Protractor: text.indexOf(...).isDisplayed is not a function

I'm aware this has been asked on a few occasions and even though I've looked through those questions, I'm not really sure how to fix this.

I'm checking if text "EUR" is contained in a div called "currency". This was working for me previously but I've started using lint and I've been getting a lot of these kind of errors.

This is the error I'm getting Failed: text.indexOf(...).isDisplayed is not a function

This is my code

checkBuyerCurrency (text, buyerCurrency) {
    let currencyPromise = new Promise(function (resolve, reject) {
    const commonUtils = new CommonUtils();
    var EC = protractor.ExpectedConditions;
    browser.wait(EC.visibilityOf(element(by.className("currency")),     4000));
    var checkCurrency = element(by.className("balances"));
      checkCurrency.getText().then(function (text) {
           expect (text.indexOf("EUR").isDisplayed()).toBe(true);
           console.log("EUR only buyer");
      });
    });
  }

Do I need to make a text a variable or convert it to string? I'm not entirely sure how to do this due to the way I'm using the Expect statement

Thanks for any help

via Edmond