Why does it throw an exception even though my condition seems correct

Question

I have created method in typescript where I pass in a string value as a param. I wanted to include an exception handler where if the 'transactionType' does not equal either 'ADD' or 'REMOVE', then throw an exception.

However, it throws an exception event through I am including the correct param value and I am not sure why that is.

Method examples:

await cashPage.completeAddRemove('ADD');
await cashPage.completeAddRemove('REMOVE');

In that method I attempted both of the following but not working:

async completeAddRemove(transactionType: string) {
    if (transactionType != 'ADD' || 'REMOVE') {
      throw new Error(`Unrecognised transaction type provided`);
    }

  async completeAddRemove(transactionType: string) {
    if (transactionType !== 'ADD' || 'REMOVE') {
      throw new Error(`Unrecognised transaction type provided`);
    }