Logical Operators: Or

The second logical operator is ||, meaning “or”. “If it's Saturday or Sunday, it's the weekend”:

const day = "Sunday";
if (day === "Saturday" || day === "Sunday") {
    console.log("It's the weekend");
}

This prints:

It's the weekend

A condition joined with || is true if at least one of its parts is true. It's false only if all the parts are false:

• true || true is true
• true || false is true
• false || true is true
• false || false is false

A typical mistake is to shorten the condition the way we speak: day === "Saturday" || "Sunday". JavaScript takes the right part on its own: it's just the text "Sunday", not a comparison. Text that isn't empty counts as true, so the whole condition is always true, even on Monday. Each part needs its own comparison: day === "Saturday" || day === "Sunday".

You can use && and || in one condition. Just as multiplication goes before addition, && goes before ||. Use parentheses to group what belongs together, as in math:

const day = "Saturday";
const temperature = 15;
if ((day === "Saturday" || day === "Sunday") && temperature > 20) {
    console.log("Let's go on a trip");
} else {
    console.log("Let's stay at home");
}

This prints:

Let's stay at home

The part in parentheses is evaluated first: it's Saturday, so it's true. But temperature > 20 is false, and true && false is false. Without the parentheses, JavaScript would first join day === "Sunday" && temperature > 20, and the condition would be true on every Saturday, whatever the weather.