Flow control statements

Flow control statements are key components in programming used to manage the order and logic of code execution. In JavaScript, common flow control statements include conditional statements, loop statements, and jump statements, which help developers implement complex logical judgments and repetitive operations.

Conditional Statements

Conditional statements are used to execute different blocks of code based on different conditions. The most common conditional statements in JavaScript are if...else and switch.

if...else Statement

The if...else statement executes a block of code if a condition is true and another block if the condition is false. The syntax is as follows:

if (condition) {
  // Code to execute if the condition is true
} else {
  // Code to execute if the condition is false
}

For example, checking if a number is positive:

let num = 10;
if (num > 0) {
  console.log("Positive");
} else {
  console.log("Non-positive");
}

You can also use else if to handle multiple conditions:

let score = 85;
if (score >= 90) {
  console.log("Excellent");
} else if (score >= 80) {
  console.log("Good");
} else if (score >= 60) {
  console.log("Pass");
} else {
  console.log("Fail");
}

switch Statement

The switch statement executes different blocks of code based on different values. The syntax is as follows:

switch (expression) {
  case value1:
    // Code block 1
    break;
  case value2:
    // Code block 2
    break;
  default:
    // Default code block
}

For example, outputting different messages based on the day of the week:

let day = 3;
switch (day) {
  case 1:
    console.log("Monday");
    break;
  case 2:
    console.log("Tuesday");
    break;
  case 3:
    console.log("Wednesday");
    break;
  default:
    console.log("Other");
}

Note: The break statement is used to exit the switch; otherwise, execution will continue to the next case.

Loop Statements

Loop statements are used to repeatedly execute a block of code until a specific condition is met. Common loop statements in JavaScript include for, while, and do...while.

for Loop

The for loop is used to repeat code execution when the number of iterations is known. The syntax is as follows:

for (initialization; condition; increment) {
  // Loop body
}

For example, printing numbers from 1 to 5:

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

The for loop can also be used to iterate through an array:

let arr = [1, 2, 3, 4, 5];
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

while Loop

The while loop repeats code execution as long as the condition is true. The syntax is as follows:

while (condition) {
  // Loop body
}

For example, printing numbers from 1 to 5:

let i = 1;
while (i <= 5) {
  console.log(i);
  i++;
}

do...while Loop

The do...while loop is similar to while but guarantees at least one execution of the loop body. The syntax is as follows:

do {
  // Loop body
} while (condition);

For example, printing numbers from 1 to 5:

let i = 1;
do {
  console.log(i);
  i++;
} while (i <= 5);

for...of and for...in Loops

for...of is used to iterate over iterable objects (e.g., arrays, strings):

let arr = [1, 2, 3];
for (let item of arr) {
  console.log(item);
}

for...in is used to iterate over the properties of an object:

let obj = { a: 1, b: 2, c: 3 };
for (let key in obj) {
  console.log(key + ": " + obj[key]);
}

Jump Statements

Jump statements are used to alter the flow of code execution and include break, continue, and return.

break Statement

The break statement is used to exit a loop or switch statement. For example, exiting a loop when a specific condition is met:

for (let i = 1; i <= 10; i++) {
  if (i === 5) {
    break;
  }
  console.log(i);
}

continue Statement

The continue statement skips the remaining part of the current loop iteration and proceeds to the next iteration. For example, skipping even numbers:

for (let i = 1; i <= 10; i++) {
  if (i % 2 === 0) {
    continue;
  }
  console.log(i);
}

return Statement

The return statement is used to return a value from a function and terminate its execution. For example:

function add(a, b) {
  return a + b;
}
let result = add(1, 2);
console.log(result); // 3

Exception Handling

In JavaScript, the try...catch statement is used to handle exceptions:

try {
  // Code that might throw an error
  let x = y + 1; // y is undefined, will throw an error
} catch (error) {
  console.log("An error occurred: " + error.message);
} finally {
  console.log("This will execute regardless of an error");
}

Labeled Statements

Labeled statements are used to identify blocks of code, often used with break or continue. For example:

outerLoop: for (let i = 0; i < 3; i++) {
  innerLoop: for (let j = 0; j < 3; j++) {
    if (i === 1 && j === 1) {
      break outerLoop;
    }
    console.log(`i=${i}, j=${j}`);
  }
}

Ternary Operator

The ternary operator is a shorthand for if...else:

let age = 18;
let message = age >= 18 ? "Adult" : "Minor";
console.log(message); // Adult