You don't know. JavaScript notes (4)

Type:

There are 7 built-in types of JavaScript

Null value (NULL)

Not defined (undefined)

Boolean value (Boolean)

Digital (number)

string (string)

Object (object)

symbols (symbol)

other than objects, collectively referred to as " basic types "

To view the type of a value using the TypeOf operator

typeof undefined = = = "undefined"; True

typeof true = = = "Boolean"; True

typeof = = = "Number"; True

typeof "a" = = = = "string"; True

typeof {Life:42} = = = "Object"; True

typeof symbol () = = = "Symbol"; True

typeof null = = = "Object"//True

we need to use compound conditions to detect the type of a null value

var a = null;

(!a && typeof a = = = "Object"); True

Null is a false value is the only basic type that will return "Object" with typeof detection

A function is a subtype of an object, a function is a callable object, and he has an internal property [Call] This property enables it to be called.

The length property of a function object is the number of arguments it declares:

function A (a,c) {

}

A.length; 2

An array is also a subtype of an object whose length is a number of elements.

typeof [N/a] = = = = "Object"; True

Values and Types

the variables in the javascrpit are of no type, only the values, and the variables can hold any type of value at any time.

The typeof operator always returns a string:

typeof typeof 42; "String"

typeof First returns the string "number" and then typeof " number" back to "string"

Undefined and undeclared

A variable is undefined when it has been declared but not assigned a value .

variables that are not declared in the scope are undeclared .

but often these two situations will be undefined, the browser handles the problem.

Global variables can be passed through window. to access.

Array:

Creating sparse arrays with both blank or empty arrays

var a = [];

A[0] = 1;

a[1] unit not set here

A[2] = [3];

A[1]; Undefined

A.length; 3

it can be indexed by numbers, or it can contain string key values and attributes. ( recommended numeric index )

var a = [];

A[0] = 1;

a["Foobar"] = 2;

A.length; 1

a["Foobar"]; 2

A.foobar; 2

Class Array

tool function slice (...) is often used for type conversions

function foo () {

var arr = Array.prototype.slice.call (arguments);

Arr.push ("Bam");

Console.log (arr);

}

Foo ("Bar", "Baz"); ["Bar", "Baz", "Bam"]

ES6 built-in Tools array.from (...) can also achieve the same function.

var arr = Array.from (argments);

String

To handle a string by borrowing the non-changing method of an array

var a = "Foo";

A.join; Undefined

A.map; Undefined

var C = Array.prototype.join.call (A, "-");

var d = Array.prototype.map.call (A,function (v) {

return v.touppercase () + ".";

}). Join ("");

C "F-o-o"

D "F.O.O"

The string is converted to an array before the result is processed back to the string.

var a = "Foo";

var C = A

Converts the value of a to a character array

. Split ("")

reverses the characters in the array

. Reverse ()

stitching the characters in an array back into a string

. Join ("");

C "Oof"

Digital

JavaScript has only one numeric type: number ( numeric );

The syntax of a number

Numeric constants are generally expressed in decimal notation

var a = 42;

var b = 42.3;

the 0 preceding the number can be ignored:

var a = 0.42;

var b =. 42;

the last 0 of the decimal part after the decimal point can be ignored.

var a = 42.0;

var B =;(. It is not recommended to write )

a particularly large or very small number is shown by default in exponential format , with the same result as the toexponential () function output.

var a = 5E10;

A

A.toexponential (); "5e+10"

var B = A * A;

b 2.5e+21

var c = 1/a;

C 2e-11

specify the number of digits to display in decimals

var a = 42.59;

a.tofixed (0); "43"

A.tofixed (1); "43.6"

A.tofixed (2); "43.59"

A.tofixed (3); "43.590"

Toprecision (..) method is used to specify the number of displays for a valid digit

var a = 42.59;

A.toprecision (1); "4e+1"

A.toprecision (2); "43"

A.toprecision (3); "42.6"

A.toprecision (4); "42.59"

A.toprecision (5); "42.590"

(for numeric variables, also for numeric constants )

A smaller number

0.1 + 0.2 = = 0.3; False

can use Number.epsilon to compare whether two numbers are equal (within the specified error range)

Security range for integers

The maximum number is 2^53-1, which is 9007199254740991 number.max_safe_integer

the minimum integer is -9007199254740991 number.min_safe_integer

Integer detection

to detect whether a value is an integer, ES6 Number.isinteger (...)

Number.isinteger (42); True

Number.isinteger (42.3); False

three-bit signed integer

Although integers can reach the maximum number of digits, some numeric operations are only used in numbers .

Special values

Value that is not a value

the undefined type has only one value, which is undefined

The null type also has only one value, which is null

Undefined

void operator

Undefined is a built-in identifier that has a value of undefined, through void

operation to get this value.

Special numbers

Numbers that are not numbers

The operand of a mathematical operation is not a numeric type, and a valid number cannot be returned, which

the value returned in the case is NaN.

A number that is not a number and is still a numeric type.

The execution of the math operation did not succeed, which is the result of a failed return.

Infinite number

Positive Infinity number Infinity

negative Infinity number -infinity

Special equations

ES6 object.is (...) to determine whether the two values are absolutely equal.

var B =-3 * 0;

Object.is (b,-0); True

Object.is (b,0); False

Values and references

There is no syntactic difference between the assignment/passing of values and references in JavaScript.

var a = 2;

var B = A; b is A copy of the value of a.

b++ ;

A 2

b 3

var c = [n/a];

var d = c; D is A reference to [all-in-one ]

D,push (4);

C [1,2,3,4]

D [1,2,3,4]

simple values are assigned, passed, unll,undefined, strings, numbers, Booleans and symbol by value copying

compound values, object functions, are assigned values/passes by reference to replication .

Because the reference points to the value itself rather than the variable, one reference cannot change the other reference to point to.

var a = [1,2,3,4];

var B = A;

A [1,2,3,4]

b [A]

then

b = [n/a];

A [A]

b [4,5,6]

You don't know. JavaScript notes (4)