A brief discussion on the magic of parseInt() in JavaScript

/**
 * @param {string} str
 * @return {number}
 */
var myAtoi = function(str) {
    str = str.trim();
    //Intercept the first character of the string var firstChar = str.charAt(),res = '',regSymbol = /^[\-\+]{1}/,regNumber = /^[0-9]{1}/;

    if(!regSymbol.test(firstChar) && !regNumber.test(firstChar)){
        return 0;
    }

    //The first character is + or -
    if(regSymbol.test(firstChar)){
        var sliceStr = str.slice(1,str.length);
        for(var i=0;i<sliceStr.length;i++){
            if (regNumber.test(sliceStr.charAt(i))) {
                res += sliceStr.charAt(i);
            }else{
                break;
            }
        }
        
        (res.length) ? res = parseInt(firstChar + res) : res = 0;

        if(firstChar == '-' && res < Math.pow(-2,31)){
            res = Math.pow(-2,31);
        }

        if(firstChar == '+' && res > Math.pow(2,31) - 1){
            res = Math.pow(2,31) - 1;
        }

        return res;
    }

    //The first character is a number if (regNumber.test(firstChar)) {
        var sliceStr = str;
        for(var i=0;i<sliceStr.length;i++){
            if (regNumber.test(sliceStr.charAt(i))) {
                res += sliceStr.charAt(i);
            }else{
                break;
            }
        }

        if (parseInt(res) > Math.pow(2,31) - 1) {
            res = Math.pow(2,31) - 1;
        }

        return res;
    }
};

/**
 * @param {string} str
 * @return {number}
 */
var myAtoi = function(str) {
    const number = parseInt(str, 10);

    if(isNaN(number)) {
        return 0;
    } else if (number < Math.pow(-2, 31) || number > Math.pow(2, 31) - 1) {
        return number < Math.pow(-2, 31) ? Math.pow(-2, 31) : Math.pow(2, 31) - 1;
    } else {
        return number;
    }
};