Draw an ASCII Rectangle

Jelly, 14 bytes

,þ%,ỊḄị“-|| ”Y

Try it online! or verify all test cases.

How it works

,þ%,ỊḄị“-|| ”Y  Main link. Left argument: w. Right argument: h

,þ              Pair table; yield a 2D array of all pairs [i, j] such that
                1 ≤ i ≤ w and 1 ≤ j ≤ h.
   ,            Pair; yield [w, h].
  %             Take the remainder of the element-wise division of each [i, j]
                by [w, h]. This replaces the highest coordinates with zeroes.
    Ị           Insignificant; map 0 and 1 to 1, all other coordinates to 0.
     Ḅ          Unbinary; convert each pair from base 2 to integer.
                  [0, 0] -> 0 (area)
                  [0, 1] -> 1 (top or bottom edge)
                  [1, 0] -> 2 (left or right edge)
                  [1, 1] -> 3 (vertex)
       “-|| ”   Yield that string. Indices are 1-based and modular in Jelly, so the
                indices of the characters in this string are 1, 2, 3, and 0.
      ị         At-index; replace the integers by the correspoding characters.
             Y  Join, separating by linefeeds.

Matlab, 69 65 56 bytes

Thanks @WeeingIfFirst and @LuisMendo for some bytes=)

function z=f(a,b);z(b,a)=' ';z([1,b],:)=45;z(:,[1,a])='|'

This is really simple in Matlab: First make a matrix of the desired size, then index the first and last row to insert the -, and do the same with the first and last column to insert |.

For example f(4,3) returns

|--|
|  |
|--|

PHP, 74 Bytes

for(;$i<$n=$argv[2];)echo str_pad("|",$argv[1]-1,"- "[$i++&&$n-$i])."|\n";