How to check that two textStyleRange have same formatting attributes

Here's another approach, depends exactly on what you are looking to do.

/**
 * [matchesCharacterProperties]                 matches textStyleRanges with a sample style
 * @param  {DOM TextStyleRange}                 sampleTextStyleRange    Sample character, textStyleRange
 * @param  {DOM Collection of TextStyleRange}   textStyleRanges         Collection of ranges that uses .everyItem() or itemByRange()
 * @param  {Array}                              properties              [Optional defaults to set list] properties you want to match the sample
 * @return {Array}                                                      Array of matching textStyleRanges
 * By Trevor http://creative-scripts.com 25 Dec 18
 * Not Optimized Uses toSource which will probably be dumped sometime
 */
function matchesCharacterProperties(sampleTextStyleRange, textStyleRanges, properties) {
    if (!properties) {
        properties = [
            'appliedFont',
            'appliedLanguage',
            'appliedCharacterStyle',
            'fillColor',
            'strokeColor',
            'baseline',
            'underline',
            'capitalization'
            // etc. etc. etc.....
        ];
    }
    if (typeof properties !== 'object') {
        properties = [properties];
    }
    var n, propertiesCount, sampleProperties, textStyleRangeProperties, textStyleCount, match, c, matches, textStyleRangeArray, propertiesCache;
    var resolveProperties = function(properties) {
        if (typeof properties !== 'object') {
            properties = [properties];
        }
        return properties.toSource().replace(/^\[/, '').replace(/]$/, '').split(', ');
    };
    propertiesCount = properties.length;
    sampleProperties = [];
    propertiesCache = [];
    matches = [];
    // cache the properties
    for (n = 0; n < propertiesCount; n++) {
        // resolveProperties returns an array and we only want the values hence the [0]
        sampleProperties.push(resolveProperties(sampleTextStyleRange[properties])[0]);
        // For the propertiesCache we want the Array of results for the whole collection
        propertiesCache.push(resolveProperties(textStyleRanges[properties]));
    }
    textStyleRangeArray = textStyleRanges.getElements();
    textStyleCount = textStyleRangeArray.length;
    for (c = 0; c < textStyleCount; c++) {
        match = true;
        for (n = 0; n < propertiesCount; n++) {
            textStyleRangeProperties = propertiesCache;
            if (textStyleRangeProperties !== sampleProperties) {
                match = false;
                break;
            }
        }
        if (match) {
            matches.push(textStyleRangeArray);
            // matches.push(c); // if you just want the indexes use this line instead of the above one
        }
    }
    sampleTextStyleRange = textStyleRanges = properties =
        n = propertiesCount = sampleProperties = propertiesCache =
        textStyleRangeProperties = textStyleCount = match =
        c = textStyleRangeArray = resolveProperties = null;
    return matches;
}
alert(
    matchesCharacterProperties(
        app.selection[0] || app.activeDocument.stories[0].characters[0], // The sample range you want to match
        app.activeDocument.stories[0].textStyleRanges.everyItem(), // the COLLECTION you want to check
        ['appliedFont', 'pointSize', 'fillColor', 'underline'] // [Optional] the properties you want to match
    ).toSource()
);