Is there a solution for the error message "Operator '+' cannot be used with types 'string | number' and 'string' | number'"?

Here's the scenario: I'm encountering an issue where I am invoking a function within another function. This inner function has the capability to return either a string or a number, and my goal is to combine that with another value. However, I keep running into an error.

I attempted to address this by implementing a check

function addNumbers(obj):number | string {
 const returnedValue = helperFunction(obj.blah) //this could be a number or string
 if (Number.isNaN(returnedValue) === false) {
  return returnedValue + anotherValue;
 }
 return ‘-’;

I thought using Number.isNaN would solve the problem but unfortunately, it did not. Any suggestions?

typescript

Answer №1

Number.isNaN specifically verifies if the value is NaN. NaN is a distinctive number generated by calculations such as 0/0. The majority of numbers and all strings will result in Number.isNaN returning false, hence not narrowing down the type to exclude strings.

My suggestion would be:

if (typeof returnedValue === 'number') {
  return returnedValue + anotherValue;
}

Answer №2

To properly define the return type, you should wrap the type with parentheses like this: (number | string)

Keep in mind that if you return null at the end, your return type should be (number | string | null)

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