How to use 'js max string length' in JavaScript

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function maxSubstringLength(currentMax, obj) {
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    var ret;
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    if (typeof (obj) === 'string') {
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        ret = Math.max(currentMax, maxSubstringLengthStr(obj));
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    }
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    else if (typeof (obj) === 'object') {
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        ret = Object.keys(obj)
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            .map(function (k) {
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            return Math.max(maxSubstringLengthStr(k), maxSubstringLength(currentMax, obj[k]));
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        })
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            .reduce(function (p, c) { return Math.max(p, c); }, currentMax);
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    }
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    else {
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        ret = currentMax;
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    }
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    return ret;
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}

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function maxChar(str) {
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    const charMap = {};
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    let result = '';
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    for (let i = 0; i < str.length; i++) {
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        const char = str[i];
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        charMap[char] = (charMap[char] || 0) + 1;
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        if (result === '' || charMap[char] > charMap[result]) {
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            result = char;
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        }
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    }
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    return result;
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}