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Conditionals in JavaScript

28 exercises

About Conditionals

General Syntax

A common way to conditionally execute logic in JavaScript is the if-statement. It consists of the if keyword, a condition wrapped in round brackets and a code block wrapped in curly brackets. The code block will only be executed if the condition evaluates to true.

if (condition) {
  // code that is executed if "condition" is true
}

It can be used stand-alone or combined with the else keyword.

if (condition) {
  // code that is executed if "condition" is true
} else {
  // code that is executed otherwise
}

Nested If-Statements

To nest another condition into the else statement you can use else if. Note that there is no elseif keyword in JavaScript. Instead, write else followed by another if statement.

if (condition1) {
  // code that is executed if "condition1" is true
} else if (condition2) {
  // code that is executed if "condition2" is true
  // but "condition1" was false
} else {
  // code that is executed otherwise
}

Theoretically, you can nest as many additional conditions as you want. In practice you would use a switch statement instead in these cases.

if (condition1) {
  // ...
} else if (condition2) {
  // ...
} else if (condition3) {
  // ...
} else if (condition4) {
  // ...
} else {
  // ...
}

Condition

When constructing complex conditions, refer to the operator precedence table to avoid unnecessary brackets.

if (num >= 0 && num < 1) {
  // ...
}

// The inner brackets are obsolete because relational operators
// have higher precedence than logical operators.
<!-- prettier-ignore-start -->

if ((num >= 0) && (num < 1)) {
  // ...
}
<!-- prettier-ignore-end -->

Also, consider using additional variables to make the code more readable.

const isPositive = num >= 0;
const isSmall = num < 1;
if (isPositive && isSmall) {
  // ...
}

In JavaScript, the condition does not have to be of type boolean. If any other type than boolean is provided in a boolean context like the if-statement, JavaScript will implicitly convert the value to boolean. Refer to the type conversion concept for details on which values are truthy and falsy, respectively.

const num = 4;
if (num) {
  // this code block will be executed because 4 is truthy
}

Short-Hand Notations

If you only want to execute one statement in the code block for if or else, it is possible in JavaScript to omit the curly brackets.

if (condition) doSomething();

// or

if (condition)
  doSomething();

This is sometimes used when checking for an error condition for example. In general, it is not recommended because it is easy to forget to add the brackets back in when adding a second statement that should depend on the same condition.

When writing functions, it is a common pattern to omit the else block and use an early return in the if block instead. In many cases, this reduces nesting and makes the code more readable and easier to follow.

function checkNumber(num) {
  let message = '';

  if (num === 0) {
    message = 'You passed 0, please provide another number.';
  } else {
    message = 'Thanks for passing such a nice number.';
  }

  return message;
}

// Can also be written as ...
function checkNumber(num) {
  if (num === 0) {
    return 'You passed 0, please provide another number.';
  }

  return 'Thanks for passing such a nice number.';
}
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