准确计算字符串的长度

思路：创造一个环境完全一样的容器和文本并计算它的长度.

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function textWidth(text) {   var span = document.createElement('span');   span.style.visibility = 'hidden';   span.style.fontSize = `${parseFloat(     window.getComputedStyle(document.documentElement).fontSize   ) * 0.875}px`;   span.style.fontFamily = window.getComputedStyle(     document.documentElement   ).fontFamily;   span.style.display = 'inline-block';   span.style.fontWeight = window.getComputedStyle(     document.documentElement   ).fontWeight;   document.body.appendChild(span);   if (typeof span.textContent != 'undefined') {     span.textContent = text;   } else {     span.innerText = text;   }     const width = Math.ceil(parseFloat(window.getComputedStyle(span).width));     return width; }<br><br>textWidth(text)