Content scripts can be declared in manifest.json to be always injected into pages that match a set of URL patterns.

Minimal example

"content_scripts" : [
  {
    "js": ["content.js"],
    "css": ["content.css"]
    "matches": ["http://example.com/*"]
  }
]

This manifest entry instructs Chrome to inject a content script content.js, along with the CSS file content.css, after any navigation to a page matching the match pattern http://example.com/*

Both js and css keys are optional: you can have only one of them or both depending on what you need.

content_scripts key is an array, and you can declare several content script definitions:

"content_scripts" : [
  {
    "js": ["content.js"],
    "matches": ["http://*.example.com/*"]
  },
  {
    "js": ["something_else.js"],
    "matches": ["http://*.example.org/*"]
  }
]

Note that both js and matches are arrays, even if you only have one entry.

More options are available in the official documentation and other Examples.

Important note

Content scripts declared in the manifest will only be injected on new navigations after the extension load. They will not be injected in existing tabs. This also applies to extension reloads while developing, and extension updates after release.

If you need to ensure that currently opened tabs are covered, consider also doing programmatic injection on startup.

If, instead of always having a content script injected based on the URL, you want to directly control when a content script is injected, you can use Programmatic Injection.

Minimal example

• JavaScript

  chrome.tabs.executeScript({file: "content.js"});
  

• CSS

  chrome.tabs.insertCSS({file: "content.css"});
  

Called from an extension page (e.g. background or popup), and assuming you have permission to inject, this will execute content.js or insert content.css as a content script in the top frame of the current tab.

Inline code

You can execute inline code instead of a file as a content script:

var code = "console.log('This code will execute as a content script');";
chrome.tabs.executeScript({code: code});

Choosing the tab

You can provide a tab ID (usually from other chrome.tabs methods or messaging) to execute in a tab other than the currently active.

chrome.tabs.executeScript({
  tabId: tabId,
  file: "content.js"
});

More options are available in the chrome.tabs.executeScript() documentation and in other Examples.

Permissions

Using chrome.tabs.executeScript() does not require "tabs" permission, but requires host permissions for the page's URL.

Checking for errors

If script injection fails, one can catch it in the optional callback:

chrome.tabs.executeScript({file: "content.js"}, function() {
  if(chrome.runtime.lastError) {
    console.error("Script injection failed: " + chrome.runtime.lastError.message);
  }
});

Same conditions, multiple scripts

If you need to inject multiple files with all other conditions being the same, for example to include a library, you can list all of them in the "js" array:

"content_scripts" : [
  {
    "js": ["library.js", "content.js"],
    "matches": ["http://*.example.com/*"]
  }
]

Order matters: library.js will be executed before content.js.

Same scripts, multiple sites

If you need to inject the same files into multiple sites, you can provide multiple match patterns:

"matches": ["http://example.com/*", "http://example.org/*"]

If you need to inject in basically every page, you can use broad match patterns such as "*://*/*" (matches every HTTP(S) page) or "<all_urls>" (matches every supported page).

Different scripts or different sites

"content_scripts" section is an array as well, so one can define more than one content script block:

"content_scripts" : [
  {
    "js": ["content.js"],
    "matches": ["http://*.example.com/*"]
  },
  {
    "js": ["something_else.js"],
    "matches": ["http://*.example.org/*"]
  }
]