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Text Search

In browsers, we are able to find specific words or phrases within a webpage by using Ctrl + F (Windows, Linux) or ⌘ + F (Mac) and entering the search term. Matches which appear will be highlighted in yellow.

Let's implement a simple version of a browser's in-webpage search with the difference being we're given a string (as opposed to HTML) and search matches appear bolded.

Given a content string and a query string, implement a function textSearch that finds all case-insensitive matches with the query string, wrapping the matches in <b>...</b> tags.


textSearch('The Quick Brown Fox Jumps Over The Lazy Dog', 'fox');
// 'The Quick Brown <b>Fox</b> Jumps Over The Lazy Dog'
textSearch('The hardworking Dog overtakes the lazy dog', 'dog');
// 'The hardworking <b>Dog</b> overtakes the lazy <b>dog</b>'
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A character will not match the same query more than once, with letters appearing earlier taking priority.

textSearch('aaa', 'aa');
// '<b>aa</b>a'
// This is because the second character cannot be used as a match again.
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Consecutive matches should be combined into a single tag.

textSearch('aaaa', 'aa');
// Correct: '<b>aaaa</b>'
// Wrong: '<b>aa</b><b>aa</b>'
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This question evaluates one's ability to manipulate arrays and strings in JavaScript, which is certainly an essential skill for Front End development.

One might think of leveraging regular expressions (regex), via RegExp. Regex is not easy to use here because we need to combine the tags for consecutive matches.

Let's try to think backwards from the desired output: we want to output a string with the substrings that exist in the query string wrapped in <b> tags. Therefore we need to know where exactly to insert the opening <b> tags and closing </b> tags. We can create a boolean array of same length as the string with every value defaulting to false. The value of boldChars[index] indicates whether the character at that index in the original string needs to be bold.

// #1: Basic case.
string: "aaabcaa", query: 'abc'
boldChars: [false, false, true, true, true, false, false]
result: "aa<b>abc</b>aa"

// #2: Multiple matches case.
string: "aaabcaabc", query: 'abc'
boldChars: [false, false, true, true, true, false, true, true, true]
result: "aa<b>abc</b>a<b>abc</b>"

// #3: Consecutive case.
string: "aababcac", query: 'ab'
boldChars: [false, true, true, true, true, false, false, false]
result: "a<b>abab</b>cac"

// #4: partial case
string: "aaa", query: 'aa'
boldChars: [true, true, false]
result: "<b>aa</b>a"
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The beginning of a span of consecutive chunks of true is where we insert the opening tag <b> and the end is where we add a closing </b>.

To identify which characters need to be bold, we do a naive substring match at each character in string for each query. Flipping the boolean value at each matching character's index to true. However, because of the "one character can only match the same query once" condition, we have to increment i to go past the current query when there's a match. Be careful of off-by-one errors here.

Because we separate out the function into two steps: (1) identification of bold characters, (2) rendering of the <b> tags, we will not run into the issue of combining the tags for consecutive matches.

 * @param {string} string
 * @param {string} query
 * @return {string}

export default function textSearch(string, query) {
  if(!string || !query) return string; // edge case

  let result = '';
  const boldChars = Array(string.length).fill(false);

  // 1. identification of bold characters
  for(let i=0; i<string.length;){
      const substr = string.slice(i, i+query.length);
      if(substr.toLowerCase() === query.toLowerCase()){
          //fill with true with same length as query's (from position i until position i+query.length)
          boldChars.fill(true, i, i+query.length);
          i += query.length;  //if found, need jump i to query.length as unit
      }else {

  // 2. rendering of the <b> tags
  for(let i=0; i<string.length; i++){
      const shouldOpenTag = boldChars[i] && !boldChars[i-1];
      const shouldCloseTag = boldChars[i] && !boldChars[i+1];

      let char = string[i];
      if(shouldOpenTag) char = `<b>${char}`;
      if(shouldCloseTag) char = `${char}</b>`;


  return result;
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