How to highlight matches within a string with JSX?

Question

I have a custom autocomplete, so when you type, it will display a list of suggestions based on the input value. In the list, I would like to bold the characters that are th

Answer 1

Writing your own highlighting code could lead down a rabbit hole. In my answer, I assume only simple text (no HTML within the strings, no charset edge cases) and valid non-escaped RegExp pattern string.

---

Instead of building a new string, you could build a new array, in which you could put JSX. React can render an array of nodes directly.

The logic behind

As a simple proof of concept, here's the logic we could use:

const defaultHighlight = s => {s};

// Needed if the target includes ambiguous characters that are valid regex operators.
const escapeRegex = v => v.replace(/[\-\[\]{}()*+?.,\\\^$|#\s]/g, "\\$&");

/**
 * Case insensitive highlight which keeps the source casing.
 * @param {string} source text
 * @param {string} target to highlight within the source text
 * @param {Function} callback to define how to highlight the text
 * @returns {Array}
 */
const highlightWord = (source, target, callback) => {
  const res = [];

  if (!source) return res;
  if (!target) return source;
  
  const regex = new RegExp(escapeRegex(target), 'gi');

  let lastOffset = 0;
  
  // Uses replace callback, but not its return value
  source.replace(regex, (val, offset) => {
    // Push both the last part of the string, and the new part with the highlight
    res.push(
      source.substr(lastOffset, offset - lastOffset),
      // Replace the string with JSX or anything.
      (callback || defaultHighlight)(val)
    );
    lastOffset = offset + val.length;
  });
  
  // Push the last non-highlighted string
  res.push(source.substr(lastOffset));
  return res;
};

/**
 * React component that wraps our `highlightWord` util.
 */
const Highlight = ({ source, target, children }) => 
  highlightWord(source, target, children);


const TEXT = 'This is a test.';

const Example = () => (
  

    
Nothing: ""
    
No target: ""
    
Default 'test': ""
    
Multiple custom with 't': 
      "
        {s => {s}}
      "
    
    
Ambiguous target '.': 
      "
        {s => {s}}
      "
    
  
);


// Render it
ReactDOM.render(
  ,
  document.getElementById("react")
);

.highlight {
  background-color: yellow;
}

No need to use dangerouslySetInnerHTML here.

This highlightWord function can take any function to wrap the matched string.

highlight(match, value) // default to `s => {s}`
// or
highlight(match, value, s => {s});

I'm doing minimal regex string escaping based on another answer on Stack Overflow.

---

The Highlight component

As shown, we can create a component so it's "more react"!

/**
 * React component that wraps our `highlightWord` util.
 */
const Highlight = ({ source, target, children }) => 
  highlightWord(source, target, children);

Highlight.propTypes = {
  source: PropTypes.string,
  target: PropTypes.string,
  children: PropTypes.func,
};

Highlight.defaultProps = {
  source: null,
  target: null,
  children: null,
};

export default Highlight;

It uses a render prop, so you'd have to change your rendering to:

  {matches.map((match, idx) => (
    

      
        {s => {s}}
      
    
  ))}