For those who like Node.JS and its util.format feature, I've just extracted it out into its vanilla JavaScript form (with only functions that util.format uses):

exports = {}; function isString(arg) { return typeof arg === 'string'; } function isNull(arg) { return arg === null; } function isObject(arg) { return typeof arg === 'object' && arg !== null; } function isBoolean(arg) { return typeof arg === 'boolean'; } function isUndefined(arg) { return arg === void 0; } function stylizeNoColor(str, styleType) { return str; } function stylizeWithColor(str, styleType) { var style = inspect.styles[styleType]; if (style) { return '\u001b[' + inspect.colors[style][0] + 'm' + str + '\u001b[' + inspect.colors[style][3] + 'm'; } else { return str; } } function isFunction(arg) { return typeof arg === 'function'; } function isNumber(arg) { return typeof arg === 'number'; } function isSymbol(arg) { return typeof arg === 'symbol'; } function formatPrimitive(ctx, value) { if (isUndefined(value)) return ctx.stylize('undefined', 'undefined'); if (isString(value)) { var simple = '\'' + JSON.stringify(value).replace(/^"|"$/g, '') .replace(/'/g, "\\'") .replace(/\\"/g, '"') + '\''; return ctx.stylize(simple, 'string'); } if (isNumber(value)) { // Format -0 as '-0'. Strict equality won't distinguish 0 from -0, // so instead we use the fact that 1 / -0 < 0 whereas 1 / 0 > 0 . if (value === 0 && 1 / value < 0) return ctx.stylize('-0', 'number'); return ctx.stylize('' + value, 'number'); } if (isBoolean(value)) return ctx.stylize('' + value, 'boolean'); // For some reason typeof null is "object", so special case here. if (isNull(value)) return ctx.stylize('null', 'null'); // es6 symbol primitive if (isSymbol(value)) return ctx.stylize(value.toString(), 'symbol'); } function arrayToHash(array) { var hash = {}; array.forEach(function (val, idx) { hash[val] = true; }); return hash; } function objectToString(o) { return Object.prototype.toString.call(o); } function isDate(d) { return isObject(d) && objectToString(d) === '[object Date]'; } function isError(e) { return isObject(e) && (objectToString(e) === '[object Error]' || e instanceof Error); } function isRegExp(re) { return isObject(re) && objectToString(re) === '[object RegExp]'; } function formatError(value) { return '[' + Error.prototype.toString.call(value) + ']'; } function formatPrimitiveNoColor(ctx, value) { var stylize = ctx.stylize; ctx.stylize = stylizeNoColor; var str = formatPrimitive(ctx, value); ctx.stylize = stylize; return str; } function isArray(ar) { return Array.isArray(ar); } function hasOwnProperty(obj, prop) { return Object.prototype.hasOwnProperty.call(obj, prop); } function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) { var name, str, desc; desc = Object.getOwnPropertyDescriptor(value, key) || {value: value[key]}; if (desc.get) { if (desc.set) { str = ctx.stylize('[Getter/Setter]', 'special'); } else { str = ctx.stylize('[Getter]', 'special'); } } else { if (desc.set) { str = ctx.stylize('[Setter]', 'special'); } } if (!hasOwnProperty(visibleKeys, key)) { name = '[' + key + ']'; } if (!str) { if (ctx.seen.indexOf(desc.value) < 0) { if (isNull(recurseTimes)) { str = formatValue(ctx, desc.value, null); } else { str = formatValue(ctx, desc.value, recurseTimes - 1); } if (str.indexOf('

') > -1) { if (array) { str = str.split('

').map(function (line) { return ' ' + line; }).join('

').substr(2); } else { str = '

' + str.split('

').map(function (line) { return ' ' + line; }).join('

'); } } } else { str = ctx.stylize('[Circular]', 'special'); } } if (isUndefined(name)) { if (array && key.match(/^\d+$/)) { return str; } name = JSON.stringify('' + key); if (name.match(/^"([a-zA-Z_][a-zA-Z_0-9]*)"$/)) { name = name.substr(1, name.length - 2); name = ctx.stylize(name, 'name'); } else { name = name.replace(/'/g, "\\'") .replace(/\\"/g, '"') .replace(/(^"|"$)/g, "'") .replace(/\\\\/g, '\\'); name = ctx.stylize(name, 'string'); } } return name + ': ' + str; } function formatArray(ctx, value, recurseTimes, visibleKeys, keys) { var output = []; for (var i = 0, l = value.length; i < l; ++i) { if (hasOwnProperty(value, String(i))) { output.push(formatProperty(ctx, value, recurseTimes, visibleKeys, String(i), true)); } else { output.push(''); } } keys.forEach(function (key) { if (!key.match(/^\d+$/)) { output.push(formatProperty(ctx, value, recurseTimes, visibleKeys, key, true)); } }); return output; } function reduceToSingleString(output, base, braces) { var length = output.reduce(function (prev, cur) { return prev + cur.replace(/\u001b\[\d\d?m/g, '').length + 1; }, 0); if (length > 60) { return braces[0] + (base === '' ? '' : base + '

') + ' ' + output.join(',

') + ' ' + braces[1]; } return braces[0] + base + ' ' + output.join(', ') + ' ' + braces[1]; } function formatValue(ctx, value, recurseTimes) { // Provide a hook for user-specified inspect functions. // Check that value is an object with an inspect function on it if (ctx.customInspect && value && isFunction(value.inspect) && // Filter out the util module, it's inspect function is special value.inspect !== exports.inspect && // Also filter out any prototype objects using the circular check. !(value.constructor && value.constructor.prototype === value)) { var ret = value.inspect(recurseTimes, ctx); if (!isString(ret)) { ret = formatValue(ctx, ret, recurseTimes); } return ret; } // Primitive types cannot have properties var primitive = formatPrimitive(ctx, value); if (primitive) { return primitive; } // Look up the keys of the object. var keys = Object.keys(value); var visibleKeys = arrayToHash(keys); if (ctx.showHidden) { keys = Object.getOwnPropertyNames(value); } // This could be a boxed primitive (new String(), etc.), check valueOf() // NOTE: Avoid calling `valueOf` on `Date` instance because it will return // a number which, when object has some additional user-stored `keys`, // will be printed out. var formatted; var raw = value; try { // the .valueOf() call can fail for a multitude of reasons if (!isDate(value)) raw = value.valueOf(); } catch (e) { // ignore... } if (isString(raw)) { // for boxed Strings, we have to remove the 0-n indexed entries, // since they just noisey up the output and are redundant keys = keys.filter(function (key) { return !(key >= 0 && key < raw.length); }); } // Some type of object without properties can be shortcutted. if (keys.length === 0) { if (isFunction(value)) { var name = value.name ? ': ' + value.name : ''; return ctx.stylize('[Function' + name + ']', 'special'); } if (isRegExp(value)) { return ctx.stylize(RegExp.prototype.toString.call(value), 'regexp'); } if (isDate(value)) { return ctx.stylize(Date.prototype.toString.call(value), 'date'); } if (isError(value)) { return formatError(value); } // now check the `raw` value to handle boxed primitives if (isString(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); return ctx.stylize('[String: ' + formatted + ']', 'string'); } if (isNumber(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); return ctx.stylize('[Number: ' + formatted + ']', 'number'); } if (isBoolean(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); return ctx.stylize('[Boolean: ' + formatted + ']', 'boolean'); } } var base = '', array = false, braces = ['{', '}']; // Make Array say that they are Array if (isArray(value)) { array = true; braces = ['[', ']']; } // Make functions say that they are functions if (isFunction(value)) { var n = value.name ? ': ' + value.name : ''; base = ' [Function' + n + ']'; } // Make RegExps say that they are RegExps if (isRegExp(value)) { base = ' ' + RegExp.prototype.toString.call(value); } // Make dates with properties first say the date if (isDate(value)) { base = ' ' + Date.prototype.toUTCString.call(value); } // Make error with message first say the error if (isError(value)) { base = ' ' + formatError(value); } // Make boxed primitive Strings look like such if (isString(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); base = ' ' + '[String: ' + formatted + ']'; } // Make boxed primitive Numbers look like such if (isNumber(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); base = ' ' + '[Number: ' + formatted + ']'; } // Make boxed primitive Booleans look like such if (isBoolean(raw)) { formatted = formatPrimitiveNoColor(ctx, raw); base = ' ' + '[Boolean: ' + formatted + ']'; } if (keys.length === 0 && (!array || value.length === 0)) { return braces[0] + base + braces[1]; } if (recurseTimes < 0) { if (isRegExp(value)) { return ctx.stylize(RegExp.prototype.toString.call(value), 'regexp'); } else { return ctx.stylize('[Object]', 'special'); } } ctx.seen.push(value); var output; if (array) { output = formatArray(ctx, value, recurseTimes, visibleKeys, keys); } else { output = keys.map(function (key) { return formatProperty(ctx, value, recurseTimes, visibleKeys, key, array); }); } ctx.seen.pop(); return reduceToSingleString(output, base, braces); } function inspect(obj, opts) { // default options var ctx = { seen: [], stylize: stylizeNoColor }; // legacy... if (arguments.length >= 3) ctx.depth = arguments[2]; if (arguments.length >= 4) ctx.colors = arguments[3]; if (isBoolean(opts)) { // legacy... ctx.showHidden = opts; } else if (opts) { // got an "options" object exports._extend(ctx, opts); } // set default options if (isUndefined(ctx.showHidden)) ctx.showHidden = false; if (isUndefined(ctx.depth)) ctx.depth = 2; if (isUndefined(ctx.colors)) ctx.colors = false; if (isUndefined(ctx.customInspect)) ctx.customInspect = true; if (ctx.colors) ctx.stylize = stylizeWithColor; return formatValue(ctx, obj, ctx.depth); } exports.inspect = inspect; // http://en.wikipedia.org/wiki/ANSI_escape_code#graphics inspect.colors = { 'bold': [1, 22], 'italic': [3, 23], 'underline': [4, 24], 'inverse': [7, 27], 'white': [37, 39], 'grey': [90, 39], 'black': [30, 39], 'blue': [34, 39], 'cyan': [36, 39], 'green': [32, 39], 'magenta': [35, 39], 'red': [31, 39], 'yellow': [33, 39] }; // Don't use 'blue' not visible on cmd.exe inspect.styles = { 'special': 'cyan', 'number': 'yellow', 'boolean': 'yellow', 'undefined': 'grey', 'null': 'bold', 'string': 'green', 'symbol': 'green', 'date': 'magenta', // "name": intentionally not styling 'regexp': 'red' }; var formatRegExp = /%[sdj%]/g; exports.format = function (f) { if (!isString(f)) { var objects = []; for (var j = 0; j < arguments.length; j++) { objects.push(inspect(arguments[j])); } return objects.join(' '); } var i = 1; var args = arguments; var len = args.length; var str = String(f).replace(formatRegExp, function (x) { if (x === '%%') return '%'; if (i >= len) return x; switch (x) { case '%s': return String(args[i++]); case '%d': return Number(args[i++]); case '%j': try { return JSON.stringify(args[i++]); } catch (_) { return '[Circular]'; } default: return x; } }); for (var x = args[i]; i < len; x = args[++i]) { if (isNull(x) || !isObject(x)) { str += ' ' + x; } else { str += ' ' + inspect(x); } } return str; };

Harvested from: https://github.com/joyent/node/blob/master/lib/util.js