JavaScript · 2 min read

Operators and Conditions

Do math, compare values, combine logic, and make decisions with if, else, and switch.

Arithmetic operators

10 + 3;   // 13
10 - 3;   // 7
10 * 3;   // 30
10 / 3;   // 3.3333...
10 % 3;   // 1   (remainder)
2 ** 3;   // 8   (exponent)
 
let count = 5;
count += 2;  // 7  (same as count = count + 2)
count++;     // 8
count--;     // 7

Comparison operators

Comparisons produce a boolean (true or false):

5 > 3;      // true
5 <= 5;     // true
5 === 5;    // true   strictly equal (same value AND type)
5 === "5";  // false  number vs string
5 !== 4;    // true   strictly not equal
5 == "5";   // true   loose equality converts types (avoid!)

Logical operators

true && false;  // false  AND: both must be true
true || false;  // true   OR: at least one must be true
!true;          // false  NOT: flips the value

Truthy and falsy

In a condition, every value acts like true or false. These values are falsy: false, 0, "" (empty string), null, undefined, and NaN. Everything else is truthy, including "0", [], and "false".

const username = "";
const displayName = username || "Guest"; // "Guest"
 
const visits = 0;
const shown = visits ?? 10; // 0  (?? only falls back for null or undefined)

if, else if, else

const score = 82;
 
if (score >= 90) {
  console.log("A");
} else if (score >= 80) {
  console.log("B");
} else {
  console.log("Keep practicing!");
}

The ternary operator

A compact if/else that produces a value:

const age = 20;
const label = age >= 18 ? "adult" : "minor";

switch

switch is handy when comparing one value against many options:

const day = "sat";
 
switch (day) {
  case "sat":
  case "sun":
    console.log("Weekend!");
    break;
  case "fri":
    console.log("Almost there");
    break;
  default:
    console.log("Weekday");
}

Try it: a grade checker

<label for="score">Your score (0-100): </label>
<input id="score" type="number" value="75" min="0" max="100">
<button id="check">Check</button>
<p id="result"></p>

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Summary

Arithmetic: + - * / % **; shortcuts like += and ++.
Compare with ===, !==, >, <, >=, <=.
Combine conditions with &&, ||, and !.
Make decisions with if/else, the ternary ? :, and switch.

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Variables and Data Types Functions