インプットメソッドエディタ (IME)

What is an IME (Input Method Editor)?

An Input Method Editor (IME) is a software component that allows users to enter characters that cannot be typed directly from a standard keyboard — most importantly the tens of thousands of Chinese, Japanese, and Korean (CJK) characters, but also Indic scripts, complex Arabic ligatures, and emoji. An IME sits between keyboard input and the application, intercepting keystrokes and converting phonetic or structural sequences into the intended characters.

The need for IMEs arises from a fundamental mismatch: keyboards have about 100 keys, but CJK writing systems require access to 2,000–50,000+ characters for everyday use. An IME resolves this by letting users type phonetic representations (pinyin for Mandarin, romaji for Japanese, phonetic hangul for Korean) and then selecting the correct characters from a candidate list.

How IMEs Work

The IME workflow has three phases:

1. Input composition: The user types phonetic or structural input. This text appears in a composition string (often underlined in the text field) — it is tentative, not yet committed to the document.

2. Candidate selection: The IME presents a list of candidate characters or words that match the phonetic input. The user navigates this list with arrow keys, Tab, or number keys.

3. Commit: The user presses Enter or Space to commit the selected candidate, replacing the composition string with the final character(s).

CJK IME Types

OS-Level IME Integration

Every major OS provides IME infrastructure:

• Windows: Text Services Framework (TSF), Windows IME API. Built-in IMEs for Simplified Chinese, Japanese, Korean.
• macOS: Input Method Kit framework. System IMEs for Chinese (Pinyin, Cangjie), Japanese, Korean, Stroke.
• Linux: Input method frameworks: IBus (most common), Fcitx, SCIM. Popular IMEs: ibus-pinyin, ibus-mozc (Japanese), ibus-hangul.
• iOS/Android: System keyboard IMEs with intelligent word prediction, gesture input (swipe), and handwriting recognition.

IME Composition Events in Web Development

Web applications must handle IME composition events to avoid processing intermediate input:

let isComposing = false;

input.addEventListener('compositionstart', () => {
  isComposing = true;
});

input.addEventListener('compositionend', (e) => {
  isComposing = false;
  // Now safe to process the committed text
  handleInput(e.data);
});

input.addEventListener('input', (e) => {
  if (!isComposing) {
    handleInput(e.target.value);
  }
  // If composing, skip — text is still tentative
});

Failing to handle composition events causes common bugs: searching or submitting forms with incomplete phonetic input, breaking character-by-character validation, etc.

Quick Facts