JavaScript: Ternary operator

Look at the definition of this function, which returns the modulus of a given number:

const abs = (number) => {
  if (number >= 0) {
    return number;
  }

  return -number;
};

abs(10); // 10
abs(-10); // 10

Can we write it more concisely? Something like return <something depending on the condition>? That would require an expression following return, but if is a statement, not an expression.

We have a construct in JavaScript that works the same way as the if-else construct, except it's an expression. It's called the ternary operator.

The ternary operator is the only JavaScript operator that takes three operands:

const abs = (number) => {
  return number >= 0 ? number : -number;
};

The general pattern looks like this: <predicate> ? <expression on true> : <expression on false>.

Here is a shortened version of the abs() function:

const abs = (number) => (number >= 0 ? number : -number);

Note the parentheses around the ternary. They are optional, but linter strongly recommends to put them to avoid ambiguities.

Let's rewrite the initial version of getTypeOfSentence() in the same way:

Before:

const getTypeOfSentence = (sentence) => {
  const lastChar = sentence.slice(-1);

  if (lastChar === '?') {
      return 'question';
  }

  return 'normal';
};

After:

const getTypeOfSentence = (sentence) => {
  const lastChar = sentence.slice(-1);

  return (lastChar === '?') ? 'question' : 'normal';
};

getTypeOfSentence('Hodor');  // normal
getTypeOfSentence('Hodor?'); // question

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If you remember the main strength of expressions, then you probably figured out by now that you can put a ternary operator into a ternary operator. Don't do it :) Such code is hard both to read and to debug, it's very bad practice.

Instructions

• Tips

Write the convertText() function that takes a string as input, and if the first letter is not capitalized, returns a reversed version of that string. If the first letter is capitalized, it returns the string unchanged. If the input is an empty string, the function must return an empty string.

Examples:

convertText('Hello'); // 'Hello'
convertText('hello'); // 'olleh'

// Be sure to take the empty line into account
convertText(''); // ''

You can reverse a string using the reverse() function. It takes a string we want to reverse as an argument:

const result = reverse('Hello!');
console.log(result); // => '!olleH'

There are various approaches to solve this problem. You may want to use the toUpperCase() method and feature that allows you to get a character from a string (for example, str[0]).

Try to write two versions of the function: with the usual if-else, and with a ternary operator.

Tips

• Consider the first condition you need to write. Is it a capitalization check or an empty line check? Which is primary?

Definitions

• Ternary operator is a way to convert a simple conditional statement into an expression, e.g., number >= 0 ? number : -number