text-transform:capitalize;

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 

作者：风雨后见彩虹
github：https://github.com/hxlmqtily1314