JavaScript — Comparisons

Every comparison returns a boolean

5 > 3        // true
2 === 1      // false
"b" >= "a"   // true

Feed results straight into if or store them:

const adult = age >= 18;

Equality: the two flavors

// STRICT (always use)
5 === 5       // true
5 === "5"     // false — different types, no conversion

// LOOSE (avoid writing; must recognize)
5 == "5"      // true  😱 — coerces "5" to number first
0 == ""       // true
null == undefined   // true

Loose equality's conversion table is famously unintuitive. Modern rule: === / !== always. ESLint's eqeqeq rule enforces it in real teams.

The full relational set

Operator	Question
=== / !==	identical? / not identical?
< / >	less / greater
<= / >=	inclusive bounds

String comparison — alphabetical-ish

"apple" < "banana"    // true — dictionary order
"Zebra" < "apple"     // true 😐 — UPPERCASE sorts before lowercase
"10" < "9"            // true as strings! ("1" < "9")

Gotchas: case sensitivity and string-vs-number confusion. For real sorting use localeCompare or numeric sort callbacks (arrays lesson).

Tricky corners worth knowing

NaN === NaN          // false! NaN equals nothing, not even itself
Number.isNaN(x)      // correct NaN test

null === undefined   // false (different types)
null == undefined    // true (loose quirk)

+0 === -0            // true

Chaining comparisons doesn't chain

18 <= age <= 65     // BUG — evaluates (18<=age) → boolean → boolean<=65
age >= 18 && age <= 65   // ✅ the correct range check

Math-class notation silently lies. Combine with &&.

Comparing objects & arrays

References, never contents:

[1,2] === [1,2]              // false — two different arrays!
{a:1} === {a:1}              // false

const a = [1,2];
const b = a;
a === b                      // true — same array, two names

Content-equality needs a helper (JSON.stringify(a) === JSON.stringify(b) for simple cases) or a library deep-equal.

Mini Practice

1. Predict-then-run five == vs === pairs from the lesson
2. Build a valid-range checker for percentage input (0–100 inclusive)
3. Show "10" < "9" and explain in one comment why
4. Prove NaN !== NaN; fix with Number.isNaN
5. Compare two arrays by contents via JSON.stringify; note when it lies (key order)

Next: booleans →