Test your knowledge: JavaScript Booleans and Conditionals

Review

1. Which list is the falsy seven?

   The falsy seven are false, 0 and -0, the empty string, null, undefined, and NaN. Empty arrays and objects are truthy, and "0" is a non-empty string, which surprises everyone exactly once.
2. A rating of 0 must survive a default. Which operator belongs in rating ?? 5 versus rating || 5?

   || replaces every falsy value, so a genuine 0 becomes the default. ?? replaces only null and undefined, which is why a real rating of 0 survives it. That distinction decides which one a default belongs to.
3. Why does the lesson retire == in favor of ===?

   == coerces its operands before comparing, so "0" == 0 comes out true and a family of surprises follows. === compares type and value with no conversion, which is why it is the spelling every serious codebase settles on.
4. A condition reads if (x = 5). What happens when the line runs?

   Assignment is an expression whose value is whatever was assigned, so the condition tests 5, which is truthy, and the branch runs every time. The bug is silent, which is why lingers in linters as an error.
5. What does orders && orders.length do when orders is an empty array?

   && evaluates left to right and stops at the first falsy operand, so when orders is empty, the length is still consulted, and 0 comes back falsy. When orders is undefined, the walk stops before the crash, which is the guard this pattern gives you for free.