为什么我无法读取未定义的属性“toString”

Question

I use this package. I've added these console.log 's at the beginning of the slug function.

function slug(string, opts) {
    console.log('log 1: -------');
    console.log('log 2: ' + string);
    console.log('log 3: ' + typeof string);
    string = string.toString();
    ....
 }

Here is the output:

log 1: -------
log 2: Yüzey Aktif Maddeler
log 3: string
/Users/------/seeder/node_modules/mongodb/lib/utils.js:99
    process.nextTick(function() { throw err; });
                                  ^

TypeError: Cannot read property 'toString' of undefined
    at slug (/Users/------/seeder/node_modules/slug/slug.js:16:20)
    at Object.createSlug (/Users/------/seeder/seeder/utils.js:4:9)
    at getContent (/Users/------/seeder/seeder/content-seeder.js:345:22)
    at topic.contents.forEach.content (/Users/------/seeder/seeder/content-seeder.js:425:31)
    at Array.forEach (native)
    at /Users/------/seeder/seeder/content-seeder.js:424:21
    at /Users/------/seeder/node_modules/mongodb/lib/collection.js:523:5
    at /Users/------/seeder/node_modules/mongodb/lib/collection.js:701:5
    at handleCallback (/Users/------/seeder/node_modules/mongodb/lib/utils.js:96:56)
    at executeCommands (/Users/------/seeder/node_modules/mongodb/lib/bulk/ordered.js:405:12)

Even if type of string parameter is a string, string.toString() throws a TypeError. It says string is undefined.

I'm not an experienced js developer. Why would this happen?

Edit: I call it from this function

function getContent(content, categoryId) {
    console.log(content.topicVideo);
    return {
        // _id: Random.id(),
        // other properties
        contentSlug: slug(content.topicVideo.contentName, {
            lower: true
        })
    };
}

New logs:

{ id: '197505065',
  categorySort: 18,
  categoryName: 'Hayatımızda Kimya',
  contentSort: 2,
  contentName: 'Yüzey Aktif Maddeler',
  fileName: 'KonuAnlatimi.mp4' }

Edit:

I've added a log at the end of the slug function.

    console.log(result);
    return result;
};

This prints the results and then throws error.

log 1: -------
log 2: Yüzey Aktif Maddeler
log 3: string
log 4: Yüzey Aktif Maddeler
yuzey-aktif-maddeler

Edit: I think this line gives a hint:

/Users/------/seeder/node_modules/mongodb/lib/utils.js:99
    process.nextTick(function() { throw err; });

I think it logs correct string, but fails on undefined one without logging the logs. Is this possible? I've found out that one of content.topicVideo.contentName is actually undefined. But error is thrown without logging it. Is this even possible?

Answer 1

To make sure that .toString() doesn't throw an error, you could:

string = "" + string; // amends the value to an string, even if its undefined or null etc

instead of:

string = string.toString();

However you could also check if the value of string is undefined or null. Also see here .

Answer 2

Maybe you should check if string has a falsy value and(optionally if it is not a string)

function slug(string, opts) {
    console.log('log 1: -------');
    console.log('log 2: ' + string);
    console.log('log 3: ' + typeof string);
    if(string)
        string = string.toString();
    ....
 }

From MDN:

Falsy

A falsy value is a value that translates to false when evaluated in a Boolean context.

More about falsy values in JavaScript here

Answer 3

The following code works fine (added slug.js module into the script to test). Perhaps this will help you analyze your code?

 (function (root) { // lazy require symbols table var _symbols, removelist; function symbols(code) { if (_symbols) return _symbols[code]; _symbols = require('unicode/category/So'); removelist = ['sign','cross','of','symbol','staff','hand','black','white'] .map(function (word) {return new RegExp(word, 'gi')}); return _symbols[code]; } function slug(string, opts) { string = string.toString(); if ('string' === typeof opts) opts = {replacement:opts}; opts = opts || {}; opts.mode = opts.mode || slug.defaults.mode; var defaults = slug.defaults.modes[opts.mode]; var keys = ['replacement','multicharmap','charmap','remove','lower']; for (var key, i = 0, l = keys.length; i < l; i++) { key = keys[i]; opts[key] = (key in opts) ? opts[key] : defaults[key]; } if ('undefined' === typeof opts.symbols) opts.symbols = defaults.symbols; var lengths = []; for (var key in opts.multicharmap) { if (!opts.multicharmap.hasOwnProperty(key)) continue; var len = key.length; if (lengths.indexOf(len) === -1) lengths.push(len); } var code, unicode, result = ""; for (var char, i = 0, l = string.length; i < l; i++) { char = string[i]; if (!lengths.some(function (len) { var str = string.substr(i, len); if (opts.multicharmap[str]) { i += len - 1; char = opts.multicharmap[str]; return true; } else return false; })) { if (opts.charmap[char]) { char = opts.charmap[char]; code = char.charCodeAt(0); } else { code = string.charCodeAt(i); } if (opts.symbols && (unicode = symbols(code))) { char = unicode.name.toLowerCase(); for(var j = 0, rl = removelist.length; j < rl; j++) { char = char.replace(removelist[j], ''); } char = char.replace(/^\\s+|\\s+$/g, ''); } } char = char.replace(/[^\\w\\s\\-\\.\\_~]/g, ''); // allowed if (opts.remove) char = char.replace(opts.remove, ''); // add flavour result += char; } result = result.replace(/^\\s+|\\s+$/g, ''); // trim leading/trailing spaces result = result.replace(/[-\\s]+/g, opts.replacement); // convert spaces result = result.replace(opts.replacement+"$",''); // remove trailing separator if (opts.lower) result = result.toLowerCase(); return result; }; slug.defaults = { mode: 'pretty', }; slug.multicharmap = slug.defaults.multicharmap = { '<3': 'love', '&&': 'and', '||': 'or', 'w/': 'with', }; // https://code.djangoproject.com/browser/django/trunk/django/contrib/admin/media/js/urlify.js slug.charmap = slug.defaults.charmap = { // latin 'À': 'A', 'Á': 'A', 'Â': 'A', 'Ã': 'A', 'Ä': 'A', 'Å': 'A', 'Æ': 'AE', 'Ç': 'C', 'È': 'E', 'É': 'E', 'Ê': 'E', 'Ë': 'E', 'Ì': 'I', 'Í': 'I', 'Î': 'I', 'Ï': 'I', 'Ð': 'D', 'Ñ': 'N', 'Ò': 'O', 'Ó': 'O', 'Ô': 'O', 'Õ': 'O', 'Ö': 'O', 'Ő': 'O', 'Ø': 'O', 'Ù': 'U', 'Ú': 'U', 'Û': 'U', 'Ü': 'U', 'Ű': 'U', 'Ý': 'Y', 'Þ': 'TH', 'ß': 'ss', 'à':'a', 'á':'a', 'â': 'a', 'ã': 'a', 'ä': 'a', 'å': 'a', 'æ': 'ae', 'ç': 'c', 'è': 'e', 'é': 'e', 'ê': 'e', 'ë': 'e', 'ì': 'i', 'í': 'i', 'î': 'i', 'ï': 'i', 'ð': 'd', 'ñ': 'n', 'ò': 'o', 'ó': 'o', 'ô': 'o', 'õ': 'o', 'ö': 'o', 'ő': 'o', 'ø': 'o', 'ù': 'u', 'ú': 'u', 'û': 'u', 'ü': 'u', 'ű': 'u', 'ý': 'y', 'þ': 'th', 'ÿ': 'y', 'ẞ': 'SS', // greek 'α':'a', 'β':'b', 'γ':'g', 'δ':'d', 'ε':'e', 'ζ':'z', 'η':'h', 'θ':'8', 'ι':'i', 'κ':'k', 'λ':'l', 'μ':'m', 'ν':'n', 'ξ':'3', 'ο':'o', 'π':'p', 'ρ':'r', 'σ':'s', 'τ':'t', 'υ':'y', 'φ':'f', 'χ':'x', 'ψ':'ps', 'ω':'w', 'ά':'a', 'έ':'e', 'ί':'i', 'ό':'o', 'ύ':'y', 'ή':'h', 'ώ':'w', 'ς':'s', 'ϊ':'i', 'ΰ':'y', 'ϋ':'y', 'ΐ':'i', 'Α':'A', 'Β':'B', 'Γ':'G', 'Δ':'D', 'Ε':'E', 'Ζ':'Z', 'Η':'H', 'Θ':'8', 'Ι':'I', 'Κ':'K', 'Λ':'L', 'Μ':'M', 'Ν':'N', 'Ξ':'3', 'Ο':'O', 'Π':'P', 'Ρ':'R', 'Σ':'S', 'Τ':'T', 'Υ':'Y', 'Φ':'F', 'Χ':'X', 'Ψ':'PS', 'Ω':'W', 'Ά':'A', 'Έ':'E', 'Ί':'I', 'Ό':'O', 'Ύ':'Y', 'Ή':'H', 'Ώ':'W', 'Ϊ':'I', 'Ϋ':'Y', // turkish 'ş':'s', 'Ş':'S', 'ı':'i', 'İ':'I', 'ğ':'g', 'Ğ':'G', // russian 'а':'a', 'б':'b', 'в':'v', 'г':'g', 'д':'d', 'е':'e', 'ё':'yo', 'ж':'zh', 'з':'z', 'и':'i', 'й':'j', 'к':'k', 'л':'l', 'м':'m', 'н':'n', 'о':'o', 'п':'p', 'р':'r', 'с':'s', 'т':'t', 'у':'u', 'ф':'f', 'х':'h', 'ц':'c', 'ч':'ch', 'ш':'sh', 'щ':'sh', 'ъ':'u', 'ы':'y', 'ь':'', 'э':'e', 'ю':'yu', 'я':'ya', 'А':'A', 'Б':'B', 'В':'V', 'Г':'G', 'Д':'D', 'Е':'E', 'Ё':'Yo', 'Ж':'Zh', 'З':'Z', 'И':'I', 'Й':'J', 'К':'K', 'Л':'L', 'М':'M', 'Н':'N', 'О':'O', 'П':'P', 'Р':'R', 'С':'S', 'Т':'T', 'У':'U', 'Ф':'F', 'Х':'H', 'Ц':'C', 'Ч':'Ch', 'Ш':'Sh', 'Щ':'Sh', 'Ъ':'U', 'Ы':'Y', 'Ь':'', 'Э':'E', 'Ю':'Yu', 'Я':'Ya', // ukranian 'Є':'Ye', 'І':'I', 'Ї':'Yi', 'Ґ':'G', 'є':'ye', 'і':'i', 'ї':'yi', 'ґ':'g', // czech 'č':'c', 'ď':'d', 'ě':'e', 'ň': 'n', 'ř':'r', 'š':'s', 'ť':'t', 'ů':'u', 'ž':'z', 'Č':'C', 'Ď':'D', 'Ě':'E', 'Ň': 'N', 'Ř':'R', 'Š':'S', 'Ť':'T', 'Ů':'U', 'Ž':'Z', // polish 'ą':'a', 'ć':'c', 'ę':'e', 'ł':'l', 'ń':'n', 'ś':'s', 'ź':'z', 'ż':'z', 'Ą':'A', 'Ć':'C', 'Ę':'E', 'Ł':'L', 'Ń':'N', 'Ś':'S', 'Ź':'Z', 'Ż':'Z', // latvian 'ā':'a', 'ē':'e', 'ģ':'g', 'ī':'i', 'ķ':'k', 'ļ':'l', 'ņ':'n', 'ū':'u', 'Ā':'A', 'Ē':'E', 'Ģ':'G', 'Ī':'I', 'Ķ':'K', 'Ļ':'L', 'Ņ':'N', 'Ū':'U', // lithuanian 'ė':'e', 'į':'i', 'ų':'u', 'Ė': 'E', 'Į': 'I', 'Ų':'U', // romanian 'ț':'t', 'Ț':'T', 'ţ':'t', 'Ţ':'T', 'ș':'s', 'Ș':'S', 'ă':'a', 'Ă':'A', // vietnamese 'Ạ': 'A', 'Ả': 'A', 'Ầ': 'A', 'Ấ': 'A', 'Ậ': 'A', 'Ẩ': 'A', 'Ẫ': 'A', 'Ằ': 'A', 'Ắ': 'A', 'Ặ': 'A', 'Ẳ': 'A', 'Ẵ': 'A', 'Ẹ': 'E', 'Ẻ': 'E', 'Ẽ': 'E', 'Ề': 'E', 'Ế': 'E', 'Ệ': 'E', 'Ể': 'E', 'Ễ': 'E', 'Ị': 'I', 'Ỉ': 'I', 'Ĩ': 'I', 'Ọ': 'O', 'Ỏ': 'O', 'Ồ': 'O', 'Ố': 'O', 'Ộ': 'O', 'Ổ': 'O', 'Ỗ': 'O', 'Ơ': 'O', 'Ờ': 'O', 'Ớ': 'O', 'Ợ': 'O', 'Ở': 'O', 'Ỡ': 'O', 'Ụ': 'U', 'Ủ': 'U', 'Ũ': 'U', 'Ư': 'U', 'Ừ': 'U', 'Ứ': 'U', 'Ự': 'U', 'Ử': 'U', 'Ữ': 'U', 'Ỳ': 'Y', 'Ỵ': 'Y', 'Ỷ': 'Y', 'Ỹ': 'Y', 'Đ': 'D', 'ạ': 'a', 'ả': 'a', 'ầ': 'a', 'ấ': 'a', 'ậ': 'a', 'ẩ': 'a', 'ẫ': 'a', 'ằ': 'a', 'ắ': 'a', 'ặ': 'a', 'ẳ': 'a', 'ẵ': 'a', 'ẹ': 'e', 'ẻ': 'e', 'ẽ': 'e', 'ề': 'e', 'ế': 'e', 'ệ': 'e', 'ể': 'e', 'ễ': 'e', 'ị': 'i', 'ỉ': 'i', 'ĩ': 'i', 'ọ': 'o', 'ỏ': 'o', 'ồ': 'o', 'ố': 'o', 'ộ': 'o', 'ổ': 'o', 'ỗ': 'o', 'ơ': 'o', 'ờ': 'o', 'ớ': 'o', 'ợ': 'o', 'ở': 'o', 'ỡ': 'o', 'ụ': 'u', 'ủ': 'u', 'ũ': 'u', 'ư': 'u', 'ừ': 'u', 'ứ': 'u', 'ự': 'u', 'ử': 'u', 'ữ': 'u', 'ỳ': 'y', 'ỵ': 'y', 'ỷ': 'y', 'ỹ': 'y', 'đ': 'd', // currency '€': 'euro', '₢': 'cruzeiro', '₣': 'french franc', '£': 'pound', '₤': 'lira', '₥': 'mill', '₦': 'naira', '₧': 'peseta', '₨': 'rupee', '₩': 'won', '₪': 'new shequel', '₫': 'dong', '₭': 'kip', '₮': 'tugrik', '₯': 'drachma', '₰': 'penny', '₱': 'peso', '₲': 'guarani', '₳': 'austral', '₴': 'hryvnia', '₵': 'cedi', '¢': 'cent', '¥': 'yen', '元': 'yuan', '円': 'yen', '﷼': 'rial', '₠': 'ecu', '¤': 'currency', '฿': 'baht', "$": 'dollar', '₹': 'indian rupee', // symbols '©':'(c)', 'œ': 'oe', 'Œ': 'OE', '∑': 'sum', '®': '(r)', '†': '+', '“': '"', '”': '"', ''': "'", ''': "'", '∂': 'd', 'ƒ': 'f', '™': 'tm', '℠': 'sm', '…': '...', '˚': 'o', 'º': 'o', 'ª': 'a', '•': '*', '∆': 'delta', '∞': 'infinity', '♥': 'love', '&': 'and', '|': 'or', '<': 'less', '>': 'greater', }; slug.defaults.modes = { rfc3986: { replacement: '-', symbols: true, remove: null, lower: true, charmap: slug.defaults.charmap, multicharmap: slug.defaults.multicharmap, }, pretty: { replacement: '-', symbols: true, remove: /[.]/g, lower: false, charmap: slug.defaults.charmap, multicharmap: slug.defaults.multicharmap, }, }; // Be compatible with different module systems if (typeof define !== 'undefined' && define.amd) { // AMD // dont load symbols table in the browser for (var key in slug.defaults.modes) { if (!slug.defaults.modes.hasOwnProperty(key)) continue; slug.defaults.modes[key].symbols = false; } define([], function () {return slug}); } else if (typeof module !== 'undefined' && module.exports) { // CommonJS symbols(); // preload symbols table module.exports = slug; } else { // Script tag // dont load symbols table in the browser for (var key in slug.defaults.modes) { if (!slug.defaults.modes.hasOwnProperty(key)) continue; slug.defaults.modes[key].symbols = false; } root.slug = slug; } }(this)); var content = { 'topicVideo': { 'id': '197505065', 'categorySort': 18, 'categoryName': 'Hayatımızda Kimya', 'contentSort': 2, 'contentName': 'Yüzey Aktif Maddeler', 'fileName': 'KonuAnlatimi.mp4' } } function getContent(content, categoryId) { console.log(content.topicVideo); return { // _id: Random.id(), // other properties contentSlug: slug(content.topicVideo.contentName, undefined) }; } console.log(getContent(content, 1));

Answer 4

You can use.

function slug(string, opts) {
    console.log('log 1: -------');
    console.log('log 2: ' + string);
    console.log('log 3: ' + typeof string);
    string = String(string);
    ....
 }