Further understanding of js closures
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The concept of closure has been confusing to me since I started learning JS a few months ago
I understood it before, but then I didn’t use it for a long time Just forgot
Closure: In layman’s terms, what most people accept is that a function has the right to use local variables in another function
I see a lot of differences
Use the simplest code to express
<span style="font-size: 14px;">function out(){<br/><br/>var age=21;<br/><br/>function inner(){<br/> <br/> console.log(age);<br/><br/>}<br/><br/>return inner;<br/><br/>}<br/><br/>var fn=out();<br/> fn(); //22</span>Very consistent with the concept
I think closure is reflecting the scope
The inner function is defined inside the out function
So console(age);
will use the variable search mechanism. First, search within the scope of your own (inner) function. If not found, go to Find
in the out function scope and then output. If not found in out, it will look for
in a larger scope. Until the scope of the window, the lower-level scope can be accessed upwards, but the upper-level scope cannot be accessed downwards
Scope refers to
{ }
And JS does not have block-level scope
for(var i=0;i<5;i){
console.log(i);// 1 2 3 4 5
}
cosole.log(i );//5
i will not be destroyed just because the for loop is out.
This should be noted
Okay, I talked about a little bit about scope. Now back to closures
The core of closures is return. Just look at the code and you will know.
My understanding is that return returns the function body of inner and the scope that inner can access!
So inner can be accessed anywhere age
Example:
<span style="font-size: 14px;">function test(){<br/> var age=23;<br/> var fn=out();<br/> fn(); //21<br/> <br/> }<br/> <br/> test();//21</span>It gets 21 instead of 22 because the function body and scope are returned together Then the nearest scope is the out function scope.
Even if age is defined in the test function, it cannot be overwritten because the existing scopes are different
It returns the scope, so it accesses all the variables in that scope and has nothing to do with the scope where your function is now.
Closure is actually a This phenomenon is that everyone who plays DNF is painting pictures and selling materials to make money. This phenomenon is called moving bricks
To sum up: it has to do with the scope of the function you define, and the role of the function you execute. Domain-independent
Contrary to this, this has nothing to do with definition time and is related to execution time. Compare memory
So if you don’t understand it well Closure
Then you can understand it like me
What is returned is the function itself and the scope that the function can access
Give one commonly used one
Closure Tab bar switching
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<script type="text/javascript"> <br/><span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);"> $(name){ <br/> <br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">return</span><span style="font-size: 12px; color: rgb(0, 0, 0);"> document.querySelectorAll(name);
} <br/> <br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> list=$(".box ul li"<span style="font-size: 12px; color: rgb(0, 0, 0);">); <br/><br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> len=<span style="font-size: 12px; color: rgb(0, 0, 0);">list.length; <br/> </span><span style="font-size: 12px; color: rgb(0, 0, 255);">for</span>(<span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> i=0;i<len;i++<span style="font-size: 12px; color: rgb(0, 0, 0);">){
list[i].onmouseover</span>=(<span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);">(n){
</span><span style="font-size: 12px; color: rgb(0, 0, 255);">return</span> <span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);">(){ <br/><br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">for</span>(<span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> j=0;j<len;j++<span style="font-size: 12px; color: rgb(0, 0, 0);">){
list[j].className</span>=""<span style="font-size: 12px; color: rgb(0, 0, 0);">;
}
list[n].className</span>="current"<span style="font-size: 12px; color: rgb(0, 0, 0);">;
}
})(i);
}<br/><br/><br/><br/></span></script>
</body>
</html></span>
Whenever the for loop executes list[i].onmouseover, the function will be executed immediately and the current variable i is passed in
Return a function. This forms a closure. Return the function and the scope that the function can access.
Whenever onmouseover is triggered, the returned function will be executed.
Then execute the for loop in the generation function to clear all li's className
This sentence is the most important when executing list[n]. The n here is the i ## passed in when defining onmouseover
# Because the function is executed immediately when it is defined and i is passed to the anonymous function. This i is within the scope of the anonymous function
Each onmouseover saves its own i
, so when triggered Onmouseover allows li to access the i
that was previously saved in the scope, thus realizing the need to change the background color of someone
Related recommendations:Understanding of actual parameters, formal parameters and closures of js functions
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