ยง Katex in duktape

Here's some code to use duktape, a lightweight JavaScript interpreter to run katex when implementing a custom static site generator [as I am doing for this website ]. This seems to be way more lightweight than relying on Node, since we don't need to pay for inter-procedure-calls. I haven't benchmarked my static site generator against others, but I would be surprised if it is much faster, since it is written entirely in C, and avoids anything 'expensive'. 
#include "duktape.h"
#include <stdio.h>
#include <assert.h>
typedef long long ll;

void vduk_print_stack(duk_context *ctx, const char *fmt, va_list args){
    char *outstr = nullptr;
    vasprintf(&outstr, fmt, args);
    assert(outstr);

    printf("\nvvv%svvv\n", outstr);
    printf("[TOP OF STACK]\n");
    const int len = duk_get_top(ctx);
    for(int i = 1; i <= len; ++i) {
        duk_dup(ctx, -i);
        printf("stk[-%2d] = %20s\n", i, duk_to_string(ctx, -1));
        duk_pop(ctx);
    }
    printf("^^^^^^^\n");
}


void duk_print_stack(duk_context *ctx, const char *fmt, ...) {
    va_list args;
    va_start(args, fmt);
    vduk_print_stack(ctx, fmt, args);
    va_end(args);
}
int main() {
    const char *latex_to_compile = "\\int_0^\\infty f(x) dx";
    const char *broken_latex_to_compile = "\\int_0^\\infty \\foobar f(x) dx";


    // FILE *fkatex = fopen("/home/bollu/blog/katex/katex.min.js", "rb");
    FILE *fkatex = fopen("/home/bollu/blog/katex/katex.min.js", "rb");
    if (fkatex == nullptr) { assert(false && "unable to open katex.min.js"); }

    fseek(fkatex, 0, SEEK_END);
    const ll len = ftell(fkatex); fseek(fkatex, 0, SEEK_SET);
    char *js = (char *)calloc(sizeof(char), len + 10);

    const ll nread = fread(js, 1, len, fkatex);
    assert(nread == len);
    duk_context *ctx = duk_create_heap_default();
    duk_push_string(ctx, "katex.min.js"); // for error message
    if (duk_pcompile_string_filename(ctx, 0, js) != 0) {
        fprintf(stderr, "===katex.min.js compliation failed===\n%s\n===\n",
                duk_safe_to_string(ctx, -1));
        assert(false && "unable to compile katex.min.js");
    } else {
        fprintf(stderr, "===katex.min.js successfully complied===\n");

        duk_print_stack(ctx, "Stack afer compilation", __LINE__);
        duk_call(ctx, 0);
        printf("program result: %s\n", duk_safe_to_string(ctx, -1));

    }



    // https://wiki.duktape.org/howtofunctioncalls
    duk_print_stack(ctx, "%d", __LINE__);
    if(duk_peval_string(ctx, "katex") != 0) {
        printf("eval failed: %s\n", duk_safe_to_string(ctx, -1));
        assert(0 && "unable to find the katex object");
    }  else {
        duk_print_stack(ctx, "%d", __LINE__);
    }

    duk_print_stack(ctx, "%d", __LINE__);
    duk_push_string(ctx, "renderToString");
    duk_push_string(ctx, latex_to_compile);

    duk_print_stack(ctx, "%d", __LINE__);
    if(duk_pcall_prop(ctx, -3, 1) == DUK_EXEC_SUCCESS) {
        printf("===katexed output:===\n%s\n", duk_to_string(ctx, -1));
    } else {
        printf("unable to katex: %s\n", duk_to_string(ctx, -1));
        assert(false && "unable to katex input");
    }

    return 0;
}