Employing a break statement after the default case within a switch statement even when the default is not

Question

Employing a break statement after the default case within a switch statement even when the default is not

According to a tutorial from w3schools that discusses switch statements, it is advised:

If the default case is not the last case in the switch block, it is important to remember to end it with a break statement.

However, the same tutorial also explains:

When JavaScript encounters a break keyword, it will exit the switch block.

This raises the question: if a default case with a break statement is placed at the beginning of a switch statement, wouldn't the default case always be executed and the block immediately exited by the interpreter? Does the interpreter not read the items in the switch statement sequentially?

javascript switch-statement

Answer 1

Answer №1

Explained in the tutorial is the concept of the default keyword

The default keyword indicates the code to be executed when there is no match with any case

The placement of the default keyword does not affect the execution. The cases following it are checked first before executing the code under the default case. If a case matches, its code is executed, therefore, the break in the default block will not be reached.

The code following default is only run if none of the predefined cases match or if the preceding case before default is chosen and there is no break before default (resulting in fall-through).

Typically, the default: case is placed last as a convention, so a break is usually not necessary there. The tutorial's warning serves as a reminder that if you place default: earlier, the rule of continuing into the next case without a break still applies; there is no special treatment for the default rule that would bypass it.

Answer 2

Explained in the tutorial is the concept of the default keyword

The default keyword indicates the code to be executed when there is no match with any case

The placement of the default keyword does not affect the execution. The cases following it are checked first before executing the code under the default case. If a case matches, its code is executed, therefore, the break in the default block will not be reached.

The code following default is only run if none of the predefined cases match or if the preceding case before default is chosen and there is no break before default (resulting in fall-through).

Typically, the default: case is placed last as a convention, so a break is usually not necessary there. The tutorial's warning serves as a reminder that if you place default: earlier, the rule of continuing into the next case without a break still applies; there is no special treatment for the default rule that would bypass it.

Answer 3

Answer №2

Check out this tutorial for more information:

The default keyword is used when there is NO MATCH in the cases provided.

For instance, if a case matches, only the associated code will run, not the code in the default block. Here's an example:

function test(n)
{
    let res;

    switch (n)
    {
        default: 
            res = "default";
            break;
        case 1:
            res = "Case 1";
            break; 
        case 0:
            res = "Case 2";
     }

     return res;
}

console.log(test(1), test(0), test(9));

.as-console {background-color:black !important; color:lime;}
.as-console-wrapper {max-height:100% !important; top:0;}

Notice that the default block will only run when there is no match, like in the case of test(9). Also, keep in mind this important reminder:

If default is not the last case in the switch block, make sure to use a break at the end of the default case.

Another important warning states:

Note: Skipping the break statement will cause the next case to run even if the evaluation doesn't match.

For example, omitting the break in the previous code will lead to unexpected results, like always returning Case 1 instead of the expected default string:

function test(n)
{
    let res;

    switch (n)
    {
        default: 
            res = "default";
        case 1:
            res = "Case 1";
            break; 
        case 0:
            res = "Case 2";
     }

     return res;
}

console.log(test(1), test(0), test(9), test(99));

.as-console {background-color:black !important; color:lime;}
.as-console-wrapper {max-height:100% !important; top:0;}

Answer 4

Check out this tutorial for more information:

The default keyword is used when there is NO MATCH in the cases provided.

For instance, if a case matches, only the associated code will run, not the code in the default block. Here's an example:

function test(n)
{
    let res;

    switch (n)
    {
        default: 
            res = "default";
            break;
        case 1:
            res = "Case 1";
            break; 
        case 0:
            res = "Case 2";
     }

     return res;
}

console.log(test(1), test(0), test(9));

.as-console {background-color:black !important; color:lime;}
.as-console-wrapper {max-height:100% !important; top:0;}

Notice that the default block will only run when there is no match, like in the case of test(9). Also, keep in mind this important reminder:

If default is not the last case in the switch block, make sure to use a break at the end of the default case.

Another important warning states:

Note: Skipping the break statement will cause the next case to run even if the evaluation doesn't match.

For example, omitting the break in the previous code will lead to unexpected results, like always returning Case 1 instead of the expected default string:

function test(n)
{
    let res;

    switch (n)
    {
        default: 
            res = "default";
        case 1:
            res = "Case 1";
            break; 
        case 0:
            res = "Case 2";
     }

     return res;
}

console.log(test(1), test(0), test(9), test(99));

.as-console {background-color:black !important; color:lime;}
.as-console-wrapper {max-height:100% !important; top:0;}

Answer 5

Answer №3

The concepts outlined in ECMA-262, 13.12.9 Runtime Semantics: CaseBlockEvaluation (which defines JavaScript) are important to understand.

While the specification may be challenging to decipher, it can be simplified as follows: When executing a switch statement, evaluate the `case` clauses before `default`, if available; then evaluate the `case` clauses after `default`, if present; and only if none match, proceed with the `default` section. The order of the `default` does not affect this process.

Answer 6

The concepts outlined in ECMA-262, 13.12.9 Runtime Semantics: CaseBlockEvaluation (which defines JavaScript) are important to understand.

While the specification may be challenging to decipher, it can be simplified as follows: When executing a switch statement, evaluate the `case` clauses before `default`, if available; then evaluate the `case` clauses after `default`, if present; and only if none match, proceed with the `default` section. The order of the `default` does not affect this process.

Employing a break statement after the default case within a switch statement even when the default is not

Answer №1

Answer №2

Answer №3

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