JavaScript: Conditional statement (if)

Logical expressions let you check various conditions, but on their own they only return true or false. To make a program perform different actions depending on the result, JavaScript has a special construct called if.

if (5 > 3) {
  console.log('Yes, it is true');
}

Here the string 'Yes, it is true' will be printed because the condition 5 > 3 is true.

┌────────────┐
│ condition? │
└─────┬──────┘
  true │
      ↓
┌────────────┐
│ if body    │
└────────────┘

Syntax

After if, the condition is written in parentheses, and the body goes in curly braces:

if (condition) {
  // the block runs if the condition is true
}

A condition is any expression that is coerced to true or false.

Code blocks

Everything inside the curly braces {} belongs to the body of the if. The code after the block runs in any case:

if (10 === 10) {
  console.log('First');
  console.log('Second');
}

console.log('Goodbye!');

Here First and Second will be printed because the condition was met. And Goodbye! is always printed since this line is already outside the block. The principle is the same as in function definitions.

Using if inside a function

Consider a function that determines the type of a given sentence. If it ends with a question mark, the function returns 'question', otherwise — 'normal':

const getTypeOfSentence = (sentence) => {
  if (sentence.endsWith('?')) {
    return 'question';
  }
  return 'normal';
};

console.log(getTypeOfSentence('Hodor'));  // => normal
console.log(getTypeOfSentence('Hodor?')); // => question

Two return statements are used here. If the condition inside if is met, return 'question' fires and the function ends. If the condition is not met, control passes to the next line with return 'normal'.

This way, the function has several possible exit points. This is a common practice: depending on the conditions, a function may end in different ways.

Even though the getTypeOfSentence function uses if, it returns strings, which means it is not a predicate. As a predicate, let's consider a function that checks whether there is enough money for a purchase:

const hasEnoughMoney = (balance, price) => {
  if (balance >= price) {
    return true;
  }
  return false;
};

console.log(hasEnoughMoney(100, 50)); // => true
console.log(hasEnoughMoney(30, 50));  // => false

if and logical expressions

We wrote the hasEnoughMoney function using if. But in this form it could do without it, because the result of the comparison is itself a logical expression:

const hasEnoughMoney = (balance, price) => balance >= price;

In simple cases, it is better to return such an expression right away. if is needed where additional actions are performed inside the block besides returning the result. The more complex the programs, the more often such situations will arise.

Write a function guessNumber(guess) that:

• returns 'You win!' if guess === 42
• returns 'Try again!' otherwise

guessNumber(42); // => 'You win!'
guessNumber(7);  // => 'Try again!'