# Combobox

> A text input joined to a filtered list — for choosing from more options than anyone should scroll through.

- Category: UI Components
- Canonical: https://www.themasterly.com/glossary/combobox

A combobox is a text input joined to a list that filters as you type. It solves the case a [dropdown](https://www.themasterly.com/glossary/dropdown-ui) cannot: too many options to scroll, or a value the person may need to create.

In B2B products it ends up everywhere, because the lists are long. Accounts, projects, users, categories, tags — a picker that was comfortable with six entries during the trial is unusable in a customer account with nine hundred.

## The variants, and choosing deliberately

| Variant | Typing does | Right for |
|---|---|---|
| **Filter only** | Narrows a fixed list | Choosing from many known options |
| **Filter and create** | Narrows, and offers to add | Tags, labels, inviting by email |
| **Autocomplete** | Suggests as you go, free text allowed | Addresses, search queries |
| **Multi-select** | Narrows, selections accumulate | Assigning several owners, filtering by several values |

The second is the one chosen by accident. Allowing free entry against a set that is meant to be fixed produces "Marketing", "marketing" and "Mktg" as three separate categories within a month, and no reasonable path to merging them later. Where creation is genuinely part of the task, allow it; where it is not, close it off.

Multi-select needs the selections to stay visible. A control reporting "12 selected" hides whether the right twelve are chosen, and [chips](https://www.themasterly.com/glossary/chip-ui) are usually worth the space.

## The keyboard is the component

Most of what makes a combobox feel finished or broken is invisible to a mouse user.

**Arrows move through options without disturbing the typed text.** Losing what was typed on the first arrow press is the single most common defect.

**Enter selects the highlighted option**, and if nothing is highlighted, it either creates or does nothing — deliberately, not by accident.

**Escape closes and restores.** Twice: once to close the list, again to clear.

**Blur with an incomplete value has to be decided.** Revert to the last valid selection, keep the text as a pending filter, or clear. Any of the three is defensible; leaving it to chance produces a control that behaves differently depending on how somebody left it.

**Announcements matter.** A screen reader needs to hear how many results are available as the list changes, and which option is active. This is where custom implementations most often fail silently.

## Matching has to forgive

The same rules as [search](https://www.themasterly.com/glossary/search-ui) apply, and for the same reason: an exact-match filter fails the moment somebody types a plural, a typo, or their own word for the thing.

**Match anywhere, not only at the start.** Typing "corp" should find "Acme Corporation".

**Tolerate a typo.** One transposed letter emptying the list is the failure people remember.

**Rank sensibly.** Exact match first, then prefix, then contains. Recent or frequent selections above alphabetical.

**Show that more exist.** "Showing 20 of 340" tells somebody to keep typing rather than to conclude their record is missing.

## Why to start from a library

This is the component with the worst record for custom builds, because it has to keep several things coherent at once: the input value, the filtered set, the active option, the announcement, and the open state.

A headless library that has solved the behaviour and the accessibility, styled to your own [design system](https://www.themasterly.com/glossary/design-system), is almost always the right trade. Writing it from scratch means reimplementing a specification that other people have already got wrong and fixed. See [component library](https://www.themasterly.com/glossary/component-library).

## In practice

A project picker starts as a native select and works through the trial.

The first real customer has four hundred projects. Scrolling is the only way to find one, the three projects anybody actually uses are scattered alphabetically, and the control takes a visible moment to open.

Replacing it with a combobox fixes the scrolling and introduces a new complaint within a week: typing a project name with a hyphen returns nothing, because the filter matches from the start of the string and the customer's naming convention puts a date first.

The fix is not the component. It is matching anywhere in the string, recent projects above the filtered list, and a count showing how many matched. The same control, behaving as though it expects people to type imperfectly.

## Where teams get it wrong

- **Free entry on a fixed set.** Three spellings of one category, permanently.
- **Arrow keys clearing the input.** The defect anybody who types fast finds immediately.
- **Prefix-only matching.** Anything not named the way you expected is invisible.
- **Undecided blur behaviour.** The same control behaving differently each time.
- **No result count.** People conclude their record is missing when it is on line 40.
- **Building it from scratch.** A specification with a long history of being got wrong.

## Related terms

- [Dropdown UI](https://www.themasterly.com/glossary/dropdown-ui)
- [Input Field](https://www.themasterly.com/glossary/input-field)
- [Search UI](https://www.themasterly.com/glossary/search-ui)
- [Form Design](https://www.themasterly.com/glossary/form-design)

## FAQ

**What is a combobox?**

A text input joined to a list that filters as you type. It exists for the case a plain dropdown fails: too many options to scroll, or a value that might not be in the list yet. The typing is what makes it different from a select.

**What is the difference between a combobox and a dropdown?**

A dropdown offers a fixed list to pick from. A combobox lets you type to narrow that list, and depending on the variant, to enter a value that is not in it. Past roughly fifteen options a dropdown stops being usable and a combobox is the answer.

**Should a combobox allow values that are not in the list?**

Only when creating one is genuinely part of the task — adding a tag, inviting an email address not yet in the account. Where the set is fixed, allowing free entry produces near-duplicates that nobody can clean up later. Decide this deliberately; it is the variant that gets chosen by accident.

**What keyboard behaviour does a combobox need?**

Arrow keys to move through options without losing the typed text, Enter to select the highlighted one, Escape to close and restore what was there, and Home and End to reach the ends. Getting this wrong is the most common reason a custom combobox feels broken to anyone who types quickly.

**Why is a combobox hard to build correctly?**

Because it combines two controls and has to keep both coherent: the text being typed, the list being filtered, which option is highlighted, what a screen reader announces as results change, and what happens on blur with an incomplete value. This is the component with the worst reputation for custom implementations, and the strongest case for starting from a headless library.

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