What is NaN?

Not a Number.

It is the returned value when Math functions fail (Math.sqrt(-1)) or when a function trying to parse a number fails (parseInt("blabla")).

What is undefined?

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.

You often test vars against undefined, rather than null.

What is null?

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values.

eval(string)

isFinite(testValue);

isNaN();

parseFloat(string);

A floating point number parsed from the given string. If the first character cannot be converted to a number, NaN is returned.

parseInt(string, radix);

An integer number parsed from the given string. If the first character cannot be converted to a number, NaN is returned. Most implementations default to base 10. strings prefixes with 0x are evaluated as hex. strings prefixed with 0 are evaluated as octal

parseInt('12a') returns 12

decodeURI(encodedURI);

decodeURIComponent(string);

encodeURIComponent()/decodeURIComponent() is for concatenating and splitting apart text strings in URI parts.

Encodes the colon and slash and plus characters, and is meant to be used in query strings. The encoding of + and ? and & is of particular importance here, as these are special chars in query strings.

encodeURI(uri);

Assumes that the URI is a complete URI, so does not encode reserved characters that have special meaning in the URI.

encodeURIComponent(str);

escape(str);

deprecated, along with unescape().

Use encodeURIComponent() or decodeURIComponent()

Object

The Object constructor creates an object wrapper.

// Object initialiser or literal { [ nameValuePair1[, nameValuePair2[, ...nameValuePairN] ] ] } 

// Called as a constructor new Object([value])

Object.assign(target, ...sources)

Copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

Does not do deep cloning. Not supported on IE

Object.create(proto[, propertiesObject)

The Object.create() method creates a new object with the specified prototype object and properties.

Object.defineProperties(obj, props);

Object.defineProperty(obj, prop, descriptor);

This method allows precise addition to or modification of a property on an object. Normal property addition through assignment creates properties which show up during property enumeration (for...in loop or Object.keys method), whose values may be changed, and which may be deleted. This method allows these extra details to be changed from their defaults. By default, values added using Object.defineProperty() are immutable.

Object.freeze(obj);

Nothing can be added to or removed from the properties set of a frozen object. Any attempt to do so will fail, either silently or by throwing a TypeError exception (most commonly, but not exclusively, when in strict mode).

Values cannot be changed for data properties. Accessor properties (getters and setters) work the same (and still give the illusion that you are changing the value). Note that values that are objects can still be modified, unless they are also frozen.

Object.getOwnPropertyDescriptor(obj, prop);

Object.getOwnPropertyDescriptors(obj);

The Object.getOwnPropertyDescriptor() method returns a property descriptor for an own property (that is, one directly present on an object and not in the object's prototype chain) of a given object.

d = Object.getOwnPropertyDescriptor(o, 'bar');
 // d is { configurable: true, enumerable: true, value: 42, writable: true }

Object.getOwnPropertyNames(obj)

Object.getOwnPropertySymbols()

Object.getPrototypeOf(obj)

var proto = {};
var obj = Object.create(proto);
Object.getPrototypeOf(obj) === proto; // true

Object.is(value1, value2);

• both undefined
• both null
• both true or both false
• both strings of the same length with the same characters
• both the same object
• both numbers and =

Object.isExtensible(obj);

Objects are extensible by default: they can have new properties added to them, and (in engines that support __proto__ their __proto__ property) can be modified. An object can be marked as non-extensible using Object.preventExtensions(), Object.seal(), or Object.freeze().

Object.isFrozen(obj);

Object.isSealed(obj);

Returns true if the object is sealed, otherwise false. An object is sealed if it is not extensible and if all its properties are non-configurable and therefore not removable (but not necessarily non-writable).

Object.keys(obj);

Object.preventExtensions(obj);

obj.hasOwnProperty(prop);

Object.getOwnPropertySymbols()


Similar to Object.getOwnPropertyNames(), you can get all symbol properties of a given object as an array of symbols. Note that Object.getOwnPropertyNames() itself does not contain the symbol properties of an object and only the string properties.

As all objects have no own symbol properties initially, Object.getOwnPropertySymbols() returns an empty array unless you have set symbol properties on your object.

Object.getPrototypeOf(obj);

The prototype of the given object. If there are no inherited properties, null is returned.

Function.length

Function.prototype.apply();

func.apply(thisArg, args);

The apply() method calls a function with a given this value and arguments provided as an array (or an array-like object).

Function.prototype.bind()

func.bind(thisArg, arg1, ...);

Function.prototype.call()

func.call(thisArg, arg1, ...);

function.toString()

The Function object overrides the toString method inherited from Object;

For Function objects, the toString method returns a string representation of the object in the form of a function declaration. That is, toString decompiles the function, and the string returned includes the function keyword, the argument list, curly braces, and the source of the function body.

Error

new Error([message[, fileName[, lineNumber]]])

Number

The Number JavaScript object is a wrapper object allowing you to work with numerical values. A Number object is created using the Number() constructor.

• If the argument cannot be converted into a number, it returns NaN.
• In a non-constructor context (i.e., without the new operator), Number can be used to perform a type conversion.

What arg is passed to the contructor of a Promise?

An 'Executor' function. The Executor is passed the resolve and reject methods.

The Executor is execute *immediately*, and normally calls an asynchronus task.

The Executor is responsible for calling resolve() or reject() to fulfill the promise.

What is __proto__ ?

let car = { engine: 'v8' };

let vehicle = { model: 'VW' }

vehicle.__proto__ = car;

What is vehicle.engine ?

What is vehicle.car ?

vehicle.engine is 'V8', accessed through __proto__

vehicle.car is undefined. 'car' is the proto (like a base class), not a property.

let car = { engine: 'v8' };

let vehicle = { model: 'VW' }

vehicle.__proto__ = car;

console.log(vehicle.__proto__);

What is the output of console.log() ?

{ engine: 'v8' }

__proto__ is the actual object that it points to. But the properties of __proto__ can be accessed as if they are properities of the object.

var alien = { kind: 'alien' };
var zippy = {};
zippy.__proto__ = alien;
zippy.kind = 'zippy';
console.log(zippy.kind);
console.log(alien.kind);

What are the values of zippy.kind and alien.kind

zippy.kind === 'zippy'

alien.kind === 'alien'

The assignmentaddeda .kind property to zippy.

It has NO effect on the value of the __proto__

let machine = {parts: ['one'], capabilities: {}};

let robot = {}; let vehicle = {};

robot.__proto__ = machine;

vehicle.__proto__ = machine;

robot.parts.push('two')

What is .parts on each object?

machine.parts === ['one', 'two'];

robot.parts === ['two'];

vehicle.parts === ['one', 'two'];

The assignment to robot overrides the connection to the parts array in the machine __proto__, AND yet adds the object on the array.

What's the function of the prototype?

Why not use inheritance?

let protoObj = { sharedProperty: 'any' }

let obj = Object.create(protoObj)

obj.sharedProperty === 'any'

The prototype allows you to have many objects sharing fuctionality. One set of functions in memory can be used by any number of objects.

Second, you can dynamically add functionality to multiple objects which share a prototype.

TRUE

It's a prototype chain. Each prototype in the chain qualifies for the object