Lessons · TypeScript · what counts as true

What counts as true

Only six values are falsy: false, 0, '', null, undefined and NaN. Everything else is truthy, including '0', 'false', [] and {}.

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What it is for

if (value) is written a hundred times a day, and it silently treats a real 0 or an empty string as missing while treating the text '0' as present.

How to think about it

Ask the question you mean. Missing: value == null. Empty text: value.length === 0. Zero: value === 0. Use bare truthiness only when 'falsy' is exactly the test you want.

Worked example

console.log(Boolean("0"), Boolean(0));

true false: a non-empty string is truthy, the number 0 is not.

console.log(Boolean(" "), Boolean(""));

true false: whitespace has length.

console.log(Boolean([]), Boolean({}));

true true: empty containers are truthy in JavaScript.

console.log(Boolean(null), Boolean(undefined), Boolean(NaN));

false false false.

Your turn

Is the text non-empty, without a truthiness surprise?

if (text.length  0)

The trap

Values from forms are text, so if (value) is true for '0' and 'false'. Convert first, or test the length.

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