Operators, Conditionals and Truthy Values

Comparison: == vs ===

0 == "0"    // true  (converts types first -- avoid this)
0 === "0"   // false (strict, checks type too -- use this)

1 === 1     // true
"a" === "a" // true

Always use === and !== in JavaScript, never == and !=. The loose versions perform confusing automatic type conversion before comparing — 0 == "0", 0 == false, and even "" == 0 are all true, which causes real bugs. This is considered a mistake in JavaScript’s original design, and the strict versions avoid it entirely.

if / else if / else

const score = 82;
let grade;
if (score >= 90) {
  grade = "A";
} else if (score >= 75) {
  grade = "B";
} else {
  grade = "C";
}

Truthy and falsy

In a boolean context — like the condition of an if statement — JavaScript converts any value to true or false automatically. Exactly six values are falsy: 0, "" (empty string), null, undefined, NaN, and false itself. Everything else, including an empty array [] and an empty object {}, is truthy. This is why you’ll often see:

if (userInput) {
  // runs only if userInput is not empty/null/undefined/0
}

const name = userInput || "Guest";   // falls back to "Guest" if userInput is falsy

The ternary operator

const status = age >= 18 ? "adult" : "minor";

A ternary is a compact if/else that produces a value directly — useful for short conditional assignments, but resist nesting more than one ternary inside another, since that quickly becomes unreadable.