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For an exact empty-string check, use value.length === 0 or value === "". If whitespace-only text should count as blank, use value.trim().length === 0 instead. For values that may be null or undefined, check those cases before accessing string properties.
Check for an exactly empty string
An empty string has zero characters. Both of these checks express that rule:
const isEmpty = (value: string): boolean => value.length === 0;
const isEmptyByLiteral = (value: string): boolean => value === "";The first checks the string’s length; the second compares it directly with the empty-string literal. Choose whichever makes the intent clearest to your team. Neither treats a string containing spaces as empty.
Decide whether whitespace counts as blank
If your validation rule considers whitespace-only input blank, trim the string before checking its length:
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const isBlank = (value: string): boolean => value.trim().length === 0;This is a different rule from exact emptiness: " " has characters, but its trimmed value has none. Use it only when that matches the behavior you want, such as rejecting a form field containing only whitespace.
Handle null or undefined before using string methods
A value typed as string | null | undefined may not have a string’s length property. With strictNullChecks, TypeScript treats null and undefined as distinct types, so narrow the union first. For example, to treat either nullish value or an empty string as absent:
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function isAbsentOrEmpty(value: string | null | undefined): boolean {
return value == null || value.length === 0;
}In JavaScript and TypeScript, value == null matches both null and undefined. The right side of the || is evaluated only when the first condition is false, so length is read only after the nullish cases have been excluded. If you want to preserve an empty string while handling null separately, compare explicitly:
if (value !== null) {
// value is not null here; it may still be an empty string
}Equality checks can narrow union types in TypeScript’s control flow. See the TypeScript Handbook’s narrowing guide and the TSConfig Reference entry for strictNullChecks.
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In JavaScript, "" is falsy. That makes if (!value) a possible shorthand when value is known to be a string and the only distinction you care about is empty versus non-empty. But truthiness is broader than an explicitly named empty-string test: if the type later includes other falsy values, the condition catches those too.
It can also accidentally skip work that should happen for an empty string. The TypeScript Handbook warns that wrapping a function body in a truthiness check can mean “we may no longer be handling the empty string case correctly.” Use an explicit length or literal comparison when that case needs deliberate handling.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the check that matches the input rule
| Check | Meaning | Use when |
|---|---|---|
value.length === 0 |
Exactly zero characters | value is a string and whitespace is meaningful. |
value === "" |
Equal to the empty-string literal | You want an explicit exact-empty comparison. |
value.trim().length === 0 |
Empty after trimming whitespace | Your rule treats whitespace-only text as blank. |
value == null || value.length === 0 |
Null, undefined, or exactly empty | The input type allows nullish values and all should count as absent. |
if (value) or if (!value) |
Truthiness or falsiness, not specifically string length | Only when the broader truthiness behavior is intentional. |
These forms are alternatives for different semantics, not performance claims; the cited documentation does not establish a speed advantage for one check.
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