Detailed explanation of js compiled language and interpreted language

This article mainly shares with you the basic knowledge of js-compiled language and interpreted language. I hope it can help everyone.

1. Primitive type and reference type 1. The difference between compiled language and interpreted language

Compiled language: Compile a file first, and the program will automatically execute this document.

Advantages: Fast;

Disadvantages: Not cross-platform.

The server requires strong stability. Linux system is used, and most clients use Windows, which causes cross-platform problems. Compiled files generated by compiled languages ​​cannot be executed on multiple platforms at the same time.

Interpreted language: Compile one sentence and execute one sentence. There is no compilation file. It is equivalent to directly compiling into 1010 machine language and then executing it.

Advantages: cross-platform;

Disadvantages: slightly slow.

Note: Java is neither a compiled language nor an interpreted language in the strict sense. After the file is compiled, the Java virtual machine interprets and executes it, making Java cross-platform.

2. The js engine is single-threaded-----can only do one thing at the same time

Asynchronous--multiple things are executed at the same time; synchronous--wait for one thing Once one thing is done, do another.

Rotation time slice: js seems to be executing two animations at the same time. In fact, js divides the process of the two animations into countless sparse time slices to form a stack. Each time one of them is executed, the content is There is no order of priority for grabbing time slices, the order is random. Then the animation is executed in the order of the stack, and it seems that both of them are moving.

3. Mainstream browsers----shell and kernel

IE----trident; Chrome-----webkit/blink; firefox---Gecko; safari-- --webkit; Opera---presto

4. Basic js knowledge points

a Variable names can be composed of $ _ English numbers, but the first letter can only be $ _ English, and another The name avoids words with special meanings while taking into account semantics.

b Original value: null undefined string number boolean; Reference value: object array function (actually the object type)

Original value---assignment is equivalent to giving a copy and placing it in a new In a variable, if you assign a value to an already assigned variable again, the index relationship between the variable and the original value will actually be cut off in the memory, and a new place in the memory will be opened to index the variable name, and the value will be the new value. . ps. Until the memory prompts that it is full, you clear some things, and then save things again, the original place will be overwritten.

var num = 1;
var num1 = num;
num = 2;
console.log(num,num1); //2,1。。。。但是这个num已经不是原来的num了

Reference value---is equivalent to the index value in the stack being the variable name, the value being the address where the real value is stored in the heap, the index in the heap being the address, and the value being the really needed value, so When assigning a variable, it is equivalent to assigning the value in the stack (address---heap index) to a new variable, causing both variables to point to the same address at the same time. Then changing the content in this address will cause the two variables to be values ​​are changed. ps If you assign a value to a variable (a new reference value or original value), the other variable will not change. It is equivalent to opening a new space in the heap and giving the address to the variable. The address of the other variable will still remain unchanged.

var arr=[1,2];
var arr1=arr;
arr.push(3);//改变同一个地址的arr的内容，两个变量都会改变
console.log(arr,arr1);//[1,2,3],[1,2,3]
arr=[1];//给arr重新赋值了一个地址，arr1的地址不会发生改变，还是原来的地址
console.log(arr,arr1)//[1],[1,2,3]

You can see the picture for details. The original value assignment is to copy a copy to another variable. The reference value is to copy the address to another variable. Modifying the content in this address will cause the values ​​of both variables to change. Reassigning the reference value is equivalent to re-opening a piece of content on the stack and then giving a new address. No It affects another element, and the original memory location is actually still occupied, but it is changed back to the default index and cannot be found.

c : 1/0----Infinity (Number type) 0/0---NaN (Number type)

d : ++ a executes a+1 before the current statement, and a++ executes a+1 after the current statement is executed. That is, (++a) is equal to a, which is equal to (a+1), (a++) is equal to the original value of a, a=a+1

The above content is a summary of the video study and personal practice understanding. If the infringement is unintentional, please notify me to make changes.

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