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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.
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The logical NOT ( ! ) operator (logical complement, negation) takes truth to falsity and vice versa. It is typically used with boolean (logical) values. When used with non-Boolean values, it returns false if its single operand can be converted to true ; otherwise, returns true .
The logical NOT operator is represented as the '!' symbol, which is used to reverse the result of any given expression or condition. If the result of an expression is non-zero or true, the result will be reversed as zero or false value.
Answer: A logical operator is a symbol or word used to connect two or more expressions such that the value of the compound expression produced depends only on that of the original expressions and on the meaning of the operator. Common logical operators include AND, OR, and NOT.
Logical operators combine relations according to the following rules: The ampersand (&) symbol is a valid substitute for the logical operator AND . The vertical bar ( | ) is a valid substitute for the logical operator OR . Only one logical operator can be used to combine two relations.
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.
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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"If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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