TypeScript Type Annotations

In TypeScript, type annotations allow you to force anything’s data type to be what you choose. If its data type is not the one you specify, the console will throw an error.

Example

Here’s an example:

function remainder(dividend: number, divisor: number) {
    return dividend % divisor;
}
const variableOne = ["Hello", "World"];
const variableTwo = [12, "Hello World"];
const variableThree = [5, 2];
const remainderOne = remainder(variableOne[0], variableOne[1]);
const remainderTwo = remainder(variableTwo[0], variableTwo[1]);
const remainderThree = remainder(variableThree[0], variableThree[1]);
const results = [remainderOne, remainderTwo, remainderThree];
for (const result of results) {
    if (isNaN(result)) {
        continue;
    }
    console.log(result);
}

The console should output something like this:

TypeError: Argument of type 'string' is not assignable to parameter of type 'number'.
TypeError: Argument of type 'string | number' is not assignable to parameter of type 'number'. 
TypeError: Type 'string' is not assignable to type 'number'.

Pros and Cons

Pros

• Ensures that you get the data type you want
• Short
• Easy to understand
• Provides a nice interface in IDEs

  Cons

• Throws an error if it gets a different data type
• Continues code execution

  Caveats

  If the data type is not the one you specify, TypeScript will throw an error. To handle this, you can use a try/catch:

  function remainder(dividend: number, divisor: number) {
    return dividend % divisor;
  }
  try {
    const result = remainder("Some Random Text", 123);
    console.log(`The result is ${result}.`);
  } catch {
    console.error("Wrong data type.");
  }
  

  Even if the data type is not the one you specify, the code will continue to execute. To handle this, you can use typeof:

  function remainder(dividend: number, divisor: number) {
    if (typeof dividend === 'number' || typeof divisor === 'number') {
        const response = dividend % divisor;
        /* if (!Number.isNaN(response)) */ return response;
    }
  }
  try {
    const result = remainder("Some Random Text", 123);
    console.log(`The result is ${result}.`);
  } catch {
    console.error("Wrong data type.");
  }