Guess indent style used in JS file (i.e. tabs, 2 spaces, 4 spaces)

detect_indent_style.js

var sample = "'use strict';\n\
\n\
/**\n\
 * @param {string} selector\n\
 * @nosideeffects\n\
 * @return {Element|null}\n\
 */\n\
HTMLElement.prototype.up = function(selector) {\n\
    var element = this;\n\
    while (element = element.parentElement) {\n\
        if (element.webkitMatchesSelector(selector)) {\n\
            return element;\n\
        }\n\
    }\n\
    return null;\n\
};";


function detectIndentStyle(input) {
    var r = /^[ |\t]+/gm;
    var indents = [];
    var match;
    var text;

    while (match = r.exec(input)) {

        text = match[0];

        if (text === '\t')
            return '\t';

        if (text === ' ')
            // Who the hell uses 1 space indent?
            continue;

        return text;
    }

    return '';
}


console.log(detectIndentStyle(sample) === '    ');