Typescript strictNullChecks error when not possible

I am attempting to turn on the strictNullChecks setting for my project, but have a rather curious error in the following snippet of code:

toasters.forEach((toster: ToasterObject) => {
  if (toaster.brandName) {
    //This line works just fine
    let y = toaster.brandName.toLowerCase() === 'test brand name';

    //This line has the error
    if (!itemsArray.some(item => item.brandName.toLowerCase() === toaster.brandName.toLowerCase())) {
        //do stuff
    }
  }
});

The error message has toaster.brandName underlined in the if statement with the following error text: error TS2532: Object is possibly 'undefined'. If the object is fine to use on the y = line above, why would it be a problem to use in the if statement? How can I fix this so it will quit erroring on this?

Control flow analysis is hard. The problem here is that the compiler does not know if or when or how a callback will be called. So the narrowing of toaster.brandName from (presumably) string | undefined to string is no longer in effect inside the array.some() callback. If you want to help the compiler out, you should assign the known-defined value of toaster.brandName to a const, which is expected by the compiler to always stay the same, narrow type:

toasters.forEach((toster: ToasterObject) => {
  if (toaster.brandName) {
    const toasterBrandName = toaster.brandName; // string, now and forever

    if (!itemsArray.some(item => item.brandName.toLowerCase() === toasterBrandName.toLowerCase())) {
        //do stuff
    }
  }
});

Hope that helps; good luck!