JavaScript's understanding Record

1, JavaScript data types are divided into two categories: primitive type and object type;

Primitive types include: numbers, strings, Booleans, nulls, and undefined; (All immutable types, values cannot be modified)

Object types: Ordinary objects and functions;

Functions can use new to create new objects, i.e. constructors; each constructor defines a class of objects-a collection of objects initialized by constructors;

A class can be considered a subtype of an object type, and the classes defined by the JavaScript language are mainly: array of arrays, function functions, date dates

The regular Regexp:var pattern=/s$/is equivalent to var pattern = new RegExp ("S $") and is used to match any string ending with ' s ';

Err Error: Defines an object that represents the error and syntax errors that are run in the program, and is used in Try--catch.

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Examples of common type judgments:

Define variables: var n;

Assign a value-----

Get type: typeof N

' Undefined '---this value is undefined;
' Boolean '---This value is a Boolean value;
' String '---This value is a string;
' Number '---this value is numeric;
' Object '---This value is an object or null;
The value of ' function '---is a function.

Null and functions are also objects, but return values are different;

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2. Numeric type:

1), all numbers are represented by floating-point numbers;

2), JavaScript predefined global Variables nan non-numeric value and infinity positive infinity;

X!=x is true only if X is Nan, and the others are false;

The IsNaN () return value is true only if the parameter is a Nan or a non-numeric value;

Isfinite () returns true only if the parameter is not Nan, infinity, or-infinity;

3), JavaScript floating-point notation is a binary notation, can not accurately represent a number similar to 0.1;

var x = 0.3-0.2; var y = 0.2-0.1;alert (x = = y); False

JavaScript in real running environment: x=0.099 999 999 999 999 98 y = 0.1;

3, string, the use of the regular example:

var text = "test:1,2,3";

var pattern =/\d+/g//Matches all instances that contain one or more numbers;

Pattern.test (text); True match succeeded;

Text.search (pattern); 6 first match success position;

Text.match (pattern); [All-in-all] an array of successful matches;

Text.replace (Pattern, ' # '); [test:#,#,#] Replace

4. Boolean value:

When judging the following values: Undefined,null,0,-0,nan, "" are false;

var y; var x= null; Alert (x = = y);//True y is undefined;

5. Type conversion:

1.number-to-string:

var n = 123456.789;

N.tofixed (2)//123456.78 specifies the number of digits after the decimal point;

N.toexponential (2)//1.23e+5 Specify the exponential counting method, the decimal money is fixed one place;

N.toprecision (10)//123456.7890 Specify a valid number of digits

N.toprecision (2)//1.23e+5 if it is less than the whole number of digits, it is converted to exponential

2.string-to-number:

parseint ()//parse integer;

Parsefloat ()//parse integers and floating-point numbers;

6, Function: function variable declaration in advance

var n = ' global ';

function () {

Console.log (n); Undefined

var n = ' scope ';

Console.log (n); Scope

}

Equivalent to:

var n = ' global ';

function () {

var n;

Console.log (n); Undefined

n = ' scope ';

Console.log (n); Scope

}

JavaScript's understanding Record