How to extract certain string characters in Javascript in a generic way?

We can try matching on the following regular expression, which captures all content we want to appear in the output:

^(.*\/shelf=\d+\/slot=\d+).*$

Note that this greedily captures all content up to, and including, the /shelf followed by /slot portions of the input path.

var inputs = ["/rack=1/shelf=1/slot=12/port=200", "/shelf=1/slot=13/port=3", "/shelf=1/slot=142/subslot=2/port=4"];
for (var i=0; i < inputs.length; ++i) {
    var output = inputs[i].replace(/^(.*\/shelf=\d+\/slot=\d+).*$/, "$1");
    console.log(inputs[i] + " => " + output);
}

If it will always be the last substring, you could use slice:

function removeLastSubStr(str, delimiter) {
  const splitStr = str.split(delimiter);

  return splitStr
    .slice(0, splitStr.length - 1)
    .join(delimiter);
}

const str = "/rack=1/shelf=1/slot=12/port=200";

console.log(
  removeLastSubStr(str, '/')
)

if you don't know where your substring is, but you know what it is you could filter it out of the split array:

function removeSubStr(str, delimiter, substr) {
  const splitStr = str.split(delimiter);

  return splitStr
    .filter(s => !s.contains(substr))
    .join(delimiter);
}

const str = "/rack=1/shelf=1/slot=12/port=200";

console.log(
  removeSubStr(str, '/', 'port=200')
)

console.log(
  removeSubStr(str, '/', 'port')
)

You could use this function. If "subslot" is always after "slot" then you can remove the "/" in indexOf("/slot")

function exractUptoSlot(str) {
    return str.substring(0,str.indexOf("/",str.indexOf("/slot")));
}