JavaScript 字符串练习题见这里。

题 1：首字母改大写

function firstLetterToUpperCase(str) {
    var res;
    if (typeof str === 'string') {
        res = str.charAt(0).toUpperCase() + str.substr(1);
    } else {
        res = str;
    }
    return res;
}

题 2：去字符串头尾空格

function trim(str) {
    var res;
    if (typeof str === 'string') {
        res = str.replace(/^\s+/g, '')
            .replace(/\s+$/g, '');
    } else {
        res = str;
    }
    return res;
}

题 3：将字符串中 _ 后面的小写字母变大写，并且删除 _

function toCamelStyle(str) {
    var res;
    if (typeof str === 'string') {
        var isFisrstLetterUnderscore = str.charAt(0) === '_';
        var wordArr;
        if(isFisrstLetterUnderscore){
            str = str.substr(1);
        }
        wordArr = str.split('_');
        wordArr = wordArr.map(function (word, index) {
            // firstLetterToUpperCase 在题目 1 中实现
            return index === 0 ? word : firstLetterToUpperCase(word);
        });
        res = wordArr.join('');
        if(isFisrstLetterUnderscore){
            res = '_' + res;
        }
    } else {
        res = str;
    }
    return res;
}

题 4：删除字符串中所有的数字

function removeNum(str) {
    var res;
    if (typeof str === 'string') {
        res = str.replace(/\d/g, '');
    } else {
        res = str;
    }
    return res;
}

题 5: 反转字符串

function reverse(str) {
    var res;
    if (typeof str === 'string') {
        res = [];
        var charArr = str.split('');
        var currIndex = charArr.length - 1;
        for(; currIndex >= 0; currIndex--){
            res.push(charArr[currIndex]);
        }
        res = res.join('');
    } else {
        res = str;
    }
    return res;
}

题 6: 统计字符串中各字符在字符串中出现的数量

function caculateExistNum(str) {
    var res = false;
    if (typeof str === 'string') {
        res = {};
        var charArr = str.split('');
        charArr.forEach(function (eachChar) {
            res[eachChar] = res[eachChar] ? res[eachChar] + 1 : 1;
        });
    }
    return res;
}

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