JS实现Base64编码、解码，即window.atob，window.btoa功能

　　window.atob()，window.btoa()方法可以对字符串精选base64编码和解码，但是有些环境比如nuxt的服务端环境没法使用window，所以需要自己实现一个base64的编码解码功能，下面是原生js实现该功能，可以作为一个常用工具使用。

// private property
let _keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

// private method for UTF-8 encoding
function _utf8_encode (string) {
    string = string.replace(/\r\n/g,"\n");
    let utftext = "";
    for (let n = ; n < string.length; n++) {
        let c = string.charCodeAt(n);
        if (c < ) {
            utftext += String.fromCharCode(c);
        } else if((c > ) && (c < )) {
            utftext += String.fromCharCode((c >> ) | );
            utftext += String.fromCharCode((c & ) | );
        } else {
            utftext += String.fromCharCode((c >> ) | );
            utftext += String.fromCharCode(((c >> ) & ) | );
            utftext += String.fromCharCode((c & ) | );
        }

    }
    return utftext;
}

// private method for UTF-8 decoding
function _utf8_decode (utftext) {
    let string = "";
    let i = ;
    let c = ;
    let c1 = ;
    let c2 = ;
    let c3 = ;
    while ( i < utftext.length ) {
        c = utftext.charCodeAt(i);
        if (c < ) {
            string += String.fromCharCode(c);
            i++;
        } else if((c > ) && (c < )) {
            c2 = utftext.charCodeAt(i+);
            string += String.fromCharCode(((c & ) << ) | (c2 & ));
            i += ;
        } else {
            c2 = utftext.charCodeAt(i+);
            c3 = utftext.charCodeAt(i+);
            string += String.fromCharCode(((c & ) << ) | ((c2 & ) << ) | (c3 & ));
            i += ;
        }
    }
    return string;
}

// public method for encoding
export const encode = (input) => {
    let output = "";
    let chr1, chr2, chr3, enc1, enc2, enc3, enc4;
    let i = ;
    input = _utf8_encode(input);
    while (i < input.length) {
        chr1 = input.charCodeAt(i++);
        chr2 = input.charCodeAt(i++);
        chr3 = input.charCodeAt(i++);
        enc1 = chr1 >> ;
        enc2 = ((chr1 & ) << ) | (chr2 >> );
        enc3 = ((chr2 & ) << ) | (chr3 >> );
        enc4 = chr3 & ;
        if (isNaN(chr2)) {
            enc3 = enc4 = ;
        } else if (isNaN(chr3)) {
            enc4 = ;
        }
        output = output +
        _keyStr.charAt(enc1) + _keyStr.charAt(enc2) +
        _keyStr.charAt(enc3) + _keyStr.charAt(enc4);
    }
    return output;
}

// public method for decoding
export const decode = (input) => {
    let output = "";
    let chr1, chr2, chr3;
    let enc1, enc2, enc3, enc4;
    let i = ;
    input = input.replace(/[^A-Za-z0-\+\/\=]/g, "");
    while (i < input.length) {
        enc1 = _keyStr.indexOf(input.charAt(i++));
        enc2 = _keyStr.indexOf(input.charAt(i++));
        enc3 = _keyStr.indexOf(input.charAt(i++));
        enc4 = _keyStr.indexOf(input.charAt(i++));
        chr1 = (enc1 << ) | (enc2 >> );
        chr2 = ((enc2 & ) << ) | (enc3 >> );
        chr3 = ((enc3 & ) << ) | enc4;
        output = output + String.fromCharCode(chr1);
        if (enc3 != ) {
            output = output + String.fromCharCode(chr2);
        }
        if (enc4 != ) {
            output = output + String.fromCharCode(chr3);
        }
    }
    output = _utf8_decode(output);
    return output;
}

　　当然github上还有很多比较好的base64转码解码插件，可以自行查找