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<html><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Keywords" name="Keywords"content="JavaScript, examples, JavaScript: The Definitive Guide"><title>Example 15-9 from JavaScript: The Definitive Guide, Fifth Edition</title><style>body { font-family: arial, sans-serif}.lefttitle { font: bold x-small arial, sans-serif; margin: 5px; letter-spacing: .15em; text-transform:uppercase;}.leftcaption { font: italic x-small sans-serif; margin-bottom: 20px;}#banner { font-family:sans-serif; color:#333; font-size:x-large; font-weight:normal; padding:15px; padding-top: 3px; border-bottom:4px double #666;}#banner h1 { font-size: x-large; margin: 0px; padding: 0px;}#banner .description { font-size:large; margin-left: 2px;}pre { border: solid black 1px; background-color: #ddd; padding: 10px; margin: 0px;} </style></script></head><body><div id="banner"><h1>davidflanagan.com/javascript5</h1><span class="description">Example 15-9 from<i>JavaScript: The Definitive Guide, Fifth Edition</i></span></div><div style="float: left; 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text-align: right"><a href="index.html" tppabs="http://www.davidflanagan.com/javascript5/index.html">Table of Examples</a></div><pre>/* * Log.js: Unobtrusive logging facility * * This module defines a single global symbol: a function named log(). * Log a message by calling this function with 2 or 3 arguments: * * category: the type of the message. This is required so that messages * of different types can be selectively enabled or disabled and so * that they can be styled independently. See below. * * message: the text to be logged. May be empty if an object is supplied * * object: an object to be logged. This argument is optional. If passed, * the properties of the object will be logged in the form of a table. * Any property whose value is itself an object may be logged recursively. * * Utility Functions: * * The log.debug() and log.warn() functions are utilities that simply * call the log() function with hardcoded categories of "debug" and * "warning". It is trivial to define a utility that replaces the built-in * alert() method with one that calls log(). * * Enabling Logging * * Log messages are *not* displayed by default. You can enable the * display of messages in a given category in one of two ways. The * first is to create a <div> or other container element with an id * of "<category>_log". For messages whose category is "debug", you might * place the following in the containing document: * * <div id="debug_log"></div> * * In this case, all messages of the specified category are appended * to this container, which can be styled however you like. * * The second way to enable messages for a given category is to * set an appropriate logging option. To enable the category * "debug", you'd set log.options.debugEnabled = true. When you * do this, a <div class="log"> is created for the logging messages. * If you want to disable the display of log messages, even if a container * with a suitable id exists, set another option: * log.options.debugDisabled=true. Set this option back to false to * reenable log messages of that category. * * Styling Log Messages * * In addition to styling the log container, you can use CSS to * style the display of individual log messages. Each log message * is placed in a <div> tag, and given a CSS class of * <category>_message. Debugging messages would have a class "debug_message" * * Log Options * * Logging behavior can be altered by setting properties of the log.options * object, such as the options described earlier to enable or disable logging * for given categories. A few other options are available: * * log.options.timestamp: If this property is true, each log message * will have the date and time added to it. * * log.options.maxRecursion: An integer that specifies the maximum number * of nested tables to display when logging objects. Set this to 0 if * you never want a table within a table. * * log.options.filter: A function that filters properties out when logging * an object. A filter function is passed the name and value of * a property and returns true if the property should appear in the * object table or false otherwise. */function log(category, message, object) { // If this category is explicitly disabled, do nothing if (log.options[category + "Disabled"]) return; // Find the container var id = category + "_log"; var c = document.getElementById(id); // If there is no container, but logging in this category is enabled, // create the container. if (!c && log.options[category + "Enabled"]) { c = document.createElement("div"); c.id = id; c.className = "log"; document.body.appendChild(c); } // If still no container, we ignore the message if (!c) return; // If timestamping is enabled, add the timestamp if (log.options.timestamp) message = new Date() + ": " + (message?message:""); // Create a <div> element to hold the log entry var entry = document.createElement("div"); entry.className = category + "_message"; if (message) { // Add the message to it entry.appendChild(document.createTextNode(message)); } if (object && typeof object == "object") { entry.appendChild(log.makeTable(object, 0)); } // Finally, add the entry to the logging container c.appendChild(entry);}// Create a table to display the properties of the specified objectlog.makeTable = function(object, level) { // If we've reached maximum recursion, return a Text node instead. if (level > log.options.maxRecursion) return document.createTextNode(object.toString()); // Create the table we'll be returning var table = document.createElement("table"); table.border = 1; // Add a Name|Type|Value header to the table var header = document.createElement("tr"); var headerName = document.createElement("th"); var headerType = document.createElement("th"); var headerValue = document.createElement("th"); headerName.appendChild(document.createTextNode("Name")); headerType.appendChild(document.createTextNode("Type")); headerValue.appendChild(document.createTextNode("Value")); header.appendChild(headerName); header.appendChild(headerType); header.appendChild(headerValue); table.appendChild(header); // Get property names of the object and sort them alphabetically var names = []; for(var name in object) names.push(name); names.sort(); // Now loop through those properties for(var i = 0; i < names.length; i++) { var name, value, type; name = names[i]; try { value = object[name]; type = typeof value; } catch(e) { // This should not happen, but it can in Firefox value = "<unknown value>"; type = "unknown"; }; // Skip this property if it is rejected by a filter if (log.options.filter && !log.options.filter(name, value)) continue; // Never display function source code: it takes up too much room if (type == "function") value = "{/*source code suppressed*/}"; // Create a table row to display property name, type and value var row = document.createElement("tr"); row.vAlign = "top"; var rowName = document.createElement("td"); var rowType = document.createElement("td"); var rowValue = document.createElement("td"); rowName.appendChild(document.createTextNode(name)); rowType.appendChild(document.createTextNode(type)); // For objects, recurse to display them as tables if (type == "object") rowValue.appendChild(log.makeTable(value, level+1)); else rowValue.appendChild(document.createTextNode(value)); // Add the cells to the row, and add the row to the table row.appendChild(rowName); row.appendChild(rowType); row.appendChild(rowValue); table.appendChild(row); } // Finally, return the table. return table;}// Create an empty options objectlog.options = {};// Utility versions of the function with hardcoded categorieslog.debug = function(message, object) { log("debug", message, object); };log.warn = function(message, object) { log("warning", message, object); };// Uncomment the following line to convert alert() dialogs to log messages// function alert(msg) { log("alert", msg); }</pre><div class="lefttitle" style="padding:0px; text-align: right"><a href="index.html" tppabs="http://www.davidflanagan.com/javascript5/index.html">Table of Examples</a></div><p><script type="text/javascript">amazon_ad_tag="davidflanagancom"; amazon_ad_width="300"; amazon_ad_height="250"; amazon_ad_logo="hide";</script><script type="text/javascript" src="../../www.assoc-amazon.com/s/ads.js" tppabs="http://www.assoc-amazon.com/s/ads.js"></script></div></body></html>⌨️ 快捷键说明
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