At a high level, an expression is a valid unit of code that resolves to a value. There are two types of expressions: those that have side effects (such as assigning values) and those that purely evaluate. The expression x = 7 is an example of the first type. This expression uses the = operator to assign the value seven to the variable x. The expression itself evaluates to 7. The expression 3 + 4 is an example of the second type. This expression uses the + operator to add 3 and 4 together and produces a value, 7. However, if it’s not eventually part of a bigger construct (for example, a variable declaration like const z = 3 + 4), its result will be immediately discarded — this is usually a programmer mistake because the evaluation doesn’t produce any effects. As the examples above also illustrate, all complex expressions are joined by operators, such as = and +.
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