What is feature detection?

The short answer

Feature detection checks if a specific feature exists in the browser before using it. Feature inference assumes that if one feature exists, a related feature also exists. UA string detection reads the browser's user agent string to identify the browser. Feature detection is the only reliable approach. The other two lead to bugs.

Feature detection (recommended)

You directly test if a feature is available:

// Check if geolocation is supported
if ('geolocation' in navigator) {
  navigator.geolocation.getCurrentPosition(handlePosition);
} else {
  showManualLocationInput();
}
// Check if localStorage is available
if (typeof localStorage !== 'undefined') {
  localStorage.setItem('key', 'value');
}
// CSS feature detection
if (CSS.supports('display', 'grid')) {
  // use grid layout
}

In CSS, you can use @supports:

@supports (display: grid) {
  .container {
    display: grid;
  }
}

This is reliable because you are testing the actual capability, not guessing.

Feature inference (unreliable)

You assume that if one feature exists, another must too:

// Bad: just because the browser has documentMode
// does not mean it supports a specific feature
if (document.documentMode) {
  // assume this is IE, assume it supports X
}

This is fragile because the assumption can be wrong. Browsers evolve independently: having one feature does not guarantee having another.

UA string detection (fragile)

You read the browser's user agent string to identify the browser:

const isChrome = navigator.userAgent.includes('Chrome');
if (isChrome) {
  // assume Chrome supports feature X
}

Problems with this approach:

• UA strings can be faked or changed by the user
• Many browsers include "Chrome" in their UA string even if they are not Chrome
• New browser versions may drop or add features, making your check outdated

When to use each

• Feature detection: always use this. It is the most reliable.
• Feature inference: avoid. It is based on assumptions.
• UA string: avoid for feature detection. Only use it for analytics (tracking which browsers visit your site).

Why interviewers ask this

This tests if you know how to write code that works across different browsers without making fragile assumptions. Feature detection is a fundamental best practice for cross-browser compatibility.