How can you determine if a css class exists with Javascript?

This should be possible to do using the document.styleSheets[].rules[].selectorText and document.styleSheets[].imports[].rules[].selectorText properties. Refer to MDN documentation.

Here is my solution to this. I'm essentially just looping through document.styleSheets[].rules[].selectorText as @helen suggested.

/**
 * This function searches for the existence of a specified CSS selector in a given stylesheet.
 *
 * @param (string) styleSheetName - This is the name of the stylesheet you'd like to search
 * @param (string) selector - This is the name of the selector you'd like to find
 * @return (bool) - Returns true if the selector is found, false if it's not found
 * @example - console.log(selectorInStyleSheet ('myStyleSheet.css', '.myClass'));
 */    

function selectorInStyleSheet(styleSheetName, selector) {
    /*
     * Get the index of 'styleSheetName' from the document.styleSheets object
     */
    for (var i = 0; i < document.styleSheets.length; i++) {
        var thisStyleSheet = document.styleSheets[i].href ? document.styleSheets[i].href.replace(/^.*[\\\/]/, '') : '';
        if (thisStyleSheet == styleSheetName) { var idx = i; break; }
    }
    if (!idx) return false; // We can't find the specified stylesheet

    /*
     * Check the stylesheet for the specified selector
     */
    var styleSheet = document.styleSheets[idx];
    var cssRules = styleSheet.rules ? styleSheet.rules : styleSheet.cssRules;
    for (var i = 0; i < cssRules.length; ++i) {
        if(cssRules[i].selectorText == selector) return true;
    }
    return false;
}

This function offers a speed improvement over other solutions in that we are only searching the stylesheet passed to the function. The other solutions loop through all the stylesheets which is in many cases unnecessary.

function getAllSelectors() { 
    var ret = [];
    for(var i = 0; i < document.styleSheets.length; i++) {
        var rules = document.styleSheets[i].rules || document.styleSheets[i].cssRules;
        for(var x in rules) {
            if(typeof rules[x].selectorText == 'string') ret.push(rules[x].selectorText);
        }
    }
    return ret;
}


function selectorExists(selector) { 
    var selectors = getAllSelectors();
    for(var i = 0; i < selectors.length; i++) {
        if(selectors[i] == selector) return true;
    }
    return false;
}

Based on the answer, I created a javascript function for searching for a CSS class in the browser's memory -

var searchForCss = function (searchClassName) {
  for (let i = 0; i < document.styleSheets.length; i++) {
    let styleSheet = document.styleSheets[i];
    try {
      for (let j = 0; j < styleSheet.cssRules.length; j++) {
        let rule = styleSheet.cssRules[j];
        // console.log(rule.selectorText)
        if (rule.selectorText && rule.selectorText.includes(searchClassName)) {
          console.log('found - ', rule.selectorText, ' ', i, '-', j);
        }
      }
      if (styleSheet.imports) {
        for (let k = 0; k < styleSheet.imports.length; k++) {
          let imp = styleSheet.imports[k];
          for (let l = 0; l < imp.cssRules.length; l++) {
            let rule = imp.cssRules[l];
            if (
              rule.selectorText &&
              rule.selectorText.includes(searchClassName)
            ) {
              console.log('found - ',rule.selectorText,' ',i,'-',k,'-',l);
            }
          }
        }
      }
    } catch (err) {}
  }
};

searchForCss('my-class-name');

This will print a line for each occurrence of the class name in any of the rules in any of the stylesheets.

Ref - Search for a CSS class in the browser memory