Employing a boolean constant to verify if a parameter has been specified

Struggling with TypeScript version 2.8.3, I'm confused as to why the code below is failing to recognize that params is defined inside the if block.

const testFunction = (params?: string) => {
  const paramIsDefined = typeof params !== 'undefined';
  if (paramIsDefined) {
    console.log(params.length);
  }
};

An error message pops up: TS2532: Object is possibly 'undefined' on the console.log line for the params variable.

However, this alternate code snippet works as expected:

const testFunction = (params?: string) => {
  if (typeof params !== 'undefined') {
    console.log(params.length);
  }
};

I can't seem to grasp what I am missing or doing incorrectly. Any insights?

Answer №1

When you encounter the construct:

if (typeof params !== 'undefined') {
    console.log(params.length);
}

It functions as a type guard, affecting the type of params.

On the other hand, if (paramIsDefined) is simply an if statement. While it's true that the boolean value being checked stems from a type check, the compiler doesn't recognize this fact. In order to narrow the type of params from string|undefined to just string, you'll need to utilize the type guard construct or make use of an assertion.

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