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JavaScript calculates the number of bytes occupied by a string

巴扎黑

Sep 15, 2017 am 09:19 AM

javascriptstringNumber of bytes

Because a recent project requires using JS to calculate the memory occupied by writing a string of strings into localStorage, this article was created. The following article mainly introduces how to use JS to calculate characters. For relevant information on the number of bytes occupied by a string, friends in need can refer to it.

Preface

Do you often encounter strings at work, but need to control the number of bytes? Because the number of bytes is often controlled in the database, it needs to be judged during the front-end process. Today we will share it.

This article mainly introduces to you the relevant content about JS calculating the number of bytes occupied by a string, and shares it for your reference and study. I won’t say much below, let’s take a look at the detailed introduction.

Sample code

/**
 * 计算字符串所占的内存字节数，默认使用UTF-8的编码方式计算，也可制定为UTF-16
 * UTF-8 是一种可变长度的 Unicode 编码格式，使用一至四个字节为每个字符编码
 * 
 * 000000 - 00007F(128个代码)  0zzzzzzz(00-7F)        一个字节
 * 000080 - 0007FF(1920个代码)  110yyyyy(C0-DF) 10zzzzzz(80-BF)    两个字节
 * 000800 - 00D7FF 
 00E000 - 00FFFF(61440个代码) 1110xxxx(E0-EF) 10yyyyyy 10zzzzzz   三个字节
 * 010000 - 10FFFF(1048576个代码) 11110www(F0-F7) 10xxxxxx 10yyyyyy 10zzzzzz 四个字节
 * 
 * 注: Unicode在范围 D800-DFFF 中不存在任何字符
 * {@link http://zh.wikipedia.org/wiki/UTF-8}
 * 
 * UTF-16 大部分使用两个字节编码，编码超出 65535 的使用四个字节
 * 000000 - 00FFFF 两个字节
 * 010000 - 10FFFF 四个字节
 * 
 * {@link http://zh.wikipedia.org/wiki/UTF-16}
 * @param {String} str 
 * @param {String} charset utf-8, utf-16
 * @return {Number}
 */
  
 var sizeof = function(str, charset){
  var total = 0,
   charCode,
   i,
   len;
  charset = charset ? charset.toLowerCase() : &#39;&#39;;
  if(charset === &#39;utf-16&#39; || charset === &#39;utf16&#39;){
   for(i = 0, len = str.length; i < len; i++){
    charCode = str.charCodeAt(i);
    if(charCode <= 0xffff){
     total += 2;
    }else{
     total += 4;
    }
   }
  }else{
   for(i = 0, len = str.length; i < len; i++){
    charCode = str.charCodeAt(i);
    if(charCode <= 0x007f) {
     total += 1;
    }else if(charCode <= 0x07ff){
     total += 2;
    }else if(charCode <= 0xffff){
     total += 3;
    }else{
     total += 4;
    }
   }
  }
  return total;
 }

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