Analyzing confusing variable assignments in JavaScript_javascript tips

Javascript is a weakly typed language. When declaring a variable, you do not need to declare its type. Var x can be equal to any type of value.
For example:

var str = "string....";
var arr = ["this","is","array"];
var obj = {name :"caizhongqi",age:26,sex:"male"};
These are all correct. It seems very simple and convenient, but this convenience will also bring some elusive surprises. Take a look below Example (Example 1):




You may know at once that the alert comes out as "this is string", yes, but for programmers who use Java language , var y=x should assign the address (pointer) of x in the memory to the y variable, so they think that alerting "ni hao" should be more consistent with the Java language habits, but this is not the case with JavaScript language. Characters String assignment is a direct operation, directly copying the data to the storage space of y.

Look at the following example (Example 2):




If you thought the alert came out as "hello", you are wrong. When var y = x, hasn’t x already given its array to y? But in fact this is not the case. When var y = x, x passes its address (pointer) in the memory! x[0]="world" modifies the data in the original storage location, so alert(y[0]) uses the new value of x to alert. Confused? Why is it measured directly and quoted directly at one time?
Don’t worry, the following example will be more confusing (Example 3):




Your eyes tell you that the alert comes out as "hello"! This is so unpredictable and weird JavaScript!

There is a similar scene in Stephen Chow's "Domestic Zero Paint":
When Mr. Xing just arrived in Hong Kong from Shenzhen on a mission, Anita Yuen found a hair dryer in his luggage. I said it was actually a beard shaver, but when I took out my leather shoes, I saw it was a hair dryer, and what looked like a mobile phone was actually a beard shaver. The shaver and hairdryer confused Anita Yuen and the audience. Hahahaha, this is one of my favorite movies. My stomach hurt when I watched it for the first time.

Looking back at the variable assignment just now, the use of direct quantities and reference quantities is like exchanging a shaver with a hairdryer, which made us all dizzy.
In fact, the problem lies in the second assignment of x = ["ni", "hao"]. Let's take a look at the changes in the memory and the difference between JavaScript's treatment of string types and object types:

We observe the following two situations:
var x = "this is string...";
var y = ["this","is","string"];

The difference between x and y lies in the type. The JavaScript parser directly assigns the string value (actually copy) to x (direct quantity), but assigns the array pointer to y (reference quantity). All this happens instantly. Fully automatic! Combined with the following picture, you may have a better understanding:



In the picture, p1, p2... are pointers to variables, and the y in var y above stores Object type variables. The pointer p1 (assumed), and x stores the string itself. Analyze Example 3 again. When executing var ni","hao"], the parser opens up a new storage space to hold this new array, and x is the pointer of this new storage space. This means that the redefinition (or reassignment) of variables in JavaScript will Create a new storage space without destroying the original space; look back at Example 2, x[0] = "world", this sentence does not define a new value for x, does not create a new storage space, but only changes its storage The data in the space, so the final alert in Example 2 is "world"; Example 1 is a string assignment, and the whole process is a direct operation.

As can be seen from the above analysis, JavaScript variables can store direct quantities or pointers. This cannot be artificially interfered with. Therefore, in daily coding, you need to pay attention to these issues, such as Details such as large string concatenation and assignment in loops can directly affect the execution efficiency of the program.

Look at two examples:
var _tmpStr="";
var str = "this is big string...";
for (i=0; i _tmpStr = a;
}
a = _tmpStr;

Because it is a string operation, using direct quantities, each loop must operate a large string. Very bulky and inefficient. If you use reference operations instead, that is, through arrays:
var str = "this is big string...";
var _tmpArray = [];
for (i=0; i _tmpArray[i]=str;
} <script> var x = "this is string"; <BR> var y = x; <BR> x="ni hao"; <BR> alert(y) <BR></script>str = _tmpArray.join(""); <script> <BR>var x = ["hello"] // 这是一个数组，只有一个元素，并且该元素为字符串类型 <BR>var y = x; <BR>x[0] = "world"; <BR>alert(y[0]); <BR></script><script> <BR>var x = ["hello"] // 这是一个数组，只有一个元素，并且该元素为字符串类型 <BR>var y = x; <BR>x = ["ni","hao"]; // x 将变成一个新的数组了。 <BR>alert(y[0]); <BR></script>Do a test, if there is a 100k string, The direct connection operation takes about 2600 milliseconds on my machine. If the array connection is used, it takes 150 milliseconds. The efficiency is more than ten times different.

I haven’t written such a long article for a long time. It took me most of the day.