Booleans and comparisons

5 == "5" is true but 5 === "5" is false. The triple equals sign also checks that both sides are the same type.

Questions with a yes or no answer

Programs constantly ask questions. Is the password long enough? Is the player out of lives? Is today a weekend? Each of these has one of two answers, and JavaScript represents them with the two boolean values, true and false. Booleans are named after George Boole, the nineteenth-century mathematician who worked out an algebra of true and false. On their own they are not very exciting, but they are what lets a program make decisions, which is the subject of the next unit.

Comparison operators

A comparison takes two values and produces a boolean. The six you will use most are below. Notice that "equal" is written with three equals signs, because a single = already means "store a value".

Like arithmetic, a comparison is an expression, so you can print it or store it in a variable: const isAdult = age >= 18; puts true or false into isAdult depending on the age. Variable names for booleans often start with is, has or can, which makes the code read like a question.

Why === and not ==

JavaScript also has a double equals, ==, called loose equality. Before comparing, it tries to convert both sides to the same type, which produces results that are hard to predict: 5 == "5" is true, and so is 0 == "". The triple equals ===, strict equality, never converts. If the types differ, the answer is simply false. MDN and most style guides recommend === and !== for everyday code, and this course uses them throughout.

console.log(5 === 5);      // true
console.log(5 === "5");    // false
// loose equality, best avoided:
console.log(5 == "5");     // true
console.log("cat" !== "dog"); // true

Strings can be compared too. === checks that two strings have exactly the same characters, capitals included, so "Yes" === "yes" is false. The < and > operators compare strings character by character using their character codes, which is close to dictionary order for lower-case words: "apple" < "banana" is true. Capital letters come before all lower-case letters in that ordering, so "Zebra" < "apple" is also true.

Combining answers with &&, || and !

Real questions often have several parts. A ride may require a height of at least 120 cm and an age of at least 8. JavaScript joins boolean answers with logical operators. && (and) is true only when both sides are true. || (or) is true when at least one side is true. ! (not) flips a boolean, so !true is false.

const height = 130;
const age = 7;
console.log(height >= 120 && age >= 8);   // false
console.log(height >= 120 || age >= 8);   // true
console.log(!(age >= 8));                 // true

Worked example. Is a day a weekend? day === "Sat" || day === "Sun". Notice that you must repeat day === on both sides. Writing day === "Sat" || "Sun" looks natural in English but does not do what you mean, because "Sun" on its own is not a comparison. Comparisons are worked out before && and ||, so height >= 120 && age >= 8 needs no extra parentheses, though adding them never hurts.

One more idea you will meet later: when JavaScript needs a boolean but gets some other value, it treats a few values as false. These falsy values are false, 0, the empty string "", null, undefined and NaN. Everything else, including "0" and "false" as strings, counts as true.