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JavaScript: switch statement

Many languages contain a switch statement in addition to the conditional if construct. This is a specialized version of if, designed for certain cases. For example, you can use it where there is an if else chain with equality checks:

if (status === 'processing') {
  // First
} else if (status === 'paid') {
  // Second
} else if (status === 'new') {
  // Third
} else {
  // Fourth
}

This compound check has one distinctive feature: each branch here is a check of the value of the status variable. 'Switch' allows you to write this code in a shorter and more expressive way:

switch (status) {
  case 'processing': // status === processing
    // First
    break;
  case 'paid': // status === paid
    // Second
    break;
  case 'new': // status === new
    // Third
    break;
  default: // else
    // Fourth
}

Switch is a rather advanced construct considering the number of elements it consists of:

  • An external definition containing the keyword switch. A variable whose values switch will use to select a behavior. And curly brackets for behavior branches.
  • The case and default constructs, where the behavior for different values of the variable is described. Each case corresponds to if in the example above. default is a special condition corresponding to the else branch in conditionals. Neither else nor default is necessary (but the linter will always ask for it).
  • break is needed to prevent it from "falling through". If break is omitted, then the program will continue to the next case statement after the required case, and carry on until the next break or the end of the switch.

The curly brackets in switch* don't specify a code block unlike elsewhere. Only the syntax shown above is acceptable. In other words, you can usecaseordefaultthere. But inside eachcase(anddefault`), the situations are different. You can execute any arbitrary code here:

switch (count) {
  case 1:
    // Do something useful
    break;
  case 2:
    // Do something useful
    break;
  default:
    // Do something
}

Sometimes the output of the case is the end of a function containing switch. If this happens, you need to return it outside somehow. There are two ways to solve this problem.

The first way. Create a variable before switch, fill it in case, and then return the value of that variable outside at the end.

(count) => {
  // Declare a variable
  let result;

  // Fill it
  switch (count) {
    case 1:
      result = 'one';
      break;
    case 2:
      result = 'two';
      break;
    default:
      result = null;
  }

  // Return
  return result;
};

The second way is easier and shorter. Instead of creating a variable, case allows us to make a regular function return inside it. And since no code is executed after return, we can get rid of break:

(count) => {
  switch (count) {
    case 1:
      return 'one';
    case 2:
      return 'two';
    default:
      return null;
  }
};

Though switch is present in the code, you always can find a way to avoid it. The key benefit of using it is that it better expresses the programmer's intention when checking the values of a particular variable. Although the code becomes a bit longer, it's easier to read compared to else if blocks.

Instructions

Write a function, getNumberExplanation(), that describes an input number passed to it. If there is no explanation available, it returns null:

getNumberExplanation(8);   // null

// Only the following numbers have descriptions
getNumberExplanation(666); // 'devil number'
getNumberExplanation(42);  // 'answer for everything'
getNumberExplanation(7);   // 'prime number'
The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

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