String to Integer Solution

myatoi.js

const WHITESPACE = ' ';
const NEGATIVE_SIGN = '-';
const POSITIVE_SIGN = '+';
const INTEGER_REGEX = /[0-9]/;
const MAX_INTEGER = (2**31) - 1;
const MIN_INTEGER = -(2**31);

function myAtoi(str) {
    const strIntArray = [];
    
    let isNegative = false;
    let resultVal = 0;
    let idx = 0;
    
    while (str[idx] === WHITESPACE) {
        idx++;
    }
    
    if (invalidChar(str[idx])) {
        return 0;
    }
    
    if (isSign(str[idx])) {
        isNegative = str[idx] === NEGATIVE_SIGN;
        idx++;
    }
    
    while (isInteger(str[idx])) {
        strIntArray.unshift(str[idx]);
        idx++;
    }

    for (let i = 0; i < strIntArray.length; i++) {
        let num = Number(strIntArray[i]);
        
        num *= 10**i;
        
        resultVal += num;
    }
    
    if (isNegative) {
        resultVal = -resultVal;
    }

    // If result value is less than min, reassign to min
    resultVal = Math.max(resultVal, MIN_INTEGER);
    
    // If result value is greater than max, reassign to max.
    resultVal = Math.min(resultVal, MAX_INTEGER);

    return resultVal;
}

function invalidChar(char) {
    return !isInteger(char) && !isSign(char);
}

function isSign(char) {
    return char === POSITIVE_SIGN || char === NEGATIVE_SIGN
}

function isInteger(char) {
    return INTEGER_REGEX.test(char);
}