javascript - 请帮我理解这个 JavaScript 片段中的 "while"循环

Question

我见过这样的片段，用于使用条件注释在 JavaScript 中检测 IE。

var ie = (function(){

    var undef, v = 3, div = document.createElement('div');

    // the while loop is used without an associated block: {}
    // so, only the condition within the () is executed.

    // semicolons arent allowed within the condition,
    //   so a comma is used to stand in for one
    // basically allowing the two separate statements 
    //   to be evaluated sequentially.

    while (
        div.innerHTML = '<!--[if gt IE '+(++v)+']><i></i><![endif]-->',
        div.getElementsByTagName('i')[0]
    );

    // each time it's evaluated, v gets incremented and
    //   tossed into the DOM as a conditional comment
    // the i element is then a child of the div.

    // the return value of the getEBTN call is used as 
    //   the final condition expression
    // if there is an i element (the IE conditional
    //   succeeded), then getEBTN's return is truthy
    // and the loop continues until there is no 
    //   more i elements.

    // In other words:  ** MAGIC**

    return v > 4 ? v : undef;

}());

上面给出的是documented (and slightly improved) version by Paul Irish在 snippet by James Padolsey 上.我发布评论版本是为了让您知道我可能需要更简单的解释。

我真的很想知道while 循环内部发生了什么。我不明白。

Answer 1

(假设我没有把它弄得一团糟)while 循环等同于以下内容:

var elt;

do
{
    v++;
    div.innerHTML = '<!--[if gt IE ' + v + ']><i></i><![endif]-->'
    elt = div.getElementsByTagName('i')[0];
} (while elt);

---

does mdc or any good ones cover this while(stmt1, stmt2) thing.

这是 MDC 对 while 的评价:

while (condition)
    statement

condition
An expression evaluated before each pass through the loop. If this condition evaluates to true, statement is executed. When condition evaluates to false, execution continues with the statement after the while loop.

我们可以准确地找出 expression 是什么 是来自 MDC 的 JavaScript:

An expression is any valid set of literals, variables, operators, and expressions that evaluates to a single value; the value can be a number, a string, or a logical value.

Conceptually, there are two types of expressions: those that assign a value to a variable, and those that simply have a value. For example, the expression x = 7 is an expression that assigns x the value seven. This expression itself evaluates to seven. Such expressions use assignment operators. On the other hand, the expression 3 + 4 simply evaluates to seven; it does not perform an assignment. The operators used in such expressions are referred to simply as operators.

如果您够勇敢，还可以查看 ECMA-262 language specification ，特别是以下部分:

• 11 表达式，尤其是 11.14 逗号运算符 ( , )
• 12.6.2 while 语句

_{抱歉，我无法提供直接链接，因为它都在 PDF 中。}