What does truthy or falsy mean in JavaScript?
Quick answer
In JavaScript, a value is truthy if a conditional treats it as true, and falsy if the conditional treats it as false. The standard falsy values are false, 0, -0, 0n, "", null, undefined, and NaN; browsers also have the special-case document.all. Empty arrays and empty objects are truthy, so use an explicit length check when you mean “not empty.” [S1][S2]
The question, the details, the next step.
What does truthy or falsy mean? I keep seeing these terms in JavaScript examples.
They describe how JavaScript interprets a value when a condition needs a Boolean result. For example, if (value) runs its block when value is truthy and skips it when value is falsy. JavaScript performs this Boolean conversion automatically in a conditional; the original value does not have to be the Boolean true or false. [S1][S2]
That distinction matters: if ("hello") runs, but the string itself is still a string. Truthiness is a convenient way to make a yes-or-no decision, not a test that a value literally equals true.
Which values should I recognize as falsy?
The standard falsy values are:
false0and-00n(BigInt zero)""(the empty string)nullundefinedNaN
MDN also lists document.all as a browser-specific falsy value. It is unusual; the standard list above covers the common cases. [S1]
A string containing a space, such as " ", is not an empty string, so it is truthy. Likewise, "0" is a nonempty string and is truthy. Those distinctions can matter when values come from form fields or other text input. [S1]
I expected an empty array or object to count as empty. Why does an if check accept them?
Because truthiness does not test whether a collection contains items or properties. Empty arrays and empty objects are truthy, just like nonempty ones. [S1][S2] So this runs its block even when the array has no entries:
const items = [];
if (items) {
console.log("This runs");
}If you mean “the array has at least one item,” check items.length > 0. That expresses the condition you intend; a plain if (items) only checks whether the value is truthy.
Can you give me a small snippet that prints the results, and a rule of thumb for choosing a check?
This example prints each value and the Boolean result of applying Boolean to it. It includes common falsy cases, empty containers, and values that can look empty:
const samples = [
false, 0, -0, 0n, "", null, undefined, NaN,
" ", "0", [], {}, ["item"], { name: "Ada" }
];
for (const value of samples) {
console.log(value, Boolean(value));
}The empty string and listed zero or missing-value cases produce false; the space string, the string "0", the empty array, and the empty object produce true. The example demonstrates Boolean conversion; it does not check whether an array or object has contents. [S1][S2]
Rule of thumb: use truthiness when any falsy value should mean “no.” Use a specific check when you mean something narrower, such as “not an empty string,” “a number other than zero,” or “an array with entries.” For example, items.length > 0 clearly expresses the last intention.
A practical checklist
When reading
if (value), identify which falsy values might occur in that variable. [S1]Remember that
" "and"0"are truthy strings; only the empty string is falsy. [S1]Do not use
if (array)to test whether an array has items. Empty arrays are truthy. [S1]Use a direct condition such as
items.length > 0when checking for array entries.Choose an explicit comparison or content check when truthiness is broader than your intention.
A few more useful answers
Is a truthy value the same as `true`?
No. A truthy value is treated like true in a Boolean context, but it can still be a string, number, array, object, or another type. [S2]
Does `if (value)` check whether a variable exists?
It checks whether the value is truthy, not just whether it exists. For example, an existing value of 0 or "" is falsy, while an empty array is truthy. [S1]
What is the quickest way to check whether an array has items?
Use a condition such as items.length > 0. A plain if (items) does not distinguish an empty array from a nonempty one. [S1]
Sources
References used for this guide.
- [S1] Truthy — MDN developer.mozilla.org · Checked
- [S2] Boolean — JavaScript — MDN developer.mozilla.org · Checked