True, false, and comparing values

Numbers are endless and text is endless, but the BOOLEAN type has exactly two values in the whole language: true and false. They are written as bare words with no quotes. The word "true" in quotes is a piece of text, not a boolean, and confusing the two causes real bugs.

Booleans are what decisions are made of. In the next module, every if, every loop condition and every filter comes down to a value that is one of these two. Getting comfortable producing them accurately now is what makes that module easy.

There is nothing special about a comparison. 3 < 5 is an expression that produces a value, in the same way 3 + 5 is; the value it produces just happens to be true rather than 8. You can name it, print it, and pass it around exactly like any other value.

The relational operators are < less than, > greater than, <= less than or equal to, and >= greater than or equal to. For equality there are === is exactly equal to and !== is not exactly equal to. There is also a two-character == which behaves differently and which the last lesson of this module is entirely about; until then, use three.

Reading them out loud helps more than it should. isReady === true reads as "is ready is exactly equal to true", which immediately sounds like something you would never say — and indeed isReady on its own already is the boolean, so comparing it to true adds nothing.

=== asks a narrow, honest question: are these the same type, and are they the same value? If the two sides are different types the answer is false immediately, with no attempt to convert anything. So 3 === 3 is true, and "3" === 3 is false, because one is a number and the other is text.

That refusal to convert is the entire point, and it is why every style guide reaches for === by default. A comparison that quietly converts its operands can be true for reasons you did not intend, and finding out why is a bad afternoon.

One exception exists, and it is the one you met in the numbers lesson: NaN === NaN is false. Every comparison involving NaN is false, including that one, which is why Number.isNaN exists.

When both sides of <, >, <= or >= are text, JavaScript compares them character by character until it finds a pair that differs, and decides based on that pair. "apple" < "banana" is true because a comes before b.

The order it uses is the numeric order of the underlying character codes, not the order a dictionary or a phone book would use. In that scheme every capital letter comes before every lower-case one, so "Zebra" < "apple" is true — which is emphatically not what a human sorting a list would do.

Digits inside text follow the same rule, with a result that catches everybody: "10" < "9" is true, because the comparison stops at the first character, and 1 comes before 9. If you want the numeric answer, compare numbers rather than text. Real alphabetical sorting of human-facing lists needs localeCompare, which the text module covers.

Chaining comparisons. In mathematics 1 < 2 < 3 means what you think it means. In JavaScript it is worked out left to right: 1 < 2 gives true, and then true < 3 is compared, which quietly turns true into 1 and asks 1 < 3. The answer happens to be true, which is worse than being wrong, because the same shape gives 3 > 2 > 1 as false. Never chain. Write two comparisons and join them, which the next module shows you how to do.

One equals sign instead of three. A single = does not compare anything: it assigns. Writing it where you meant a comparison either changes a value you did not mean to change or fails outright, depending on what is on the left. If you remember one piece of punctuation from this lesson, remember that = means becomes and === means is equal to.

An exclamation mark in front of a boolean flips it: !true is false and !false is true. It is pronounced not, so !isOutOfStock reads as not out of stock.

Two of them in a row, !!, flip it twice and therefore change nothing about a boolean. You will see !!value in real code, where it is being used on a value that is not a boolean to force it into one — a trick that only makes sense once you have met truthiness in the next module.

A style note that will save you: name booleans so they read as a claim that is either true or false. isLow, hasEnough, wasFound. A boolean named status or check tells the next reader nothing, and the next reader is usually you.

The boolean type holds exactly true and false. Comparisons are ordinary expressions that produce them. === compares type and value with no conversion, and is the default. Text compares by character code, so capitals sort before lower case and "10" is less than "9". Never chain comparisons, never write = where you meant ===.