Initializing ArrayData from characters and byte values as in C

Question

I have to implement a binary protocol that is supposed to go over WebSockets.

In C I'd write test data such as :

const char test_message[16] = { '/', 'f', 'o', 'o', 0, 0, 0, 0, ',', 'i', 0, 0, 123, 0, 0, 0 };

(this is a valid message in the protocol I'm implementing).

How to generate a JavaScript ArrayData containing the same bytes in the easiest way (for the programmer) ?

• I do not want to look up ASCII codes corresponding to the bytes I want to send.

• I do not want to index each character one by one, e.g. data[0] = '/'; data[1] = 'f'; ....

Answer 1

Assuming you want to end up with a Uint8Array or similar, and assuming the only textual characters you'll use are ASCII, you could do:

const message = toMessage(["/", "f", "o", "o", 0, 0, 0, 0, ",", "i", 0, 0, 123, 0, 0, 0]);
// or even
const message = toMessage(["/foo", 0, 0, 0, 0, ",i", 0, 0, 123, 0, 0, 0]);

For the first one, toMessage might be:

function toMessage(source) {
    const array = new Uint8Array(source.length);
    source.forEach((element, index) => {
        array[index] = typeof element === "string" ? element.charCodeAt(0) : element;
    });
    return array;
}

Live example:

function toMessage(source) {
    const array = new Uint8Array(source.length);
    source.forEach((element, index) => {
        array[index] = typeof element === "string" ? element.charCodeAt(0) : element;
    });
    return array;
}

const message = toMessage(["/", "f", "o", "o", 0, 0, 0, 0, ",", "i", 0, 0, 123, 0, 0, 0]);

console.log(message);

If you want the second one that lets you do runs of simple characters as strings, it's slightly more complicated.

function toMessage(source) {
    let length = 0;
    for (const element of source) {
        length += typeof element === "string" ? element.length : 1;
    }
    const array = new Uint8Array(length);
    let index = 0;
    for (const element of source) {
        if (typeof element === "string") {
            for (const ch of element) {
                array[index++] = ch.charCodeAt(0);
            }
        } else {
            array[index++] = element;
        }
    }
    source.forEach((element, index) => {
    });
    return array;
}

Live Example:

function toMessage(source) {
    let length = 0;
    for (const element of source) {
        length += typeof element === "string" ? element.length : 1;
    }
    const array = new Uint8Array(length);
    let index = 0;
    for (const element of source) {
        if (typeof element === "string") {
            for (const ch of element) {
                array[index++] = ch.charCodeAt(0);
            }
        } else {
            array[index++] = element;
        }
    }
    source.forEach((element, index) => {
    });
    return array;
}

const message = toMessage(["/foo", 0, 0, 0, 0, ",i", 0, 0, 123, 0, 0, 0]);

console.log(message);

Those are both off-the-cuff and can be optimized if necessary, or adapted to produce something other than a Uint8Array, but the give you the idea.

---

Alternatively, if the text characters you want to use are just in tye 7-bit ASCII printable range, that's only 96 characters. You could easily have consts for them:

const L_a = 65;
const L_b = 66;
// ...
const L_A = 97;
const L_B = 98;
// ...

(The list is easily generated, you don't have to type it out.)

Then you wouldn't need a function at all:

const message = UInt8Array.of([L_SLASH, L_f, L_o, L_o, 0, 0, 0, 0, L_COMMA, L_i, 0, 0, 123, 0, 0, 0]);

There is no literal form for typed arrays, so it does involve a function call.