Try the following code.

const addCart = () => {
  for (let i = 0; i < props.cart.length; i++) {
    if (props.cart[i].domainName === props.domainName) {
      return true;
    }
  }
  return false;
};

The function returns true when the first equal property has been found. It gives you the possibility to obtain the result without checking all elements of the array.

You can use some here

const addCart = () => {
  if (props.cart.some((o) => o.domainName === props.domainName)) return true;
  return false;
};

or simply using single liner solution

const addCart = () => props.cart.some((o) => o.domainName === props.domainName);

I'd suggest:

const addCart = () => {
  // here we use Array.prototype.some(),
  // which returns a Boolean; true if any
  // of the array-elements match the provided
  // assessment, or false if not:
  return props.cart.some(
    (cart) => cart.domainName === props.domainName
  );
}

References:

• Arrow functions.
• Array.prototype.some().