's a good thing especially if you're checking between 0 and false and null.
For example, if you have:
$a = 0;
Then:
$a==0;
$a==NULL;
$a==false;
All returns true and you may not want this. Let's suppose you have a function that can return the 0th index of an array or false on failure. If you check with "==" false, you can get a confusing result.
So with the same thing as above, but a strict test:
$a = 0;
$a===0; // returns true
$a===NULL; // returns false
$a===false; // returns false

=== operator checks the values as well as the types of the variables for equality.
== operator just checks the value of the variables for equality.

JSLint sometimes gives you unrealistic reasons to modify stuff. === has exactly the same performance as == if the types are already the same.
It is faster only when the types are not the same, in which case it does not try to convert types but directly returns a false.
So, IMHO, JSLint maybe used to write new code, but useless over-optimizing should be avoided at all costs.
Meaning, there is no reason to change == to === in a check like if (a == 'test') when you know it for a fact that a can only be a String.
Modifying a lot of code that way wastes developers' and reviewers' time and achieves nothing.

A simple example is
2 == '2' -> true, values are SAME because of type conversion.
2 === '2' -> false, values are NOT SAME because of no type conversion.

The top 2 answers both mentioned == means equality and === means identity. Unfortunately, this statement is incorrect.
If both operands of == are objects, then they are compared to see if they are the same object. If both operands point to the same object, then the equal operator returns true. Otherwise,
the two are not equal.
var a = [1, 2, 3];
var b = [1, 2, 3];
console.log(a == b) // false
console.log(a === b) // false
In the code above, both == and === get false because a and b are not the same objects.
That's to say: if both operands of == are objects, == behaves same as ===, which also means identity. The essential difference of this two operators is about type conversion. == has conversion before it checks equality, but === does not.

The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...
var x = 0;
var isTrue = x == null;
var isFalse = x === null;
Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...
CString x;
delete x;
Which obviously during runtime does very undefined things...
Google for implicit conversions in C++ and STL to get some of the arguments against it...

From the core javascript reference
=== Returns true if the operands are strictly equal (see above)
with no type conversion.

Equality comparison:
Operator ==
Returns true, when both operands are equal. The operands are converted to the same type before being compared.
>>> 1 == 1
true
>>> 1 == 2
false
>>> 1 == '1'
true
Equality and type comparison:
Operator ===
Returns true if both operands are equal and of the same type. It's generally
better and safer if you compare this way, because there's no behind-the-scenes type conversions.
>>> 1 === '1'
false
>>> 1 === 1
true

Here is a handy comparison table that shows the conversions that happen and the differences between == and ===.
As the conclusion states:
"Use three equals unless you fully understand the conversions that take
place for two-equals."
http://dorey.github.io/JavaScript-Equality-Table/

null and undefined are nothingness, that is,
var a;
var b = null;
Here a and b do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.

What's the difference between ( | ) and ( || )?

What's the difference between | and || in Javascript?
Furthermore, what's the difference between & and &&?

| is a bitwise or, || is a logical or.
A bitwise or takes the two numbers and compares them on a bit-by-bit basis, producing a new integer which combines the 1 bits from both inputs. So 0101 | 1010 would produce 1111.
A logical or || checks for the "truthiness" of a value (depends on the type, for integers 0 is false and non-zero is true). It evaluates the statement left to right, and returns the first value which is truthy. So 0101 || 1010 would return 0101 which is truthy, therefore the whole statement is said to be true.
The same type of logic applies for & vs &&. 0101 & 1010 = 0000. However 0101 && 1010 evaluates to 1010 (&& returns the last truthy value so long as both operands are truthy).

& is the bitwise AND operator
| is the bitwise OR operator
&& is the logical AND operator
|| is the logical OR operator
The difference is that logical operators only consider each input at face value, treating them as whole, while bitwise operators work at the bit level:
var thetruth = false;
var therest = true;
var theuniverse = thetruth && therest; //false
var theparallel = thetruth && thetruth; //true
var theindifferent = thetruth || therest; //true
var theideal = thetruth || thetruth; // false
var thematrix = 5346908590;
var mrsmith = 2354656767;
var theoracle = thematrix & mrsmith; //202445230
var theone = thematrix | mrsmith; //7499120127

Another difference is that || uses shortcut evaluation. That is, it only evaluates the right side if the left side is false (or gets converted to false in a boolean context, e.g. 0, "", null, etc.). Similarly, && only evaluates the right side if the left side is true (or non-zero, non-empty string, an object, etc.). | and & always evaluate both sides because the result depends on the exact bits in each value.
The reasoning is that for ||, if either side is true, the whole expression is true, so there's no need to evaluate any further. && is the same but reversed.
The exact logic for || is that if the left hand side is "truthy", return that value (note that it is not converted to a boolean), otherwise, evaluate and return the right hand side. For &&, if the left hand side is "falsey", return it, otherwise evaluate and return the right hand side.
Here are some examples:
false && console.log("Nothing happens here");
true || console.log("Or here");
false || console.log("This DOES get logged");
"foo" && console.log("So does this");
if (obj && obj.property) // make sure obj is not null before accessing a property

To explain a little more in layman's terms:
&& and || are logical operators. This means they're used for logical comparison;
if (a == 4 && b == 5)
This means "If a equals to four AND b equals to five"
| and & are bitwise operators. They operate on bits in a specific fashion which the wiki article explains in detail:
http://en.wikipedia.org/wiki/Bitwise_operation

In Javascript perspective, there is more to it.
var a = 42;
var b = "abc";
var c = null;
a || b; // 42
a && b; // "abc"
c || b; // "abc"
c && b; // null
Both || and && operators perform a boolean test on the first operand (a or c). If the operand is not already boolean (as it's not, here), a normal ToBoolean coercion occurs, so that the test can be performed.
For the || operator, if the test is true, the || expression results in the value of the first operand (a or c). If the test is false, the || expression results in the value of the second operand (b).
Inversely, for the && operator, if the test is true, the && expression results in the value of the second operand (b). If the test is false, the && expression results in the value of the first operand (a or c).
The result of a || or && expression is always the underlying value of one of the operands, not the (possibly coerced) result of the test. In c && b, c is null, and thus falsy. But the && expression itself results in null (the value in c), not in the coerced false used in the test.

Single | is bit wise OR operator .
If you do 2 | 3 , it converts to binary and performs OR operation.
01
11
Results in 11 equal to 3.
Where as || operator checks if first argument is true, if it is true it returns else it goes to other operator.
2 || 3 returns 2 since 2 is true.

one more point i want to add is || operator is used to assign default value in case the value you are assigning is undefined. So for Exapmle you are assigning a obj to some object test and if you dont want test to be undefined then you can do the following to ensure that value of test wont be undefined.
var test = obj || {};
so in case obj is undefined then test's value will be empty object.
So it is also being used for assigning default value to your object.

Why don't logical operators (&& and ||) always return a boolean result?

Why do these logical operators return an object and not a boolean?
var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );
var _ = obj && obj._;
I want to understand why it returns result of obj.fn() (if it is defined) OR obj._ but not boolean result.

In JavaScript, both || and && are logical short-circuit operators that return the first fully-determined “logical value” when evaluated from left to right.
In expression X || Y, X is first evaluated, and interpreted as a boolean value. If this boolean value is “true”, then it is returned. And Y is not evaluated. (Because it doesn’t matter whether Y is true or Y is false, X || Y has been fully determined.) That is the short-circuit part.
If this boolean value is “false”, then we still don’t know if X || Y is true or false until we evaluate Y, and interpret it as a boolean value as well. So then Y gets returned.
And && does the same, except it stops evaluating if the first argument is false.
The first tricky part is that when an expression is evaluated as “true”, then the expression itself is returned. Which counts as "true" in logical expressions, but you can also use it. So this is why you are seeing actual values being returned.
The second tricky part is that when an expression is evaluated as “false”, then in JS 1.0 and 1.1 the system would return a boolean value of “false”; whereas in JS 1.2 on it returns the actual value of the expression.
In JS false, 0, -0, "", null, undefined, NaN and document.all all count as false.
Here I am of course quoting logical values for discussion’s sake. Of course, the literal string "false" is not the same as the value false, and is therefore true.

In the simplest terms:
The || operator returns the first truthy value, and if none are truthy, it returns the last value (which is a falsy value).
The && operator returns the first falsy value, and if none are falsy, it return the last value (which is a truthy value).
It's really that simple. Experiment in your console to see for yourself.
console.log("" && "Dog"); // ""
console.log("Cat" && "Dog"); // "Dog"
console.log("" || "Dog"); // "Dog"
console.log("Cat" || "Dog"); // "Cat"

var _ = ((obj.fn && obj.fn() ) || obj._ || ( obj._ == {/* something */}))? true: false
will return boolean.
UPDATE
Note that this is based on my testing. I am not to be fully relied upon.
It is an expression that does not assign true or false value. Rather it assigns the calculated value.
Let's have a look at this expression.
An example expression:
var a = 1 || 2;
// a = 1
// it's because a will take the value (which is not null) from left
var a = 0 || 2;
// so for this a=2; //its because the closest is 2 (which is not null)
var a = 0 || 2 || 1; //here also a = 2;
Your expression:
var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );
// _ = closest of the expression which is not null
// in your case it must be (obj.fn && obj.fn())
// so you are gettig this
Another expression:
var a = 1 && 2;
// a = 2
var a = 1 && 2 && 3;
// a = 3 //for && operator it will take the fartest value
// as long as every expression is true
var a = 0 && 2 && 3;
// a = 0
Another expression:
var _ = obj && obj._;
// _ = obj._

In most programming languages, the && and || operators returns boolean. In JavaScript it's different.
OR Operator:
It returns the value of the first operand that validates as true (if any), otherwise it returns the value of the last operand (even if it validates as false).
Example 1:
var a = 0 || 1 || 2 || 3;
^ ^ ^ ^
f t t t
^
first operand that validates as true
so, a = 1
Example 2:
var a = 0 || false || null || '';
^ ^ ^ ^
f f f f
^
no operand validates as true,
so, a = ''
AND Operator:
It returns the value of the last operand that validates as true (if all conditions validates as true), otherwise it returns the value of the first operand that validates as false.
Example 1:
var a = 1 && 2 && 3 && 4;
^ ^ ^ ^
t t t t
^
last operand that validates as true
so, a = 4
Example 2:
var a = 2 && '' && 3 && null;
^ ^ ^ ^
t f t f
^
return first operand that validates as false,
so, a = ''
Conclusion:
If you want JavaScript to act the same way how other programming languages work, use Boolean() function, like this:
var a = Boolean(1 || 2 || 3);// a = true

You should think of the short-circuit operators as conditionals rather than logical operators.
x || y roughly corresponds to:
if ( x ) { return x; } else { return y; }
and x && y roughly corresponds to:
if ( x ) { return y; } else { return x; }
Given this, the result is perfectly understandable.
From MDN documentation:
Logical operators are typically used with Boolean (logical) values. When they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they will return a non-Boolean value.
And here's the table with the returned values of all logical operators.

I think you have basic JavaScript methodology question here.
Now, JavaScript is a loosely typed language. As such, the way and manner in which it treats logical operations differs from that of other standard languages like Java and C++. JavaScript uses a concept known as "type coercion" to determine the value of a logical operation and always returns the value of the first true type. For instance, take a look at the code below:
var x = mystuff || document;
// after execution of the line above, x = document
This is because mystuff is an a priori undefined entity which will always evaluate to false when tested and as such, JavaScript skips this and tests the next entity for a true value. Since the document object is known to JavaScript, it returns a true value and JavaScript returns this object.
If you wanted a boolean value returned to you, you would have to pass your logical condition statement to a function like so:
var condition1 = mystuff || document;
function returnBool(cond){
if(typeof(cond) != 'boolean'){ //the condition type will return 'object' in this case
return new Boolean(cond).valueOf();
}else{ return; }
}
// Then we test...
var condition2 = returnBool(condition1);
window.console.log(typeof(condition2)); // outputs 'boolean'

We can refer to the spec(11.11) of JS here of:
Semantics
The production LogicalANDExpression :LogicalANDExpression &&BitwiseORExpression is evaluated as follows:
Evaluate LogicalANDExpression.
2.Call GetValue(Result(1)).
3.Call ToBoolean(Result(2)).
4.If Result(3) is false, return Result(2).
5.Evaluate BitwiseORExpression.
6.Call GetValue(Result(5)).
7.Return Result(6).
see here for the spec

First, it has to be true to return, so if you are testing for truthfulness then it makes no difference
Second, it lets you do assignments along the lines of:
function bar(foo) {
foo = foo || "default value";

Compare:
var prop;
if (obj.value) {prop=obj.value;}
else prop=0;
with:
var prop=obj.value||0;
Returning a truthy expression - rather than just true or false - usually makes your code shorter and still readable. This is very common for ||, not so much for &&.