An article analyzing process control in JavaScript

As a scripting language, JavaScript is very suitable for web programming. In fact, it is one of the most commonly used languages ​​in web development. Flow control is one of the most commonly used programming methods in JavaScript. This article will analyze flow control in JavaScript from the perspectives of if statements, switch statements, for loops, while loops and do-while loops.

if statement

The if statement is used to control the execution flow of the program based on conditions. Its basic syntax is as follows:

if (condition) {
  // code block to be executed if condition is true
}

If the condition (condition) is met, then the code block in the curly braces will be executed; otherwise, the statements in the code block will be skipped. The following example demonstrates the use of if statements:

var x = 10;

if (x > 5) {
  console.log("x is greater than 5");
}

In this example, because the value of x is 10, the condition x > 5 is satisfied, and the statements in the code block are executed. The console will display "x is greater than 5".

if...else statement

The if...else statement is a flow control structure that executes one code block when a condition is met and another code block when the condition is not met. . The basic syntax is as follows:

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

In the above code, if the condition is met, the code in the if statement block will be executed; otherwise, the code in the else statement block will be executed. The following is an example:

var x = 20;

if (x < 10) {
  console.log("x is less than 10");
} else {
  console.log("x is greater than or equal to 10");
}

Since the value of x is 20, the condition x < 10 in the if statement is not met, so the program executes the code in the else statement block and prints the console as "x is greater than or equal to 10".

switch statement

The switch statement is a multi-way branch statement used to execute different code blocks based on different conditions. The basic syntax is as follows:

switch (expression) {
  case value1:
    // code block to be executed if expression matches value1
    break;
  case value2:
    // code block to be executed if expression matches value2
    break;
  ...
  default:
    // code block to be executed if no case matches expression
}

As mentioned above, expression is the parameter of the switch statement and can be any expression that can calculate a value. Inside the switch statement block, the case label specifies the value to be tested, and the break statement is used to jump out of the statement block. If there are no matching cases, the default statement will be executed. The following is an example:

var day = "Monday";

switch (day) {
  case "Monday":
    console.log("Today is Monday");
    break;
  case "Tuesday":
    console.log("Today is Tuesday");
    break;
  case "Wednesday":
    console.log("Today is Wednesday");
    break;
  default:
    console.log("Today is not Monday, Tuesday, or Wednesday");
}

In the above code, since the value of day is "Monday", the output is "Today is Monday".

for loop

The for loop is a loop that executes a specified number of times step by step, testing a condition on each loop iteration. Its basic syntax is as follows:

for (initialization; test condition; update) {
  // code block to be executed
}

In a for loop, the initialization statement usually declares a counter variable and assigns it an initial value. Test condition is used to test whether the counter variable meets the condition. The update statement updates the counter variable each iteration. Inside the for loop block, the program executes all statements in sequence. Here is an example:

for (var i = 0; i < 5; i++) {
  console.log(i);
}

The output is:

0
1
2
3
4

while loop

The while loop is a loop that repeatedly executes a block of code when a condition is true. The basic syntax is as follows:

while (condition) {
  // code block to be executed
}

Inside the while loop, the program will execute the code block repeatedly only when the condition (condition) is true. The program exits the while loop when the condition is no longer met. The following is an example:

var i = 0;

while (i < 5) {
  console.log(i);
  i++;
}

The output result is the same as the example using a for loop:

0
1
2
3
4

do-while loop

do-while loop is a condition that is true A loop that repeatedly executes a block of code while, unlike a while loop, always executes the block of code once and then checks the condition after executing the block of code. The basic syntax is as follows:

do {
  // code block to be executed
} while (condition)

The following is an example:

var i = 0;

do {
  console.log(i);
  i++;
} while (i < 5);

The output result of this example is also the same as the example using for loop and while loop:

0
1
2
3
4

In summary As mentioned, flow control in JavaScript is the key to writing efficient programs. end

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