Boolean Evaluation

What is the output here?

console.log("to be" || "not to be"); // to be

Explanation: The value "to be" evaluates to true. The OR logical operator returns the first operand if that operand evaluates to true.

• 0, NaN, null, undefined, and empty string ("" or '') are all falsy.
• All other values are truthy.

console.log(null && "Ali"); // null:  null evaluates to false
console.log(undefined || "Ali"); // Ali:   undefined evaluates to false
console.log(!"Ali"); // false: "Ali" evaluates to true

The JavaScript community uses these Boolean-ish values too. For instance, it is common to see code like this

if (user) {
  // Skipped if user is any falsy value (undefined, null, "", 0, false, NaN)
}

That code can be converted to this

if (user !== undefined && user !== null) {
}

Another everyday use is to set a default value to a function argument.

function calcWage(hoursWorked, hourlyPayRate) {
  hourlyPayRate = hourlyPayRate || 12.5;
  // calculate the wages!
}

calcWage(40); // uses default hourly pay rate.
calcWage(35, 15);

That trick can be avoided by using default function parameters. (Default function parameters are covered in the chapter on functions.)

function calcWage(hoursWorked, hourlyPayRate = 12.5) {
  // calculate the wages!
}

There are cases where it might be more work; for example, the following expression

query = (query && query.trim()) || "";

does the same job as the code below:

if (query !== undefined && query !== null) {
  query = query.trim(); // removes whitespace from both ends
} else {
  query = "";
}

Another example:

denominator = denominator || 1;

which does the same job as the following code:

if (
  denominator === undefined ||
  denominator === null ||
  Number.isNaN(denominator) ||
  denominator === 0
) {
  denominator = 1;
}