freeCodeCamp/guide/english/javascript/if-else-statement/index.md

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If-Else Statement

Introduction

The if statement executes a statement if a specified condition is true. If the condition is false, then the statement inside else block gets executed.

Note: The else statement is optional.

if (condition) {
    /* do this if condition is TRUE */
} else {
    /* do this if condition is FALSE (or not TRUE) */
}

The conditions can be as simple as just checking booleans, but you can also check the value of variables using operators. One of the most commonly used operators is the equality operator, "==". It checks if two values are equal to each other. Note that there are two equal signs and not just one. This is because of the fact that one equal sign would set the first value to equal the second, not compare them.

Some other common operators are the greater/smaller than operator (> and < respectively) and the not equal to operator (!=).

Multiple if...else statements can be chained to create an else if clause. This specifies a new condition to test and can be repeated to test multiple conditions, checking until a true statement is presented to execute.

if (condition1) {
    /* do something */
} else if (condition2) {
    /* do something else */
} else if (condition3) {
    /* do something else */
} else {
    /* final statement */
}

Note: If you want to execute more than one statement in the if, else or else if part, curly braces are required around the statements:

if (condition) {
    /* do */
    /* something */
    /* with multiple statements */
} else {
    /* do something */
    /* else */
}

MDN link | MSDN link

Additional ternary operator

For a simple operation like assigning a value to a variable conditionally you can use a ternary operator, which is a shorthand syntax for an if...else clause in JavaScript

// Normal if...else
let num  = 1;
if (someCondition){
    num = 1;
} else {
    num = 0;
}

// Using ternary operator
const num = someCondition ? 1 : 2;
// condition ? true case : false case

Examples

Using if...else:

// If x is equal to 5, then the condition is TRUE. This results in z being set to 7 and q being set to 42.
// If x does not equal to 5, then the condition is FALSE. This results in z being set to 19.
if (x == 5) {
  z = 7;
  q = 42;
} else {
  z = 19;
}

Using else if:

if (x < 10) {
  return "Small number";
} else if (x < 50) {
  return "Medium number";
} else if (x < 100) {
  return "Large number";
} else {
  flag = 1;
  return "Invalid number";
}

Using if alone:

    // This function can also act as a Boolean
    if (x < 30) {
        return "true";
    }