react-native-qrcode/node_modules/qr.js/example/index.js

var qr = require('qr.js');

var qrcode = qr('foo');

var width = 200;
var height = 200;

var canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;

var ctx = canvas.getContext('2d');

var cells = qrcode.modules;

var tileW = width  / cells.length;
var tileH = height / cells.length;

for (var r = 0; r < cells.length ; ++r) {
    var row = cells[r];
    for (var c = 0; c < row.length ; ++c) {
        ctx.fillStyle = row[c] ? '#000' : '#fff';
        var w = (Math.ceil((c+1)*tileW) - Math.floor(c*tileW));
        var h = (Math.ceil((r+1)*tileH) - Math.floor(r*tileH));
        ctx.fillRect(Math.round(c*tileW), Math.round(r*tileH), w, h);
    }
}

canvas // =>