Javascript is 100 faster than Classical C in simple for loop test, why?

Question

How JavaScript could do much faster than C for the following simple for loop example. It's almost 100 times faster than C after I tested those two codes. How JavaScript do string concatenation faster than C in the loop? Somebody said JavaScript is heavy dynamic language and it change variable, function on run-time, What's that meaning ? From the console.log or printf for str variable, it proved the for-loop is executed in both code without any compiler optimization that I guess.

JavaScript Loop Time: 205ms
C loop time: 32500ms

javascript:

 var i, a, b, c, max,str;
 max = 2e5;
 str="";
 var a = new Date();
 var myvar =   Date.UTC(a.getFullYear(),a.getMonth(),a.getDay(),a.getHours(),a.getMinutes(),a.getSeconds(),a.getMilliseconds());
 for (i=0;i< max;i++) {
     str= str+i+"=";  //just concat string
 }
 var a = new Date();
 var myvar2 = Date.UTC(a.getFullYear(),a.getMonth(),a.getDay(),a.getHours(),a.getMinutes(),a.getSeconds(),a.getMilliseconds());
 console.log("loop time:",myvar2-myvar);  //show for-loop time
 console.log("str:",str);  //for checking the for-loop is executed or not

classical c

#include <string.h>
#include <limits.h>
#include <stdio.h>
#include <time.h>
int main() {
    int i, a, b, c, max;
    clock_t t1, t2;
    t1 = clock();
    max = 2e5;
    char f[9];
    char str[10000000] = {""};
    for (i = 0; i < max; i++) {
        sprintf(f, "%ld", i); // convert integer to string
        strcat(str, "="); // just concat
        strcat(str, f);
    } // just concat
    t2 = clock();
    float diff = (((float)t2 - (float)t1) / 1000000.0F) * 1000;
    printf("loop time output in ms= :%.2fms\n", diff); // show for-loop time
    printf("str:%s\n", str); // check whether the for loop is executed or not
    return 0;
}

Answer 1

Javascript is 100 faster than Classical C in simple for loop test, why?

Because C does not have strings in the same sense javascript has.

To make the test more fair, do these changes:

Outside the loop add

char *strptr;
strptr = str;

and replace the loop with

for (i = 0; i < max; i++) {
    strptr += sprintf(strptr, "=%d", i);
}

Of course now, after these changes, the javascript version may be doing more work than the C version. C has no buffer overflow checks. The javascript version, apparently, is checking the size of the string and expanding it when needed.