Conditional statements allow you to change the behavior of a program depending on certain conditions. Thanks to them, we can build complex programs that behave differently under various scenarios.

The if statement is a control structure in programming that manages the sequence of operations. It contains a predicate expression inside parentheses, followed by a block of code within curly braces. This block executes only if the predicate returns true.

If the predicate returns false, the program skips the block and continues execution. In this example, the next line return 'general'; will execute, causing the function to return a string and terminate. Note that the return statement can be used anywhere in a function, including inside an if block.

If the if block only contains one line of code, curly braces can be omitted, for example:

const getTypeOfSentence = (sentence) => {
  const lastChar = sentence[sentence.length - 1];
  if (lastChar === '?')
    return 'question';
  return 'general';
}; 
console.log(getTypeOfSentence('Hodor'));  // => general
console.log(getTypeOfSentence('Hodor?')); // => question

It’s recommended to always use curly braces, even for one-line statements. This makes the structure of your code more visible and easier to understand.

else

Let’s create a getTypeOfSentence() function that analyzes the tone of a sentence: for declarative sentences – it returns General sentence, for questions – Question sentence.

getTypeOfSentence('Hodor');  // General sentence
getTypeOfSentence('Hodor?'); // Question sentence

Here’s the function:

const getTypeOfSentence = (sentence) => {
  // Declare a variable to hold the sentence type
  let sentenceType;

  // Predicate to check the sentence ending
  // If it ends with '?', it returns true
  if (sentence.endsWith('?')) {
    sentenceType = 'Question';
  } else {
    // Otherwise it's a general sentence
    sentenceType = 'General';
  }

  // Return the sentence type using interpolation
  return `${sentenceType} sentence`;
};

We added the else keyword with a new block. This block only executes if the if condition returns false.

else if

The previous getTypeOfSentence() function only detects question and general sentences. Let’s expand it to also detect exclamatory sentences.

const getTypeOfSentence = (sentence) => {
  const lastChar = sentence[sentence.length - 1];
  let sentenceType; 
  if (lastChar === '!') {
    sentenceType = 'exclamation';
  } else {
    sentenceType = 'normal';
  }
  if (lastChar === '?') {
    sentenceType = 'question';
  }
  return `Sentence is ${sentenceType}`;
};

getTypeOfSentence('Who?'); // Sentence is question
getTypeOfSentence('No');   // Sentence is normal
getTypeOfSentence('No!');  // Sentence is exclamation

We added a check for “exclamation” (!). Technically the function works, but semantically there are issues:

1. The question mark is checked regardless of whether an exclamation has already been detected.
2. The second condition is not part of the else branch.

It’s better to use else if for cleaner logic:

const getTypeOfSentence = (sentence) => {
  const lastChar = sentence[sentence.length - 1];
  let sentenceType;
  if (lastChar === '?') {
    sentenceType = 'question';
  } else if (lastChar === '!') {
    sentenceType = 'exclamation';
  } else {
    sentenceType = 'normal';
  }
  return `Sentence is ${sentenceType}`;
};

getTypeOfSentence('Who?'); // Sentence is question
getTypeOfSentence('No');   // Sentence is normal
getTypeOfSentence('No!');  // Sentence is exclamation

Now all conditions are merged into a single if block. The else if clause means: “if the previous condition wasn’t met, but this one is – then do this.”

The execution logic will only trigger one of the branches inside the if statement.