Question

我正在从“学习jQuery”一书中学习jQuery，并发现部分代码无法理解。

有一部分JSON：

[
    {
        "term": "BACCHUS",
        "part": "n.",
        "definition": "A convenient deity invented by the...",
        "quote": [
            "Is public worship, then, a sin,",
            "That for devotions paid to Bacchus",
            "The lictors dare to run us in,",
            "And resolutely thump and whack us?"
        ],
        "author": "Jorace"
    },
    {
        "term": "BACKBITE",
        "part": "v.t.",
        "definition": "To speak of a man as you find him when..."
    },
    {
        "term": "BEARD",
        "part": "n.",
        "definition": "The hair that is commonly cut off by..."
    },

这是jQuery代码：

$(document).ready(function() {
    $('#letter-b a').click(function() {
        $.getJSON('b.json', function(data) {
            $('#dictionary').empty();
            $.each(data, function(entryIndex, entry) {
                var html = '<div class="entry">';
                html += '<h3 class="term">' + entry['term'] + '</h3>';
                html += '<div class="part">' + entry['part'] + '</div>';
                html += '<div class="definition">';
                html += entry['definition'];
                if (entry['quote']) {
                    html += '<div class="quote">';
                    $.each(entry['quote'], function(lineIndex, line) {
                        html += '<div class="quote-line">' + line + '</div>';

有人能解释一下这条线的含义：

if (entry['quote'])

P.S。我试图搜索stackoverflow和谷歌，但无法找到解释。

Answer 1

您的JSON结构具有可选键quote。

如果密钥存在，entry['key']在布尔上下文（if）中求值为true。如果它不存在，则计算结果为false，并且不执行后续的if-block。

所以，总结一下：

if (entry['quote']) {        // This block will only run if the JSON contains
                             //  a key "quote"
    html += '<div class="quote">';

如果您不确定布尔上下文中变量的值，请使用双重感叹号将其转换为布尔值（双重否定）：

alert("Quote exists? True or false: " + !!entry["quote"]);

Answer 2

所有if（entry ['quote']）正在检查输入结构中是否存在此键。

Answer 3

entry只是$.each引用的匿名函数中的第二个参数。 ['quote']部分是括号表示法，用于引用该特定属性（即键），在这种情况下似乎是

"quote": [
"Is public worship, then, a sin,",
"That for devotions paid to Bacchus",
"The lictors dare to run us in,",
"And resolutely thump and whack us?"]

使用JSON时jQuery中if语句的说明

3 个答案: