[JS] Assigning a person to a faint javascript variable

Assigning values to variables

This article is reproduced from http://hellobug.github.io/blog/javascript-variable-assignment/

Let's start with a few examples to make sure the starting point is Halo State ~:P

Example 1.1

1234	var a = "apple";var b = a;a = "banana";b

Supposedly, B = A After, A is what value B should be followed by what value ~
However, the b result is a "apple" value at the beginning of the assignment.

Example 1.2

1234	var a = Span class= "p" >{name: "apple" }; var b = a; a. Name = "banana" ; b. Name

This time, B and then a change, the b.name result is"banana"

Example 1.3

12 34

var a = {name: "apple" }; var b = a;< Span class= "line" >a = {name: span class= "s2" > "banana" }; b. Name

This time B insisted on himself, the b.name result is"apple"

Example 1.4

1234	var a = Span class= "p" >{count: 2}; var b = a. Count; a. Count = 3; b

The same b result is the beginning 2 , b in the end is to make what?!

====================== begins to explain the split line ==============================

In fact, B is very innocent, this is from the ECMAScript variable value type to say ~

There are two kinds of types:

• basic type (primitive values) -includes undefined, Null, Boolean, number, and string five basic data types
• reference types (reference values) -objects that are stored in memory cannot be manipulated directly, but can only be manipulated by address references stored in variables

Now take a look at the first example例1.1

1	var a = "apple";

"apple"is a string type, which is the base type, at which point the value is stored:

1	var b = a;

A copy of the value of a is then assigned to B:

So, from A and b squarely, how to modify each other will not affect the other ~

Let's look at a second example.例1.2

1	var a = {name: "apple"};

{name: "apple"}is an object that belongs to the reference type, and the value before and after the assignment is stored like this:

So when it's a.name = "banana"; time to change the property value of the object in memory that everyone points to, the b.name value is changed.

例1.3In

Example 1.3

123456

var a = {name: "apple" }; var b = a;< Span class= "line" >a = {name: span class= "s2" > "banana" }; //{name: "Banana"} is a new object in memory, // The address of the A variable is also pointed to the new object, so it's irrelevant to B. b.//or "apple"

例1.4In

Example 1.4

123456

var a = {count: 2};< Span class= "line" >var b = a.< Span class= "NX" >count; //A.count is the number type, so the value is copied to B,//from then on how to modify the B has nothing to do with a. Count = 3; b //or 2

Small summary, variable assignment always copy a copy, if it is the basic type, copy is the actual value, if it is a reference type, copy is the address value pointing to object, so point to the same object.

Variable comparison

Take a closer look at the comparison of variables ~

Example 2.1

123	var a = "apple";var b = "apple";a == b

This is no problem, the result must be true .

What about this one?

Example 2.2

123	var a = ["apple"];var b = ["apple"];a == b

Although the two numbers are identical, the result is still false .

In fact, the principle is the same, for the basic type, compared to the actual value, and for the reference type (array is an object), compared to the address value , although the two array content is the same, but they are in memory is two object, the address is not the same, So the result of the comparison is false .

Posted by Hellobug Jan 27th, javascript

[JS] Assigning a person to a faint javascript variable