For a string, read text.length. It returns the number of UTF-16 code units in the string—not necessarily the number of Unicode code points or characters a person sees. If a value might not be a string, check it with typeof value === "string" before reading its length.
Get the length of a string
TypeScript builds on JavaScript, so a string’s length property works the same way in both languages and returns a number. A function whose input is already typed as string can access the property directly:
function getStringLength(text: string): number {
return text.length;
}For example, getStringLength("hello") returns 5. The TypeScript handbook also uses length in an example involving strings and arrays: TypeScript for JavaScript Programmers.
Check a value before reading its length
A TypeScript type annotation does not validate data at runtime. When a value comes from an API, a form, or another source that may provide a non-string, accept it as unknown and narrow it with a type check:
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function checkedStringLength(value: unknown): number | undefined {
if (typeof value === "string") {
return value.length;
}
return undefined;
}Inside the if branch, TypeScript narrows value to string, so accessing value.length is safe. Returning undefined is one possible policy; an application can instead return a validation error or a discriminated result.
Do not use a type assertion such as value as string to make unchecked input appear valid. An assertion changes what TypeScript assumes; it does not check the runtime value. Also prefer the primitive string type over the boxed String type, as the TypeScript handbook recommends: TypeScript: Do’s and Don’ts.
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Choose what “length” should count
The right method depends on the unit your application requires. JavaScript’s String.length measures UTF-16 code units, while spreading a string counts Unicode code points, and Intl.Segmenter can count grapheme clusters that more closely correspond to user-perceived characters. MDN explains these distinctions in its String.length reference.
| Counting method | Example | What it counts | Use it when |
|---|---|---|---|
text.length |
"😄".length is 2 |
UTF-16 code units; a supplementary code point such as this emoji occupies two. | The API or limit is defined in JavaScript UTF-16 code units. |
[...text].length |
[..."😄"].length is 1 |
Unicode code points; a surrogate pair is iterated as one point. Combined sequences can still contain multiple points. | You need a simple code-point count rather than a count of displayed characters. |
Intl.Segmenter with granularity: "grapheme" |
MDN’s family-emoji example, "👨👩👧👧", has one grapheme cluster. |
Grapheme clusters, which are closer to user-perceived characters and can include multiple code points. | Your product’s rule is based on perceived characters; confirm support in the target runtime. |
A grapheme-cluster count can be computed like this:
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function graphemeCount(text: string): number {
const segmenter = new Intl.Segmenter(undefined, { granularity: "grapheme" });
return Array.from(segmenter.segment(text)).length;
}The locale argument is undefined here, so the runtime uses its default locale. Check that Intl.Segmenter is supported in the environments where your code runs, and follow the exact counting rule required by your product.
Do not confuse a string’s length with the String function’s arity
Read text.length to measure a particular string. String.length refers instead to the number of parameters the built-in String function declares; it is not the length of a string value.
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