Given two expressions var1 and var2, what are two valid ways to return the concatenation...

We want to guarantee that the result is of type string regardless of what var1 and var2 are (numbers, booleans, etc.).

Option A:

String(var1).concat(var2)

String(var1) converts var1 explicitly to a string.

.concat(var2) calls String.prototype.concat, which:

Converts var2 to string if needed.

Returns a new string that is the concatenation of the two.

Result is always a string. So A is valid.

Option B:

String.concat(var1 + var2)

String.concat (as a static method) does not exist. concat is an instance method on string objects, not on the String constructor.

var1 + var2 might be a number (if both numeric) and used incorrectly here. So B is invalid.

Option C:

var1 + var2

+ can mean:

String concatenation (if at least one operand is a string).

Numeric addition (if both operands are numbers).

If var1 and var2 are both numbers, var1 + var2 will be a number, not a string.

This does not ensure the result is always a string. So C is not guaranteed to satisfy the requirement.

Option D:

var1.toString() + var2.toString()

var1.toString() converts var1 to a string (for most primitive/object types).

var2.toString() does the same.

Now both operands of + are strings, so the result is string concatenation.

Result is guaranteed to be a string. So D is valid.

Therefore, the two valid ways are A and D.

Study Guide / Concept References (no links):

String() conversion vs .toString()

String.prototype.concat

+ operator overload: numeric addition vs string concatenation

Type coercion rules for primitives