Conditional checks
Run code only when a value is present or enabled.
JS Puzzles · Const
Truthy and Falsy
JavaScript converts values to Boolean in conditions and logical expressions. Most values are truthy; false, zero, the empty string, null, undefined, and NaN are falsy.
How it works
JavaScript converts an expression to a Boolean when it is used as a condition or with Boolean conversion. Falsy values include false, 0, -0, 0n, the empty string, null, undefined, and NaN; other values, including empty arrays and objects, are truthy.
Syntax
Boolean(value)Key points
- Truthiness is contextual: a value can be used directly as a condition without first being explicitly converted with Boolean().
- The default rule is that values are truthy; only the language's defined falsy values convert to false.
- The standard falsy set includes false, numeric zero forms, BigInt zero, the empty string, null, undefined, NaN, and the browser-specific document.all value.
- An if condition, logical operator, or Boolean conversion can coerce a non-Boolean expression to a Boolean result for testing.
- && processes operands left to right and short-circuits as soon as it encounters a falsy operand.
- Logical operators preserve selected operand values, which lets expressions produce data such as a fallback or a guarded result.
Common mistakes
- Logical operators return an operand rather than necessarily returning true or false: && stops at the first falsy operand, while || stops at the first truthy operand.
Example: Testing truthiness
const values = [false, 0, "", null, undefined, NaN, [], {}];
for (const value of values) {
console.log(Boolean(value));
}
Boolean contexts convert values without changing the original value; objects and arrays are truthy, even when empty.
When to use it
Fallback selection with ||
Choose the first available value from several candidates.
Guarded evaluation with &&
Stop a computation when an earlier prerequisite is absent.
Interview questions
QWhat are truthy and falsy values in JavaScript?
A truthy value is converted to true in a Boolean context, while a falsy value is converted to false. Falsy values include false, 0, -0, 0n, the empty string, null, undefined, and NaN.
Follow-up: What does Boolean([]) return? It returns true because arrays are objects, and objects are truthy.
QHow does && use truthiness?
The && operator evaluates from left to right and returns the first falsy operand. If every operand is truthy, it returns the last operand, preserving that operand's original value.
Follow-up: What does 0 && "fallback" return? It returns 0 and does not evaluate the right operand because 0 is falsy.
QHow does || use truthiness?
The || operator evaluates from left to right and returns the first truthy operand. If none is truthy, it returns the last operand, so it is commonly used to select a fallback.
Follow-up: Why can || be unsuitable for defaulting a valid zero? Because 0 is falsy; use ?? when only null or undefined should trigger the fallback.
Related
Logical operatorsLogical NOT (!)if...elseBoolean