如果我理解正确，则希望将字符串缩短为一定的长度（例如，缩短"The quick brown fox jumps over the lazy dog"为6个字符而不切断任何单词）。

在这种情况下，您可以尝试以下操作：

var yourString = "The quick brown fox jumps over the lazy dog"; //replace with your string.
var maxLength = 6 // maximum number of characters to extract

//Trim and re-trim only when necessary (prevent re-trim when string is shorted than maxLength, it causes last word cut) 
if(yourString.length > trimmedString.length){
    //trim the string to the maximum length
    var trimmedString = yourString.substr(0, maxLength);

    //re-trim if we are in the middle of a word and 
    trimmedString = trimmedString.substr(0, Math.min(trimmedString.length, trimmedString.lastIndexOf(" ")))
}