This question already has answers here:
JavaScript || or operator with an undefined variable
(6 answers)
Closed 7 years ago.
what does this means?
var q = [];
var q2 = ["canada","usa","mexico"];
var w = q||q2;
document.writeln(w);
the value of the variable w is : [] the empty list.
can someone explain to me why it is displaying this [] instead of ["canada","usa","mexico"].
You should read Logical Operators from MDN. According to the documentation
The logical operator OR (||) expr1 || expr2
Returns expr1 if it can be converted to true; otherwise, returns
expr2. Thus, when used with Boolean values, || returns true if either
operand is true; if both are false, returns false.
In a nutshell, logical operators evaluate from left to right. In your situation, since you have declare q as an empty array ([]), it evaluates to true and immediately goes and assigns w to that.
If you want q2 to take precedence, you can simply do
var w = q2 || q;
so that way only if q2 evaluates to falsey will it assign w to be q instead. The other option is to not declare q at all or declare it as something falsey. You can find out what evaluates to false here.
Related
This question already has answers here:
Logical operators in JavaScript — how do you use them?
(2 answers)
Javascript AND operator within assignment
(7 answers)
Closed 1 year ago.
I am practising the logical operators in JavaScript and fully understood the concepts, but It seems that I didn't with this equation.
const one = 1;
const two = 5;
console.log(one && two);
Why in this case it returns five and not one, shouldn't be returned the first value since both are true ?
From MDN on the && operator:
"If expr1 can be converted to true, returns expr2; else, returns
expr1."
So in this case, 1 can be converted to true, so it returns the second value, 5.
The LOGICAL && operator returns the last value if all other values are true, or else it will return the first non truthy value.
i.e. Java != JavaScript
This question already has answers here:
Is there a "null coalescing" operator in JavaScript?
(19 answers)
Closed 4 years ago.
I have a function that may return a value or may return null and I want to assign it to a variable. So at the moment I've got
someVar = (someFun())
? someFun()
: "foo";
Is there a shorterer way of doing it, where I don't have to call the function twice, or add an extra variable like:
funResult = someFun();
someVar = (funResult)
? funResult
: "foo";
Basically, something like:
someVar = someFun() | "foo"
The idiomatic way is
someVar = someFun() || "foo";
The || operator will yield the value of the left-hand expression if it's truthy. If not, it will move on to evaluate the right-hand side, and return its value.
For those unfamiliar with the term "truthy", in JavaScript it means values that are not considered by the language to be implicitly false. The implicitly false values are 0, NaN, false (of course), "", null, and undefined. Any reference to an object, even if it's completely empty, is considered to be "truthy".
In your specific example, the || approach is to be preferred because it means you only call someFun() once.
This question already has answers here:
Why don't logical operators (&& and ||) always return a boolean result?
(9 answers)
Closed 5 years ago.
There is one interview question below.
The logical AND of two truths should be true. But the output is 3. Why?
var a = 2;
var b = 3;
var c = a && b; // value of c = 3
console.log(c);
Check this out.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators
If you use && with non-boolean values, it returns the first element if it can be converted to false. If it cannot converted to false, it returns second element
Returns a if it can be converted to false; otherwise, returns b.
This question already has answers here:
Is there a "null coalescing" operator in JavaScript?
(19 answers)
Closed 5 years ago.
I just saw this syntax in PHP:
// Fetches the value of $_GET['user'] and returns 'nobody'
// if it does not exist.
$username = $_GET['user'] ?? 'nobody';
Why don't we have the same in JavaScript?
I am tired of doing:
var name = obj['name'] ? obj['name'] : 'GOD';
The Null coalescing operator is a recent addition to PHP. It was introduced in PHP 7 (released in December 2015), more than 10 years after the feature was proposed for the first time.
In Javascript, the logical OR operator can be used for this purpose for ages (since Javascript was created?!).
As the documentation explains:
Logical OR (||)
expr1 || expr2
Returns expr1 if it can be converted to true; otherwise, returns expr2.
Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.
Instead of writing
var name = obj['name'] ? obj['name'] : 'GOD';
you can use the shorter:
var name = obj['name'] || 'GOD';
The || operator can be used several times to create a longer expression that evaluates to the value of the first operand that is not empty:
var name = obj['name'] || obj['desc'] || 'GOD';
In javascript you can do the following:
var name = obj['name'] || "GOD"
If the first value is false (null, false, 0, NaN, "" or undefined), then the second value is going to be assigned.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
JavaScript: Is “z=(x||y)” same as “z=x?x:y” for non-boolean?
Are the following two lines of code equivalent in javascript?
a = b ? b : c
a = b || c
I want to express: "a should be assigned b if b is truthy, otherwise a should be assigned c"
I expect they would both work exactly the same, but I'm not 100% sure.
Yes. The two are almost exactly identical.
Both will first evaluate b. If it's truthy, it'll return b. Else, it'll return c.
As pointed out by #thesystem, if you have a getter method on b, it'll be called twice for the ternary, but only once for the or statement.
Test it using the following snippet:
var o = {};
o.__defineGetter__("b", function() {
console.log('ran');
return true;
});
var d = o.b || o.not;
console.log('-----');
var d = o.b ? o.b : o.not;
Here's the fiddle: http://jsfiddle.net/bqsey/
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 may
return a non-Boolean value.
ref: Logical Operators - MDN