QuickJS: Handle Typescript Sourcemap

I'm currently using Bellard's QuickJS engine on a new TypeScript project.

Since QuickJS only support JS, we need to transpile our TS first (qjs --std typescript.js path/to/your/project)

We can debug quickjs execution but stacktraces are relativ to JS so we need to parse our sourcemap files to get the TS line back. Those sourcemap use VLQ encoding, so let's make a VLQ parser:

const VLQ_BASE = 5;
const VLQ_CONT = 1 << VLQ_BASE;  // 100000
const VLQ_MASK = VLQ_CONT - 1;   // 011111
const b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
function vlq(segment) {
    const fields = [];
    for (let segIdx = 0; segIdx < segment.length;) {
        let result = 0;
        for (let shift = 0; ; shift += VLQ_BASE) {
            const digit = b64.indexOf(segment.charAt(segIdx++));
            result += ((digit & VLQ_MASK) << shift);
            if (!(digit & VLQ_CONT)) break;
        };
        fields.push(result & 1 ? -(result >> 1) : result >> 1)
    }
    return fields; // col,srcIdx,lineSrc,colSrc,nameIdx
};

this allows us to lookup the original TS line of a given JsLine using a myMap file

// JsLine = 42
// myMap = await fetch("my.map").then(r=>r.json())
const fields = myMap.mappings.split(';').map(g=>g.split(',').map(vlq));
let sum = 0;
const tsLine = fields.map(f => sum += f[0][2])[JsLine];

enjoy !