Test your knowledge: JavaScript Functions

Review

1. A function body ends without a return statement. What does calling it evaluate to?

   A function without a return returns undefined, the value, not an error. Python hands back None in the same spot; JavaScript hands back undefined, and the difference in spelling is the whole difference.
2. When does an arrow function need braces and an explicit return?

   A brace-less arrow body is one expression, and its value is the return value, no keyword needed. The moment braces appear for multiple statements, the return must be written, or the call quietly evaluates to undefined.
3. One receipt function carried a defaulted tax rate, and changing it changed every receipt. Why is that a win?

   A default puts the decision in the signature, where every call site can see it. Change the default and every caller that relied on it follows, while callers that pass their own value are untouched.
4. What does ...items do in a parameter list?

   In a parameter list, the dots gather every remaining argument into a real array, so the function can accept as many values as it is handed. The same dots inside a call spread an array back out, which is how the two are told apart.
5. What can a function body see, according to the lesson's rule of scope?

   A function sees values declared in its enclosing scopes as well as its own, which is what lets a receipt function read a rate defined above it. What it cannot do is reach into scopes that do not enclose it, no matter how near they look in the file.