JavaScript prefix calculator

Question

For this test we need expected to write a function that evaluates an expression written in prefix notation and returns a value

For example

expression: + 1 5
variables: {}

result_expression: 6​​

expression: + 1 2 3
variables: {}

result_expression: null

expression: + 1
variables: {}

result_expression: null

expression: 9
variables: {}

result_expression: 9

expression: * + 1 2 3
variables: {}

result_expression: 9
Although negative numeric operands are invalid in expression, intermediate and final results may be negative:

expression: + 6 * - 4 + 2 3 8
variables: {}

result_expression: -2

Operators and operands must be separated by one or more white spaces:

expression: -+1 5 3
variables: {}

result_expression: null

expression: + 1 2
variables: {}

result_expression: 3

Expression containing variables:

expression: * + 2 x y
variables: { "x": 1, "y": 3 }

result_expression: 9

Answer 1

How is this evaluated as -2?

+ 6 * - 4 + 2 3 8

Answer 2

6 + ((4 - (2 + 3)) * 8) like this