Insight into heap and stack stored in new string() /string() in JS

stay javascript There are two ways to store variables in memory , Value type and Reference type ;

Value type ： character string （string）、 The number （number）、 Boolean value （boolean）、undefined、null

(ECMAScript 2016 Added a basic data type ：symbol )
The main features are as follows ：

• . The footprint is fixed , Save it in the stack
• What is saved and copied is the value itself
• Use typeof Type of detection data

Reference type ： object （Object）、 Array （Array）、 function （Function）

• The occupied space is not fixed , Keep it in the pile
• Saving and copying is a pointer to an object
• instanceof Test data type
• new() Method is a reference type

To explore the String() and new String() The difference of

There are three ways to define a string ;

var csdn1 = 'csdn';
var csdn2 = String('csdn');
var csdn3 = new String('csdn');

And judge whether the values and types of three variables are equal , Here we use === Congruence

console.log(csdn1 === csdn2); //true
console.log(csdn1 === csdn3); //false
console.log(csdn2 === csdn3); //false

Mainly because csdn1 And csdn2 Value type , and csdn3 Is a reference type

console.log(typeof csdn1); // string
console.log(typeof csdn2); // string
console.log(typeof csdn3); //object

Values of basic types can call the corresponding length Method , But I can't define myself ;

console.log(csdn1.length); // 4
csdn1.say = 'hello';
console.log(csdn1.say); // undefined
 
console.log(csdn3.lengh); // 4
csdn3.say = 'hello';
console.log(csdn3.say); // hello

Because of the csdn1 Value type ( Source material type ) , and csdn3 Is a reference type ( Packaging object ), adopt new String() created string Instance object of , So has the

csdn3 instanceof String // true

but csdn1 And csdn2 Value type , Nature does not belong to String The method in

csdn1 instanceof String // false
csdn2 instanceof String // false

however csdn1,csdn2 But have String Properties and methods in

console.log(csdn1.indexOf === String.prototype.indexOf); // true
console.log(csdn2.indexOf === String.prototype.indexOf); // true
console.log(csdn3.indexOf === String.prototype.indexOf); // true

Value type ( Source material type ) Accessible reference types ( Packaging object ) All methods and properties in , But they don't have
That's why csdn1 And csdn2( Value type ) You can visit length And indexof, Is itself stored on the stack , Therefore, it does not have the characteristics of setting properties and methods ; and csdn3( Reference object ) Is the storage heap pointer , Can be used to add properties and methods ;

Whenever a string is accessed in read mode , Three steps that will be performed in the background ：

1. Create a String Instance of type
2. Call a specific method on the instance
3. Consumption instance

Think of these three steps as performing the following 3 That's ok ECMAScript Code for ：

let s1 = new String("some text");
let s2 = s1.substring(2);
s1 = null;

This behavior allows the original value to have the behavior of the object ;

additional ：

.toString() and String() Can convert data into strings ,undefined and null No, toString() Method , You have to use String() Method returns these two Literal text of the value .

toString() Can receive a base parameter , That is to What is the base number to output the string representation of the value . The default is decimal . By passing in the parameter Count , You can get the binary value 、 octal 、 Hexadecimal , Or anything else String representation of the valid cardinality , such as ：

let num = 10; console.log(num.toString()); // 10
console.log(num.toString(2));   // 1010
console.log(num.toString(8));   // 12
console.log(num.toString(10));  // 10
console.log(num.toString(16));  // a

If you are not sure whether a value is null or undefined , You can make use String() Transformation function , It always returns a string representing the value of the corresponding type ;

let value1 = 10;
let value2 = true;
let value3 = null;
let value4; 
 
console.log(String(value1));  // 10
console.log(String(value2));  // true
console.log(String(value3));  // null
console.log(String(value4));  // undefined