Several methods and efficiency Summary of for loop in JavaScript, javascriptfor

Several methods and efficiency Summary of for loop in JavaScript, javascriptfor

Preface

For the for loop, I believe it is not common. But this is because I didn't understand the meaning of a for loop when I read the code.

This for loop is written as follows:

for (var i = 0, rule; rule = rules[i++];) { //do something}

What does this statement mean? Later, I am selling a customs gateway. I think it is quite good.

Impact of for loop writing on Efficiency

Before talking about the above Code, let's talk about the efficiency of the for loop. There are quite a lot of articles about the for loop writing method and the impact on efficiency when dealing with js. But in general, there are two ways to write for a for Loop:

1. Do not write declaration variables:for(var i = 0;i<arr.length;i++){}
2. Writing declaration variables:for(var i = 0,len = arr.length;i < len;i++){}
   

Besides the for loopforEach() Also, some articles have said:forEach()Most efficient, recommendedforEach()Which of the following statements is highly efficient? Let's take a test.

Test Plan

The overall test scheme is as follows:

1. Make a test array variable that can hold tens of millions.
2. Use the for loop and foreach statements to traverse the test variable.
3. Perform 10 tests on the same stable machine and obtain the average value.
4. Test environment: CPU: Inter (R) Core i5-3210M, RAM: 12GM, system: win10 (x64)
   

Test procedure

Create Test variables

Use the while loop to make a test variable. This is very simple. The details are as follows:

var testArrs = [], i = 0;while(i<40000000){ testArrs.push(i); i++;}

Compile corresponding test functions

Code for measuring and executing the time, which isconsole.time()Andconsole.timeEnd().

For these three for loops, we first make three functions. They are

Foreach loop test:

function testForeach(testArrs){ console.time('foreach'); var newArrs = []; testArrs.forEach(function(i){ newArrs.push(i); }); console.timeEnd('foreach');}

For Loop without declaring variables:

function testNoDeclare(testArrs){ console.time('no declare'); var newArrs = []; for(var i = 0;i<testArrs.length;i++){ newArrs.push(i); } console.timeEnd('no declare');}

Variable Declaration

function testUseDeclare(testArrs){ console.time('use declare'); var newArrs = []; for(var i = 0,len = testArrs.length;i<len;i++){ newArrs.push(i); } console.timeEnd('use declare');}

Execute the test function

It's easy to execute the test function. You just need to call the function.

testForeach(testArrs);testNoDeclare(testArrs);testUseDeclare(testArrs);

Test Results

After 10 tests, the following results are obtained:

Foreach	Do not write a statement	Write statement
2372.891 ms	672.530 ms	743.974 ms
2431.821 ms	710.275 ms	805.676 ms
2422.448 ms	729.287 ms	741.014 ms
2330.894 ms	730.200 ms	755.390 ms
2423.186 ms	703.255 ms	769.674 ms
2379.167 ms	689.811 ms	741.040 ms
2372.944 ms	712.103 ms	710.524 ms
2316.005 ms	726.518 ms	726.522 ms
2535.289 ms	733.826 ms	747.427 ms
2560.925 ms	793.680 ms	817.098 ms
Average Value	Average Value	Average Value
2414.56 ms	720.15 ms	755.83 ms

I wonder if the results have surprised you? I did not expect that the most common writing method is the most efficient. Why? I don't want to understand either of them. If anyone knows, let me know, but I guess the statement writing is meaningless. Becauselen = arr.lengthThisarr.lengthIt may have been cached, so it is meaningless to declare a len variable to store it.

Complete the test code and copy it to your computer to test the Code. If there is anything unreasonable, please let me know.

var testArrs = [], i = 0;while(i<40000000){ testArrs.push(i); i++;}function testForeach(testArrs){ console.time('foreach'); var newArrs = []; testArrs.forEach(function(i){ newArrs.push(i); }); console.timeEnd('foreach');}function testNoDeclare(testArrs){ console.time('no declare'); var newArrs = []; for(var i = 0;i<testArrs.length;i++){ newArrs.push(i); } console.timeEnd('no declare');}function testUseDeclare(testArrs){ console.time('use declare'); var newArrs = []; for(var i = 0,len = testArrs.length;i<len;i++){ newArrs.push(i); } console.timeEnd('use declare');}testForeach(testArrs);testNoDeclare(testArrs);testUseDeclare(testArrs);

Special for loop writing

Next, let's talk about the code that I didn't understand at the beginning of the article, and explain the for loop syntax that I was not familiar with before. The basic syntax of the for Loop is:

For (Statement 1; Statement 2; Statement 3) {executed code block}

1. Statement 1: run the statement before the loop (code block) starts.
2. Statement 2: define conditions for running cycles (code blocks)
3. Statement 3: executed after the loop (code block) has been executed
   

If we use the for loop to output 1 to 10, we can write it like this:

for(var i=0;i<10;i++){console.log(i);}

But! According to the preceding syntax, we can also write

for(var i=10;i--;){console.log(i);}

I was wondering at the beginning. How can I write this? Statement 2 places the loop condition and I-the judgment condition. Otherwise, in statement 2, if true is returned, the execution will continue. In js, when 0, null, undefined, false, ''," is used as a condition, the result is false, that is, when I-to 0 is false, the loop ends.

Return to the code at the beginning of the article

for (var i = 0, rule; rule = rules[i++];) { //do something}

This rule = rules [I ++] is the judgment condition. When it becomes undefined, the loop will be terminated. Therefore, this code is written as follows:

for(var i = 0;i < rules.length;i++){ var rule = rules[i]}

In fact, it is to put the judgment and value assignment together, and assign values while repeating. Is it quite simple?

Summary

The above is all the content of this article. I hope the content of this article will help you learn or use Javascript. If you have any questions, please leave a message.