What are truthy and falsy values in JavaScript?

When a value is used as a yes or no, as in an if, JavaScript treats a short list of values as false: false, 0, -0, 0n, "", null, undefined and NaN. Everything else counts as true, including "0", [] and {}.

An if does not need a real true or false. Give it any value and JavaScript decides whether that value counts as true (truthy) or false (falsy). The falsy list is short enough to learn; everything not on it is truthy.

The example prints Boolean(value) for each sample. 0, '', null, undefined and NaN are falsy. The string '0', a price string like '₹0', an empty array [] and an empty object {} are all truthy. The last two surprise people: a check like if (rows) passes even when the list of rows is empty. Test the length instead, rows.length > 0.

The second half shows the bug that falsiness causes in test code. A quantity of 0 read from the page is a real value, but !qtyOnPage treats it as missing. When you mean "was a value provided?", say so: qtyOnPage == null is true only for null and undefined, so 0 is correctly reported as present.

Truthy checks are fine for genuine yes/no flags. For counts, prices and text from the page, compare against the value you actually mean.

truthy-falsy.mjs

const samples = [
  ['0', 0], ["''", ''], ['null', null], ['undefined', undefined], ['NaN', NaN],
  ["'0'", '0'], ["'₹0'", '₹0'], ['[]', []], ['{}', {}],
];
for (const [label, value] of samples) {
  console.log(label.padEnd(9), Boolean(value));
}

const qtyOnPage = 0;
if (!qtyOnPage) {
  console.log('!qty says missing, but qty is', qtyOnPage);
}
if (qtyOnPage == null) {
  console.log('missing');
} else {
  console.log('== null says present:', qtyOnPage);
}

Output

0         false
''        false
null      false
undefined false
NaN       false
'0'       true
'₹0'      true
[]        true
{}        true
!qty says missing, but qty is 0
== null says present: 0

Test yourself

A helper returns an empty array when no orders match. Does if (orders) { ... } run its block?

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What are truthy and falsy values in JavaScript?

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