Logical Operators

If we use the && and || in a conditional, this is how they work:

console.log(10 < 20 && 30 > 15 && 40 > 30); // Must all be true
console.log(10 > 20 || 30 < 15); // Only one has to be true

There may be cases where you want to conditionally assign a value based on if a value is truthy or falsy. We can use logical operators and logical assignment for this.

AND Operator (&&)

If we assign a value, using the && operator, it will execute from left to right. If any of the values are falsy, that value will be returned; otherwise, the last value will be returned.

x = 10 && 20; // 20
x = 10 && 20 && 30; // 30
x = 10 && 0 && 30; // 0 (30 is not evaluated)
x = 10 && false && 30; // false (30 is not evaluated)

OR Operator (||)

The || operator is the opposite of the && operator. It will return the first value that is truthy. This is more common than using the && operator.

y = 10 || 20; // 10 (20 is not evaluated)
y = 0 || 20; // 20
y = 10 || 0 || 30; // 10 (0 and 30 are not evaluated)
y = false || false || 30; // 30

Nullish Coalescing Operator (??)

The ?? operator will return the right side operand when its left side is either null or undefined. It is similar to || operator except, it doesn't look at all falsey values, only null and undefined.

let c;
c = 10 ?? 20; // 10
c = null ?? 20; // 20
c = undefined ?? 20; // 20
c = null ?? 0 ?? 20; // 0 (it will return 0 because it is not null or undefined)