String.prototype.firstUpperCase = function(){
    return this.replace(/\b(\w)(\w*)/g,function($0,$1,$2){
      return $1.toUpperCase() + $2.toLowerCase();
    })
}
var result = "i'm hello world".firstUpperCase();;
console.log(result); //I'M Hello World

String.prototype.firstUpperCase = function(){
  let arr = this.split(' ');
  let uppserCase = ([first,...rest]) => first.toUpperCase() + rest.join('');
  let result = '';
  arr.forEach((val) => {
    result += uppserCase(val) + ' ';
  })
  return result;
}
//结果：//I'm Hello World 

String.prototype.firstUpperCase = function(){
  let arr = this.split(' ');
  let result = '';
  arr.forEach((val) => {
    result += val.charAt(0).toUpperCase() + val.slice(1) + ' ';
  })
  return result;
}
//结果：//I'm Hello World 

String.prototype.firstUpperCase = function(){
  let arr = this.split(' ');
  let result = '';
  arr.forEach((val) => {
    result += `${val[0].toUpperCase()}${val.slice(1)} `;
  })
  return result;
}
//结果：//I'm Hello World