Converting JavaScript code into TypeScript

In this section, we will see how to transform some JavaScript code into TypeScript.

Let’s suppose we have to check whether a word is a palindrome. The JavaScript code for this algorithm will be as follows:

function isPalindrome(word) {
  const lowerCaseWord = word.toLowerCase()
  const reversedWord = lowerCaseWord.split('').reverse().join('')
  return lowerCaseWord === reversedWord
}

You can name this file palindrome.ts.

As you can see, we are receiving a string variable (word), and we are returning a boolean value. So, how will this be translated into TypeScript?

function isPalindrome(word: string): boolean {
  const lowerCaseWord = word.toLowerCase()
  const reversedWord = lowerCaseWord.split('').reverse().join('')
  return lowerCaseWord === reversedWord
}

You’re probably thinking, “Great, I just specified the string type as word and the boolean type to the function returned value, but now what?”

If you try to run the function with some value that is different from string, you will get a TypeScript error:

console.log(isPalindrome('Level')) // true
console.log(isPalindrome('Anna')) // true
console.log(isPalindrome('Carlos')) // false
console.log(isPalindrome(101)) // TS Error
console.log(isPalindrome(true)) // TS Error
console.log(isPalindrome(false)) // TS Error

So, if you try to pass a number to the function, you will get the following error:

Figure 2.1: Type number is not assignable to parameter of type string

That’s why TypeScript is very useful, because it will force you to be stricter and more explicit with your code.