JavaScript Comparisons

Equal in two different ways

=== is strict equality: same value and same type. == is loose equality: JavaScript converts one side, then compares. That conversion is called coercion, and it is how"5" == 5 becomes true.

Prefer === in this tutorial and in your own scripts. When a string and a number should stay different, strict equality is the tool that keeps them different.

=== checks value and type

Numbers compare as numbers. Strings compare as strings. A number is never strictly equal to a string of digits. true is not 1 under ===.

console.log(5 === 5);
console.log("5" === "5");
console.log("5" === 5);
console.log(true === 1);
console.log(null === undefined);

document.body.textContent =
  String("5" === 5) + " strict / " + String(5 === 5) + " number";

== coerces

Loose equality converts before it compares. "5" == 5 is true because the string becomes the number 5. true == 1 is true. null == undefined istrue, which is a special case in the spec.

console.log("5" == 5);
console.log(true == 1);
console.log(false == 0);
console.log("" == 0);
console.log(null == undefined);
console.log(null == 0);

document.body.textContent =
  String("5" == 5) + " loose / " + String("" == 0);

Not equal

!== is the strict opposite of ===. != is the loose opposite of==. Use !== with the same discipline you use for ===.

Ordering

<, <=, >, and >= compare numbers in the obvious way. They also coerce: "10" > 2 is true because the string becomes a number. If both sides are strings, they compare alphabetically: "10" > "2" isfalse because "1" comes before "2".

console.log(10 > 2);
console.log("10" > 2);
console.log("10" > "2");
console.log(7 <= 7);
console.log("apple" < "banana");

document.body.textContent =
  String("10" > 2) + " coerced / " + String("10" > "2") + " text";

NaN is never equal

NaN === NaN is false. NaN == NaN is also false. After a failed Number(...) conversion, use Number.isNaN(value), not an equality check.

const n = Number("wait");
console.log(n === n);
console.log(n == n);
console.log(Number.isNaN(n));

document.body.textContent = "isNaN " + String(Number.isNaN(n));

What to remember

• Default to === and !==.
• "5" and 5 are different under strict equality.
• Ordering coerces too. Convert on purpose, then compare numbers to numbers.
• Never test NaN with ===.

Next: if, else if, and else — run a block when a test is true.