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Properties of Objects and Their Prototypes
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index.md

Properties of Objects and Their Prototypes

In JavaScript, there is a bit of a mixture between utility methods and enums existing as inherited prototype properties as opposed to utility methods and enums that only exist on the built-in object itself.

For example, Date is a constructable object, but constructed Date instances that inherit the built-in Date prototype object's properties will not have the now property. The now property only exists on (and is only callable from) the global Date object.

const aNewDate = new Date()

Date.now() // fine
aNewDate.now() // TypeError

Listing Properties of Objects and Their Prototypes

If we construct a new object from a built-in object, we won't be able to directly see the properties that the object inherits from the original built-in object's prototype. For example:

// create a new array, which should inherit properties from the Array prototype
const arr = [1,2,3]

// if we try to log the properties of arr, we don't see the prototype properties
console.log(Object.getOwnPropertyNames(arr))
// output: [ '0', '1', '2', 'length' ]

// this, of course, works, even though the reverse() property doesn't show up when
// calling Object.getOwnPropertyNames(arr)
arr.reverse() // -> [3,2,1]

This usually isn't an issue since we generally know the type of an object (or we can use typeof to get this information), and we can infer what utility properties are available on the object. But, we can print an object's own properties as well as its prototype properties side by side, anyway:

const arr = [1,2,3]

console.log({
  props: Object.getOwnPropertyNames(arr),
  prototypeProps: Object.getOwnPropertyNames(Object.getPrototypeOf(arr)),
})

/* output:
{ props: [ '0', '1', '2', 'length' ],
  prototypeProps: 
   [ 'length',
     'constructor',
     'concat',
     'copyWithin',
     'fill',
     'find',
     'findIndex',
     'lastIndexOf',
     'pop',
     'push',
     'reverse',
     'shift',
     'unshift',
     'slice',
     'sort',
     'splice',
     'includes',
     'indexOf',
     'join',
     'keys',
     'entries',
     'values',
     'forEach',
     'filter',
     'flat',
     'flatMap',
     'map',
     'every',
     'some',
     'reduce',
     'reduceRight',
     'toLocaleString',
     'toString' ] }
*/

Common Built-in Objects and Their Prototypes

Below is the logged output of using Object.getOwnPropertyNames() on a variety of common JavaScript built-ins as well as their prototypes as they exist in an ESNext 2020 context. We can see that some properties exist solely on the built-in object itself, while others will be available on the object's prototype and will, by association, be available on any instances of that object prototype.

Built-ins logged:

  • Array
  • Boolean
  • Date
  • Error
  • Function
  • JSON
  • Math
  • Number
  • Object
  • Promise
  • RegExp
  • String
  • Symbol
// common built-in objects and their prototypes
console.log({
  ArrayProperties: Object.getOwnPropertyNames(Array),
  ArrayPrototypeProperties: Object.getOwnPropertyNames(Array.prototype),

  BooleanProperties: Object.getOwnPropertyNames(Boolean),
  BooleanPrototypeProperties: Object.getOwnPropertyNames(Boolean.prototype),

  DateProperties: Object.getOwnPropertyNames(Date),
  DatePrototypeProperties: Object.getOwnPropertyNames(Date.prototype),

  ErrorProperties: Object.getOwnPropertyNames(Error),
  ErrorPrototypeProperties: Object.getOwnPropertyNames(Error.prototype),

  FunctionProperties: Object.getOwnPropertyNames(Function),
  FunctionPrototypeProperties: Object.getOwnPropertyNames(Function.prototype),

  // this built-in object doesn't have a prototype and is therefore not constructable
  JSONProperties: Object.getOwnPropertyNames(JSON),

  // this built-in object doesn't have a prototype and is therefore not constructable
  MathProperties: Object.getOwnPropertyNames(Math),

  NumberProperties: Object.getOwnPropertyNames(Number),
  NumberPrototypeProperties: Object.getOwnPropertyNames(Number.prototype),

  ObjectProperties: Object.getOwnPropertyNames(Object),
  ObjectPrototypeProperties: Object.getOwnPropertyNames(Object.prototype),

  PromiseProperties: Object.getOwnPropertyNames(Promise),
  PromisePrototypeProperties: Object.getOwnPropertyNames(Promise.prototype),

  RegExpProperties: Object.getOwnPropertyNames(RegExp),
  RegExpPrototypeProperties: Object.getOwnPropertyNames(RegExp.prototype),

  StringProperties: Object.getOwnPropertyNames(String),
  StringPrototypeProperties: Object.getOwnPropertyNames(String.prototype),

  SymbolProperties: Object.getOwnPropertyNames(Symbol),
  SymbolPrototypeProperties: Object.getOwnPropertyNames(Symbol.prototype),
})

Output:

{ ArrayProperties: [ 'length', 'name', 'prototype', 'isArray', 'from', 'of' ],
  ArrayPrototypeProperties: 
   [ 'length',
     'constructor',
     'concat',
     'copyWithin',
     'fill',
     'find',
     'findIndex',
     'lastIndexOf',
     'pop',
     'push',
     'reverse',
     'shift',
     'unshift',
     'slice',
     'sort',
     'splice',
     'includes',
     'indexOf',
     'join',
     'keys',
     'entries',
     'values',
     'forEach',
     'filter',
     'flat',
     'flatMap',
     'map',
     'every',
     'some',
     'reduce',
     'reduceRight',
     'toLocaleString',
     'toString' ],
  BooleanProperties: [ 'length', 'name', 'prototype' ],
  BooleanPrototypeProperties: [ 'constructor', 'toString', 'valueOf' ],
  DateProperties: [ 'length', 'name', 'prototype', 'now', 'parse', 'UTC' ],
  DatePrototypeProperties: 
   [ 'constructor',
     'toString',
     'toDateString',
     'toTimeString',
     'toISOString',
     'toUTCString',
     'toGMTString',
     'getDate',
     'setDate',
     'getDay',
     'getFullYear',
     'setFullYear',
     'getHours',
     'setHours',
     'getMilliseconds',
     'setMilliseconds',
     'getMinutes',
     'setMinutes',
     'getMonth',
     'setMonth',
     'getSeconds',
     'setSeconds',
     'getTime',
     'setTime',
     'getTimezoneOffset',
     'getUTCDate',
     'setUTCDate',
     'getUTCDay',
     'getUTCFullYear',
     'setUTCFullYear',
     'getUTCHours',
     'setUTCHours',
     'getUTCMilliseconds',
     'setUTCMilliseconds',
     'getUTCMinutes',
     'setUTCMinutes',
     'getUTCMonth',
     'setUTCMonth',
     'getUTCSeconds',
     'setUTCSeconds',
     'valueOf',
     'getYear',
     'setYear',
     'toJSON',
     'toLocaleString',
     'toLocaleDateString',
     'toLocaleTimeString' ],
  ErrorProperties: 
   [ 'length',
     'name',
     'prototype',
     'captureStackTrace',
     'stackTraceLimit' ],
  ErrorPrototypeProperties: [ 'constructor', 'name', 'message', 'toString' ],
  FunctionProperties: [ 'length', 'name', 'prototype' ],
  FunctionPrototypeProperties: 
   [ 'length',
     'name',
     'arguments',
     'caller',
     'constructor',
     'apply',
     'bind',
     'call',
     'toString' ],
  JSONProperties: [ 'parse', 'stringify' ],
  MathProperties: 
   [ 'abs',
     'acos',
     'acosh',
     'asin',
     'asinh',
     'atan',
     'atanh',
     'atan2',
     'ceil',
     'cbrt',
     'expm1',
     'clz32',
     'cos',
     'cosh',
     'exp',
     'floor',
     'fround',
     'hypot',
     'imul',
     'log',
     'log1p',
     'log2',
     'log10',
     'max',
     'min',
     'pow',
     'random',
     'round',
     'sign',
     'sin',
     'sinh',
     'sqrt',
     'tan',
     'tanh',
     'trunc',
     'E',
     'LN10',
     'LN2',
     'LOG10E',
     'LOG2E',
     'PI',
     'SQRT1_2',
     'SQRT2' ],
  NumberProperties: 
   [ 'length',
     'name',
     'prototype',
     'isFinite',
     'isInteger',
     'isNaN',
     'isSafeInteger',
     'parseFloat',
     'parseInt',
     'MAX_VALUE',
     'MIN_VALUE',
     'NaN',
     'NEGATIVE_INFINITY',
     'POSITIVE_INFINITY',
     'MAX_SAFE_INTEGER',
     'MIN_SAFE_INTEGER',
     'EPSILON' ],
  NumberPrototypeProperties: 
   [ 'constructor',
     'toExponential',
     'toFixed',
     'toPrecision',
     'toString',
     'valueOf',
     'toLocaleString' ],
  ObjectProperties: 
   [ 'length',
     'name',
     'prototype',
     'assign',
     'getOwnPropertyDescriptor',
     'getOwnPropertyDescriptors',
     'getOwnPropertyNames',
     'getOwnPropertySymbols',
     'is',
     'preventExtensions',
     'seal',
     'create',
     'defineProperties',
     'defineProperty',
     'freeze',
     'getPrototypeOf',
     'setPrototypeOf',
     'isExtensible',
     'isFrozen',
     'isSealed',
     'keys',
     'entries',
     'fromEntries',
     'values' ],
  ObjectPrototypeProperties: 
   [ 'constructor',
     '__defineGetter__',
     '__defineSetter__',
     'hasOwnProperty',
     '__lookupGetter__',
     '__lookupSetter__',
     'isPrototypeOf',
     'propertyIsEnumerable',
     'toString',
     'valueOf',
     '__proto__',
     'toLocaleString' ],
  PromiseProperties: 
   [ 'length',
     'name',
     'prototype',
     'all',
     'race',
     'resolve',
     'reject',
     'allSettled',
     'any' ],
  PromisePrototypeProperties: [ 'constructor', 'then', 'catch', 'finally' ],
  RegExpProperties: 
   [ 'length',
     'name',
     'prototype',
     'input',
     '$_',
     'lastMatch',
     '$&',
     'lastParen',
     '$+',
     'leftContext',
     '$`',
     'rightContext',
     '$\'',
     '$1',
     '$2',
     '$3',
     '$4',
     '$5',
     '$6',
     '$7',
     '$8',
     '$9' ],
  RegExpPrototypeProperties: 
   [ 'constructor',
     'exec',
     'dotAll',
     'flags',
     'global',
     'ignoreCase',
     'multiline',
     'source',
     'sticky',
     'unicode',
     'compile',
     'toString',
     'test' ],
  StringProperties: 
   [ 'length',
     'name',
     'prototype',
     'fromCharCode',
     'fromCodePoint',
     'raw' ],
  StringPrototypeProperties: 
   [ 'length',
     'constructor',
     'anchor',
     'big',
     'blink',
     'bold',
     'charAt',
     'charCodeAt',
     'codePointAt',
     'concat',
     'endsWith',
     'fontcolor',
     'fontsize',
     'fixed',
     'includes',
     'indexOf',
     'italics',
     'lastIndexOf',
     'link',
     'localeCompare',
     'match',
     'matchAll',
     'normalize',
     'padEnd',
     'padStart',
     'repeat',
     'replace',
     'search',
     'slice',
     'small',
     'split',
     'strike',
     'sub',
     'substr',
     'substring',
     'sup',
     'startsWith',
     'toString',
     'trim',
     'trimStart',
     'trimLeft',
     'trimEnd',
     'trimRight',
     'toLocaleLowerCase',
     'toLocaleUpperCase',
     'toLowerCase',
     'toUpperCase',
     'valueOf',
     'replaceAll' ],
  SymbolProperties: 
   [ 'length',
     'name',
     'prototype',
     'for',
     'keyFor',
     'asyncIterator',
     'hasInstance',
     'isConcatSpreadable',
     'iterator',
     'match',
     'matchAll',
     'replace',
     'search',
     'species',
     'split',
     'toPrimitive',
     'toStringTag',
     'unscopables' ],
  SymbolPrototypeProperties: [ 'constructor', 'toString', 'valueOf', 'description' ] }
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