Comparisons

Comparison asks a yes-or-no question

A comparison is an expression that answers with a boolean: true or false. You use comparisons when code needs to make a choice: is the password long enough, did the score reach 100, is the value missing, or should this item come before that one?

You already met values in the Data types, Numbers & math, Strings, and Booleans, null & undefined lessons. This lesson focuses on the operators that compare those values.

In this lesson you will compare values with a live equality matrix, step through string order, untangle null and undefined, and learn why NaN is the one value equality cannot find.

JavaScript has several equality-looking operators. The one you want most of the time is strict equality, written ===. It returns true only when both sides have the same type and the same value.

• 5 === 5 is true: both are numbers, both are 5.
• "5" === 5 is false: string on the left, number on the right.
• false === 0 is false: boolean on the left, number on the right.

Loose equality, written ==, may convert one side first. You learned the idea of implicit conversion in the Type conversion lesson. Here you only need the warning label: == can make different-looking values match.

1const left = "0";2const right = 0;3 4console.log(left == right);5console.log(left === right);6console.log(Object.is(left, right));

Try "0" against 0, then "" against false, then null against undefined. Those are real results, not guesses. The third column uses Object.is, which you met in Numbers & math and will study deeply in “Equality & sameness.”

The ordering operators <, >, <=, and >= also return booleans. With two numbers, they feel like math. With two strings, JavaScript compares text character by character using character codes.

Step through the examples. In string mode, "10" < "9" is true because JavaScript compares the first characters: "1" comes before "9". In number mode, 10 < 9 is false. In mixed mode, a string-number pair is converted to numbers before the relational comparison.

1console.log("apple" < "banana");Run2console.log("Apple" < "apple");3console.log("10" < "9");4console.log("a".charCodeAt(0));

null means “intentionally empty.” undefined means “no value was provided or found.” They are different values, but loose equality gives them one special handshake: null == undefined is true.

The tricky part is that equality and relational comparisons do not share one simple rule. Watch null == 0, null >= 0, and undefined >= 0 carefully.

1console.log(null == undefined);Run2console.log(null === undefined);3console.log(null == 0);4console.log(null >= 0);5console.log(null > 0);6console.log(undefined == 0);7console.log(undefined >= 0);

NaN stands for “Not-a-Number,” a special number value that means a numeric operation failed. The Numbers & math lesson introduced Number.isNaN. Here is why it matters: equality cannot detect NaN.

1console.log(NaN === NaN);Run2console.log(NaN == NaN);3console.log(Number.isNaN(NaN));4console.log(Object.is(NaN, NaN));5console.log([NaN].includes(NaN));6console.log([NaN].indexOf(NaN));

The array examples tease a future arrays lesson. For now, remember the professional habit: when the question is “is this NaN?”, ask Number.isNaN(value).

Where you’ll use this

Comparisons show up anywhere your program chooses a path. In front-end code, the values often start as text: form fields, URL pieces, stored settings, and data from a server. Safe code makes the conversion obvious before comparing.

1const ageText = "18";2const age = Number(ageText);3 4if (Number.isNaN(age)) {5  console.log("Please enter a number.");6} else if (age >= 18) {7  console.log("Adult ticket");8} else {9  console.log("Child ticket");10}

This is also why linters often warn about ==. They are not saying the operator is useless. They are saying hidden conversion is a common bug factory.

Common misconceptions

Try each one before opening the hints. The exact outputs have been checked against real JavaScript.

console.log("0" == 0);
console.log("0" === 0);
console.log(null == undefined);
console.log(NaN === NaN);

const entered = "5";

if (entered == 5) {
  console.log("Level 5 unlocked");
}

const entered = "5";
const level = Number(entered);

if (level === 5) {
  console.log("Level 5 unlocked");
}

console.log("item10" < "item9");
console.log("10" < "9");
console.log(10 < 9);

function isMissing(value) {
  return value == null;
}
console.log(isMissing(null));
console.log(isMissing(undefined));
console.log(isMissing(0));

function isMissing(value) {
  return value == null;
}
console.log(isMissing(null));
console.log(isMissing(undefined));
console.log(isMissing(0));

const value = Number("nope");
console.log(Number.isNaN(value));

const value = Number("nope");
console.log(Number.isNaN(value));

Your score counts first tries. Read every explanation; the wrong choices are designed to catch common comparison bugs.

Key takeaways

• === is your default equality operator because it checks type and value.
• == converts before comparing; use it only deliberately, especially for value == null.
• String ordering uses character codes, so capitals and number-looking strings can surprise you.
• Check possibly missing values before using relational operators like >=.
• NaN never equals itself; use Number.isNaN(value).

Up next: Logical operators.