Javascript AND operator within assignment

Basically, the Logical AND operator (&&), will return the value of the second operand if the first is truthy, and it will return the value of the first operand if it is by itself falsy, for example:

true && "foo"; // "foo"
NaN && "anything"; // NaN
0 && "anything";   // 0

Note that falsy values are those that coerce to false when used in boolean context, they are null, undefined, 0, NaN, an empty string, and of course false, anything else coerces to true.

&& is sometimes called a guard operator.

variable = indicator && value

it can be used to set the value only if the indicator is truthy.

Beginners Example

If you are trying to access "user.name" but then this happens:

Uncaught TypeError: Cannot read property 'name' of undefined 

Fear not. You can use ES6 optional chaining on modern browsers today.

const username = user?.name;

See MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

Here's some deeper explanations on guard operators that may prove useful in understanding.

Before optional chaining was introduced, you would solve this using the && operator in an assignment or often called the guard operator since it "guards" from the undefined error happening.

Here are some examples you may find odd but keep reading as it is explained later.

var user = undefined;
var username = user && user.username;
// no error, "username" assigned value of "user" which is undefined

user = { username: 'Johnny' };
username = user && user.username;
// no error, "username" assigned 'Johnny'

user = { };
username = user && user.username;
// no error, "username" assigned value of "username" which is undefined

Explanation: In the guard operation, each term is evaluated left-to-right one at a time. If a value evaluated is falsy, evaluation stops and that value is then assigned. If the last item is reached, it is then assigned whether or not it is falsy.

falsy means it is any one of these values undefined, false, 0, null, NaN, '' and truthy just means NOT falsy.

Bonus: The OR Operator

The other useful strange assignment that is in practical use is the OR operator which is typically used for plugins like so:

this.myWidget = this.myWidget || (function() {
   // define widget
})();

which will only assign the code portion if "this.myWidget" is falsy. This is handy because you can declare code anywhere and multiple times not caring if its been assigned or not previously, knowing it will only be assigned once since people using a plugin might accidentally declare your script tag src multiple times.

Explanation: Each value is evaluated from left-to-right, one at a time. If a value is truthy, it stops evaluation and assigns that value, otherwise, keeps going, if the last item is reached, it is assigned regardless if it is falsy or not.

Extra Credit: Combining && and || in an assignment

You now have ultimate power and can do very strange things such as this very odd example of using it in a palindrome.

function palindrome(s,i) {
 return (i=i || 0) < 0 || i >= s.length >> 1 || s[i] == s[s.length - 1 - i] && isPalindrome(s,++i);
}

In depth explanation here: Palindrome check in Javascript

Happy coding.