js tag syntax usage details

Preface:

In daily development, we often use recursion, break , continue , return and other statements to change the location of program execution. In fact, JavaScript also provides label statements to mark specified code blocks, making it easier to jump to specified locations. This article records the use of label statements.

1. Introduction to label statements

Label statements are used to label statements. Labels can have the same name as variables. They are independent syntax elements (neither variables nor types). They are used to identify " labeled statement ", which are equivalent to locators and are used to jump to any position in the program. The syntax is as follows:

label: statement

For example:

    hello:console.log("hello")

Label statements can change the execution flow of a program, similar to break , continue , and return . break and continue can be used with labels.

2. Label statement usage

(1) Use label statements and break statements together to jump out of a specific loop

    let num = 0;
    mylabel:
    for (let i = 0; i < 10; i++) {
      for (let j = 0; j < 10; j++) {
        if (i == 5 && j == 5) {
          break mylabel;
        }
        num++;
      }
    }
    console.log(num); // 55

In this example, outermost label identifies the first for statement. Normally, each loop executes 10 times, which means that the num++ statement will be executed 100 times, and the result of console.log should be 100 when the loop ends. However, the break statement takes a variable, which is the label to exit to. Adding a label causes break to exit not only the inner loop (which uses the variable j), but also the outer loop (which uses the variable i). When i and j are both equal to 5, the loop stops executing and the value of num is 55.

(2) Using label statements with continue

    let num = 0;
    mylabel:
    for (let i = 0; i < 10; i++) {
      for (let j = 0; j < 10; j++) {
        if (i == 5 && j == 5) {
          continue mylabel;
        }
        num++;
      }
    }
    console.log(num); // 95

The continue statement forces the loop to continue executing, but instead of continuing the inner loop, it continues the outer loop. When i and j are both equal to 5, continue will be executed, jumping to the outer loop to continue execution, resulting in the inner loop being executed 5 times less, and the result is num equal to 95.

Summarize:

Combining label statements with break and continue can implement complex logic, but it is also prone to errors. Be careful to use descriptive text for your tags and not to nest them too deeply.

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