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Use Array.prototype.join() to turn an array into a string in TypeScript. Pass a separator to control how the items appear: colors.join(', ') produces red, blue, green. The conversion runs at JavaScript runtime; TypeScript checks the types used in your code.
Use join() to choose the separator
join() returns a string containing the array elements, with the chosen separator between them. If you omit the separator, it uses a comma.
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const colors: string[] = ['red', 'blue', 'green'];
const text = colors.join(', ');
// "red, blue, green"TypeScript infers that text is a string, so an explicit : string annotation is unnecessary. Array types such as string[] describe the values at compile time; the conversion itself uses JavaScript’s array method. See the TypeScript Handbook for array type forms and primitive types.
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Choose a separator that fits the output you need:
| Code | Result |
|---|---|
['red', 'blue'].join() |
red,blue |
['red', 'blue'].join(', ') |
red, blue |
['red', 'blue'].join(' | ') |
red | blue |
['red', 'blue'].join('') |
redblue |
The separator appears between elements, not after the last element. An empty array joined with any separator returns an empty string. MDN’s join() reference documents the method and its default comma separator.
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What happens with empty, nullish, or nested values?
In a joined string, null, undefined, and empty array slots become empty fields—not the literal words null or undefined.
const values: (string | null | undefined)[] = ['a', null, undefined, 'b'];
values.join(', ');
// "a, , , b"Nested arrays are converted recursively. The outer separator applies between the outer elements; nested arrays use commas internally, so their structure can look flattened in the resulting text.
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[[1, 2], [3, 4]].join(';');
// "1,2;3,4"These behaviors, including empty slots and nested arrays, are described in MDN’s method reference.
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Joining objects directly usually does not produce useful labels. Map each object to the field intended for display, then join those strings.
type User = { name: string };
const users: User[] = [{ name: 'Ada' }, { name: 'Lin' }];
const names = users.map(user => user.name).join(', ');
// "Ada, Lin"This makes the desired representation explicit instead of relying on default object coercion.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When should you use toString() instead?
For arrays, toString() produces the default comma-separated form: it calls join() without an argument. Use join() when you need to choose the delimiter or make that formatting choice clear in the code. String(array) also gives the default string conversion, so neither is a substitute for a deliberately formatted display string.
A display string is not necessarily a serialization format. If another system needs structured data, choose a format designed for that requirement rather than assuming comma-separated output preserves the array’s structure.
The TypeScript Handbook recommends lowercase primitive type names such as string rather than the special built-in type String. See Everyday Types.
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