JS Bin// source http://jsbin.com/tewaren

jsbin.tewaren.js

'use strict';

function atoi(str) {
  var sign = 1;
  var base = 0;
  var i = 0;
  var INT_MIN = Number.MIN_VALUE;
  var INT_MAX = Number.MAX_VALUE;
  var strArr = str.split('');

  while (i < strArr.length && strArr[i] === ' ') {
    i++;
  }

  if (i >= strArr.length) {
    return 0;
  }

  if (strArr[i] === '+' || strArr[i] === '-') {
    if (strArr[i] === '-') {
      sign = -1;
    }
    i++;
  }

  while (i < strArr.length && strArr[i] >= '0' && strArr[i] <= '9') {
    if (base > INT_MAX / 10 || base === INT_MAX / 10 && strArr[i] - '0' > 7) {
      if (sign === -1) {
        return INT_MIN;
      } else {
        return INT_MAX;
      }
    }

    base = 10 * base + (strArr[i++] - '0');
  }

  return base * sign;
}

console.log('atoi(-42)', atoi('-42'));
console.log('atoi(4193 with words)', atoi('4193 with words'));