Number#
Numbers are divided into integers and floating-point numbers, which can be represented using decimal, octal, hexadecimal, and scientific notation.
var a = 1
var b = 1.1
var c = 067
var d = 0xA
var e = 1.23e7
console.log(a,b,c,d,e)
NaN#
Represents a non-number, and any operation involving NaN will return NaN. NaN is not equal to any value (including itself).
BigInt#
BigInt is used to represent integers of arbitrary length.
The number type cannot safely represent integers greater than (i.e., 9007199254740991), or less than .
Warning
bigInt type is not compatible with number type and cannot be directly calculated.
const bigInt = BigInt(1234567890123456789012345678901234567890);
String#
String types must be enclosed in quotes.
let str = "Hello"; // Double quotes
let str2 = 'Single quotes are ok too'; // Single quotes
let phrase = `can embed another ${str}`; // Backticks
Boolean#
Only two values: true and false
Null#
Represents nothing, empty, unknown.
Undefined#
Represents not being assigned a value.
var str;
console.log(str) // undefined
Symbol#
Represents a unique identifier.
let user = {
name: "zhang"
};
var id = Symbol('id');
user[id] = 1
console.log(user, user[id], user.id)
Hidden properties#
Global Symbol.for#
Get the same symbol by using the same description.
let id = Symbol.for("id"); // Create it if it doesn't exist
let idAgain = Symbol.for("id");
console.log( id === idAgain );
Symbol.keyFor#
Get the description by using the symbol.
Warning
Symbol.keyFor only works for symbol created by Symbol.for.
let id = Symbol.for('id')
let name = Symbol('id')
console.log(Symbol.keyFor(id),Symbol.keyFor(name))
- Symbol.hasInstance: Called when other objects use instanceof to determine if they are instances of the object.
- Symbol.isConcatSpeardable: Determines if concat() should be spread.
- Symbol.species: Allows manual setting of the constructor for derived objects.
- Symbol.match: Called when match() is invoked.
- Symbol.search: Called when search() is invoked.
- Symbol.replace: Called when replace() is invoked.
- Symbol.split: Called when split() is invoked.
- Symbol.iterator: Called when iterator methods are invoked.
- Symbol.toPrimitive: Called when an object is converted to a primitive type.
- Symbol.toStringTag: Called when toString() is invoked to customize the output.
- Symbol.unscopables: Excludes properties from being included in a with statement.
Object type#
Stores collections of data and more complex entities.
6 ways to determine types#
typeof#
Can only recognize primitive types and reference types. typeof x and typeof(x) are the same, the parentheses here are not part of typeof. They are parentheses for mathematical grouping.
Note
JavaScript programming language design error, JavaScript converts data to 32-bit storage, the null tag type is the same as object, both are 000 link.
console.log(typeof 1)
console.log(typeof '1')
console.log(typeof undefined)
console.log(typeof true)
console.log(typeof Symbol())
console.log(typeof null)
console.log(typeof [])
console.log(typeof {})
console.log(typeof console)
console.log(typeof console.log)
constructor#
Points to the constructor of the current instance.
let str = 'Covid-19'
console.log(str.constructor)
let number = 123
console.log(number.constructor)
let arr = [1,2,3]
console.log(arr.constructor)
let fun = function(){}
console.log(fun.constructor)
let obj = {}
console.log(obj.constructor)
instanceof#
Checks if it is an instance of the constructor in the prototype chain.
Warning
Primitive types do not have a prototype chain in JavaScript. So the instanceof operator will only return false for primitive types.
let arr = [1,2,3]
console.log(arr instanceof Array)
let fun = function(){}
console.log(fun instanceof Function)
let obj = {}
console.log(obj instanceof Object)
let number = 123;
console.log(number instanceof Number);
let string = '123'
console.log(number instanceof String);
let boolean = true
console.log(number instanceof Boolean);
Object.prototype.toString#
Can determine the data type well, wrap it into a method.
console.log(Object.prototype.toString({}))
console.log(Object.prototype.toString.call({}))
console.log(Object.prototype.toString.call(1))
console.log(Object.prototype.toString.call('1'))
console.log(Object.prototype.toString.call(true))
console.log(Object.prototype.toString.call(function(){}))
console.log(Object.prototype.toString.call(null))
console.log(Object.prototype.toString.call(undefined))
console.log(Object.prototype.toString.call(/123/g))
console.log(Object.prototype.toString.call(new Date()))
console.log(Object.prototype.toString.call([]))
Duck typing#
Determine by checking specific properties of itself.
let str = 'Covid-19'
console.log(str.toLowerCase())
let arr = [1,2,3]
console.log(arr.join(','))
Equality comparison#
console.log(null === null)
console.log(undefined === void 0)