returning with &&

Question

What does it mean to return a value with &&?

else if (document.defaultView && document.defaultView.getComputedStyle) {

    // It uses the t         


        

Answer 1

The AND && operator does the following:

• Evaluate operands from left to right.
• For each operand, convert it to a boolean. If the result is false, stop and return the original value of that result.
• If all other operands have been assessed (i.e. all were truthy), return the last operand.

As I said, each operand is convert to a boolean, if it's 0 it's falsy and every other value different than 0 (1, 56, -2, etc etc) are truthy

In other words, AND returns the first falsy value or the last value if none were found.

// if the first operand is truthy,
// AND returns the second operand:
return 1 && 0 // 0
return 1 && 5 // 5

// if the first operand is falsy,
// AND returns it. The second operand is ignored
return null && 5 // null
return 0 && "no matter what"  // 0

We can also pass several values in a row. See how the first falsy one is returned:

return 1 && 2 && null && 3 // null

When all values are truthy, the last value is returned:

return 1 && 2 && 3 // 3, the last one

You can learn more about the logical operator here https://javascript.info/logical-operators

Answer 2

return a && b means "return a if a is falsy, return b if a is truthy".

It is equivalent to

if (a) return b;
else return a;

Answer 3

The logical AND operator, &&, works similarly. If the first object is falsy, it returns that object. If it is truthy, it returns the second object. (from https://www.nfriedly.com/techblog/2009/07/advanced-javascript-operators-and-truthy-falsy/).

Interesting stuff!

EDIT: So, in your case, if document.defaultView.getComputedStyle(elem, " ") does not return a meaningful ("truthy") value, that value is returned. Otherwise, it returns s.getPropertyValue(name).