Replace all instances of one word with another in a web page

replace_words.js

// ### Replace words in document
//
// Update all instances of `fromWord` to `toWord` within the text
// in the current document.
//
function replaceWordsInDocument(fromWord, toWord) {
  if (/\s/.test(fromWord)) {
    throw new Error('You must enter a single word without whitespace');
  }
  const textNodes = iterAllTextNodes();
  replaceInNodes(fromWord, toWord, textNodes);
}


// ### Iterate all text nodes
//
// Returns a generator that yields all the text nodes within a
// document (excluding those within link, style, and script tags)
//
function* iterAllTextNodes() {
  const ignoreNodes = new Set(['LINK', 'STYLE', 'SCRIPT']);

  for (const elt of document.querySelectorAll('*')) {
    if (!ignoreNodes.has(elt.nodeName)) {

      for (const node of elt.childNodes) {
        if (node.nodeType === 3) {
          yield node;
        }
      }
    }
  }
}


// ### Replace in nodes
//
// Replaces all instances of fromText with toText within
// the textNodes. Matches whole words and expects
// `fromText` to have no whitespaces. Matches are also
// case-insensitve, and replacements attempt to replicate
// the original capitalization found in the node.
//
// -   fromText (string) representing one word with no spaces
// -   toText (string)
// -   textNodes (nodeList of text nodes)
//
function replaceInNodes(fromText, toText, textNodes) {
  const fromLower = fromText.toLowerCase();
  const replacements = {
    [fromLower]: toText.toLowerCase(),
    [fromText.toUpperCase()]: toText.toUpperCase(),
    [capitalize(fromText)]: capitalize(toText)
  }

  for (const node of textNodes) {
    const newText = node.nodeValue.split(/(\s+)/).map((token, i) => {
      if (i % 2 === 0) {
        const tokenLower = token.toLowerCase();
        if (tokenLower === fromLower) {
          return replacements[token] || toText;
        }
      }
      return token;
    }).join('');

    // looks faster to only set if changed https://jsperf.com/node-nodevalue
    if (newText !== node.nodeValue) {
      node.nodeValue = newText;
    }
  }
}


// ### Helpers
//

const capitalize = (text) => text.substring(0, 1).toUpperCase() + text.toLowerCase().substring(1);


// ### Run it
//

replaceWordsInDocument('you', 'y\'all');