Can pass Test the website and complete the performance test .

One . Use global variables with caution

1. Global variables are defined in the global execution context , It's the top of all the scope chains , Variables that are not found in the local scope will be found in the global variable , So the search

Time consuming .

2. The global execution context always exists in the context execution stack , Until the program exits .

3. If a variable with the same name appears in a certain and local scope, it will obscure or pollute the global scope .

Next, we will analyze it through the code Global variable storage and local variable storage Performance comparison local variable storage is faster

Two . Cache global variables

Cache the global variables that cannot be avoided in use to local , As shown in the figure below, local variable caching is faster than global variable caching

3、 ... and . Adding methods through prototypes

As shown in the figure below, the method of adding a prototype chain is relatively fast

Four . Avoid closure traps

Closure characteristics :

1. The outside has a reference to the inside

2. Access the data of the internal scope in the external scope

As shown in the figure below , Call function foo,foo The function points inside fn The existence of , That is to say, the external refers to the internal reference

When calling foo Function , We are foo The scope can access foo Inside name Variable , That is, the external scope accesses the data of the internal scope

As shown in the figure below, improper use of closures can easily lead to memory leaks

First ,btn The element itself exists in dom nodes , It's just for btn1 This variable is stored inside the function , So onclick Events are the outside pointing to the inside , full

That's the first thing about closures ,onclick When an event is called , The scope is a separate scope , Follow foo Scope is not a scope , It uses foo do

Use the el, So satisfy the second of closures . There's a problem with this code , According to our previous GC The reference mechanism of garbage collection ,btn This dom Object is referenced 2 Time ,

Once in html in , Once in foo In variables , If we take this dom Node slave html Delete in , That's one less reference , But in foo There's another internal reference

It won't go away , That's why garbage can't be recycled , That's what we call a memory leak .

The solution is as follows , When the operation is finished, put btn1 Set as null

5、 ... and . Avoid using property access methods

according to JS Characteristics of , All the properties and methods inside the constructor are exposed to the outside , Try to avoid using methods to return properties inside constructors , Instead, we should visit them directly

attribute , As shown in the figure below , Obviously Direct access to properties is faster .

6、 ... and .for Cycle optimization

As shown in the figure below ,

7、 ... and . Choose the best cycle method

As shown in the figure below , If we just traverse the data of the array ,foreach Best performance , The second is for, And finally for in

8、 ... and . Document fragment optimization node add

As shown in the figure below , Adding document fragment nodes is faster

Nine . Clone optimizes node operations

Cloning is mainly about adding Dom node , It can be based on the fact that similar Dom node , replicate , So as to reduce the consumption of performance, as shown in the figure below , clone DOM Node ratio

establish DOM Nodes should be faster .

Ten . Direct substitution new object

As shown in the figure below : It's better to define arrays directly with variables than with new Object The way to create arrays is obviously faster

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