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, another statement can be executed using the else statement.

Note: The else statement is optional.

if (condition)
    /* do something */
else
    /* do something else */

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=5 then z=7 and q=42 else z=19.
    if (x === 5) {
      z = 7;
      q = 42;
    }else{
      z = 19;
    }

Using else if:

    // Categorising any number x as a small, medium, or large number  
    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";
    }