A C compiler in TypeScript, Written by Claude

To understand what AI is capable of today (February 2026), I asked Claude (Opus 4.5) to write a C compiler in TypeScript. I was expecting a week-long project with lots of human intervention. To my surprise, Claude did it in about a minute, and it worked on the first try!

This project is a great way to learn how compilers work and explore x86-64 assembly language. Despite being under 1,500 lines of straightforward, readable code, the compiler is capable enough to handle array sorting and binary search routines.

Prompt given to Claude

Write a simple C compiler in typescript that can compile the program below
to GNU compatible assembly:

int puts(const char *);

void sort(int arr[], int n)
{
    int i;
    int j;
    int temp;

    for (i = 0; i < n - 1; i = i + 1)
    {
        for (j = 0; j < n - 1 - i; j = j + 1)
        {
            if (arr[j] > arr[j + 1])
            {
                temp = arr[j];
                arr[j] = arr[j + 1];
                arr[j + 1] = temp;
            }
        }
    }
}

int main()
{
    int arr[5];
    char buffer[20];
    int i;

    arr[0] = 5;
    arr[1] = 2;
    arr[2] = 9;
    arr[3] = 1;
    arr[4] = 3;

    sort(arr, 5);

    for (i = 0; i < 5; i = i + 1)
    {
        int_to_string(arr[i], buffer);
        puts(buffer);
    }

    return 0;
}

Claude wrote the compiler in one shot and it worked the first time!

What the compiler supports:

• Types: int, char, void, pointers, arrays
• Declarations: functions, function prototypes, local/global variables
• Statements: if/else, for, while, return, blocks
• Expressions: arithmetic (+, -, *, /, %), comparisons, logical operators (&&, ||, !), array indexing, function calls, assignments, address-of (&)

How to run

System requirements

• You need a x64 machine because the compiler produces x64 code.
• I have only tested on a Windows box, but it should run on Linux and macOS as long as it is x64.
• Docker. (I used Docker to avoid having to install TypeScript and GNU assembler/linker.)

Build and run

To build (run this command in project folder):

docker build -t c-compiler .

To run (run this command in project folder):

docker run --rm -v .:/workspace c-compiler test.c

Examine the compiler output

The input to the compiler is in test.c. The compiler outputs x64 assembly language. The output is placed in output.s. It is then assembled and linked and the final native executable is placed in the file program. The executable is then run and the output is displayed on the screen.