A small script for recording and feeding values into `Math.random` in JavaScript

feedrandom.js

// MIT License
//
// Copyright (c) 2018 Yuze Huang (hi@yuzeh.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

exports.feed = function (fn, {values = [], stack = true} = {}) {
  const random = Math.random;
  const output = [];

  let idx = 0;

  Math.random = function () {
    let rv;
    if (idx < values.length) {
      rv = values[idx];
      ++idx;
      if (rv.value) {
        rv = rv.value;
      }
    } else {
      rv = random.apply(null, arguments);
    }

    const log = {value: rv};
    if (stack) {
      log.stack = new Error().stack.split('\n');
    }

    output.push(log);

    return rv;
  };

  const done = () => {
    Math.random = random;
    return output;
  };

  const result = fn();
  if (result && typeof(result.then) === 'function') {
    return result.then(done);
  } else {
    return done();
  }
};