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What is the difference between && and ||? *?

The && and || Operators in JavaScript. If applied to boolean values, the && operator only returns true when both of its operands are true (and false in all other cases), while the || operator only returns false when both of its operands are false (and true in all other cases).

mariusschulz.com - The && and || Operators in JavaScript - Marius Schulz

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Similar to other C-like programming languages, JavaScript defines the two operators && and || which represent the logical AND and OR operations, respectively. Using only the two boolean values true and false , we can generate the following truth tables: // Logical AND operation true && true ; // true true && false ; // false false && true ; // false false && false ; // false // Logical OR operation true || true ; // true true || false ; // true false || true ; // true false || false ; // false If applied to boolean values, the && operator only returns true when both of its operands are true (and false in all other cases), while the || operator only returns false when both of its operands are false (and true in all other cases).

#Using Logical Operators with Non-Boolean Values

In JavaScript, the logical operators have different semantics than other C-like languages, though. They can operate on expressions of any type, not just booleans. Also, the logical operators do not always return a boolean value, as the specification points out in section 12.12: The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

The following examples showcase some values produced by && and || :

"foo" && "bar" ; // "bar" "bar" && "foo" ; // "foo" "foo" && "" ; // "" "" && "foo" ; // "" "foo" || "bar" ; // "foo" "bar" || "foo" ; // "bar" "foo" || "" ; // "foo" "" || "foo" ; // "foo" Both && and || result in the value of (exactly) one of their operands: A && B returns the value A if A can be coerced into false ; otherwise, it returns B.

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returns the value A if A can be coerced into ; otherwise, it returns B. A || B returns the value A if A can be coerced into true ; otherwise, it returns B. They select one of their operands, as noted by Kyle Simpson in his You Don't Know JS series: In fact, I would argue these operators shouldn't even be called "logical operators", as that name is incomplete in describing what they do. If I were to give them a more accurate (if more clumsy) name, I'd call them "selector operators," or more completely, "operand selector operators."

#Control Flow Structures and Truthy Values

In practice, you might not even notice that && and || don't always produce a boolean value. The body of control flow structures like if -statements and loops will be executed when the condition evaluates to a "truthy" value, which doesn't have to be a proper boolean: let values = [ 1 , 2 , 3 ]; while (values. length ) { console. log (values. pop ()); } // 3 // 2 // 1 So when is a value considered truthy? In JavaScript, all values are considered truthy, except for the following falsy values:

false

undefined

null

NaN

0 -0

0n

""

The above while -loop works because after popping the last element, values.length returns the "falsy" value 0 . Therefore, the loop body won't be executed and the loop terminates.

#Truthy and Falsy Return Values

Let's look at an example where it actually matters that && and || don't necessarily produce a boolean value. Imagine you're developing a web application. Users can be signed out, in which case the user object is null , or they can be signed in, in which case the user object exists and has an isAdmin property. let user = { isAdmin : true }; // ... if (user && user. isAdmin ) { // ... }

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function isAdministrator (user) { return user && user. isAdmin ; } let user = { isAdmin : true }; if ( isAdministrator (user)) { // ... }

But what happens if the user object is null ?

The expression user && user.isAdmin evaluates to null , its first operand, because user contains a "falsy" value. Now, a function called isAdministrator should only return boolean values, as the prefix is in the name suggests.

#Coercion to Boolean Values

In JavaScript, a common way to coerce any value into a boolean is to apply the logical NOT operator ! twice: The ! operator, produces the value false if its single operand can be coerced into true ; otherwise, it returns true . The result is always a proper boolean, but the truthiness of the operand is flipped. Applying the ! operator twice undoes the flipping: !! true = ! false = true ; !! false = ! true = false ; !! 0 = ! true = false ; !! 1 = ! false = true ; Another option would've been to call the Boolean function, which is a slightly more explicit way to convert a given value to a proper boolean value:

function isAdministrator (user) { return Boolean (user && user. isAdmin ); }

mariusschulz.com - The && and || Operators in JavaScript - Marius Schulz

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