Two alternatives come to mind.

switch/case statement

This doesn't exist in Python, but in JavaScript you can use a switch statement as follows:

const x = 3;
let a;


switch (x) {
  case 2:
    a = 1;
    break;
  case 3:
    a = 2;
    break;
  default:
    a = 1;
}

console.log(a);

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It's a little verbose, but you can get rid of some of the verbosity by wrapping it in a function:

function val(x) {
  switch (x) {
    case 2:
      return 3
    case 3:
      return 2;
    default:
      return 1;
  }
}

const x = 3;
const a = val(x);

console.log(a);

Lookup object

You can populate an object with lookup values. You can use a regular object, but since you're dealing with numeric keys, a Map is more suited:

const values = new Map([
  [2, 1],
  [3, 2]
]);

const x = 3;
const a = values.get(x);

console.log(a);

You may be talking about expressions - the conditional operator. Here is the example.

The conditional (ternary) operator is the only JavaScript operator that takes three operands: a condition followed by a question mark (?), and then an expression to execute if the condition is truthy, followed by a colon (:), and finally the expression to execute if the condition is false. This operator is frequently used as a shortcut for the if statement.

function fun(var) {
  return (var ? '2' : '10');
}

console.log(fun(true));
// Expected output: "2"

console.log(fun(false));
// Expected output: "10"

console.log(fun(null));
// Expected output: "10"