JavaScript supports control flow statements similar to other programming languages. These statements allow you to control the flow of your program’s execution based on conditions.
JavaScript has an if statement similar to languages like Java and C++, except that the condition of an if statement can be anything that could be coerced to a boolean value (see the previous section on Boolean-ish values)
if (condition) {
// statements
}The statements inside an if block can contain other control structures, including other if statements (nested if statements).
const age = 17;
if (age >= 13) {
if (age <= 19) {
console.log("You are a teenager!");
}
}An if condition can be expanded with an else block
const num = 37;
if (num % 2 === 0) { // check if num is even
console.log(num + " is even!");
} else {
console.log(num + " is odd!");
}If you have only one statement, the braces around that statement are optional. However, consider it good practice always to include braces.
const age = 12;
if (age >= 13)
if (age <= 19)
console.log("You are a teenager!");
else
console.log("You are NOT a teenager!");
// The else is paired with the nearest if
// unless you enclose the second if in brackets.You can chain several if statements:
const score = 73;
// Convert score to letter grade
if (score >= 85) {
console.log("Grade: A");
} else if (score >= 70) {
console.log("Grade: B");
} else if (score >= 55) {
console.log("Grade: C");
} else {
console.log("Grade: F");
}The if-else-if ladder exits at the first success. If the conditions overlap, the first criteria occurring in the flow of execution is executed.
Consider the following expression that uses the ternary operator ?:
let max = a > b ? a : b;which is the same as
let max;
if (a > b) {
max = a;
} else {
max = b;
}When needing to make a lot of comparisons for a single value, instead of using many if-else-if statements, you can use the switch statement:
const letter = "B-";
let gpa;
switch (letter) {
case "A+":
case "A":
gpa = 4.0;
break;
case "A-":
gpa = 3.7;
break;
case "B+":
gpa = 3.3;
break;
case "B":
gpa = 3.0;
break;
case "B-":
gpa = 2.7;
break;
case "C+":
gpa = 2.3;
break;
case "C":
gpa = 2.0;
break;
case "C-":
gpa = 1.7;
break;
case "D+":
gpa = 1.3;
break;
case "D":
gpa = 1.0;
break;
case "D-":
gpa = 0.7;
break;
case "F":
gpa = 0.0;
break;
default:
gpa = null;
}
if (gpa !== null) {
console.log("Your GPA is " + gpa);
} else {
console.error(letter + " cannot be converted to GPA value");
}
// Your GPA is 2.7default case in a switch statement is like the last else in an ifelseif chain. It will be reached if none of the previously tested conditions are true.break statement is needed to break out of the switch statement. If you omit break, switch will run all the following cases until it encounters break or exits. This behavior can be useful for grouping cases.Keep in mind that JavaScript uses strict equality for switch statements.