Implementation of Alex Yakushev's JavaScript type nightmare (https://twitter.com/unlog1c/status/879421477036183552) feat. a fun pun!

undefinedSin.js

(function undefinedSin() {
  // Save the original sine function
  const originalSin = Math.sin;

  // Encapsulate a scaled sine function
  const sinScaled = function sinScaled(value, xScale, yScale) {
    return originalSin((value * Math.PI) / xScale) * yScale;
  };

  // Function that returns a graph formed by the string representation of the input
  const sinUndefined = function sinUndefined(input) {
    // Sine parameters
    const yScale = 2.5;
    const xScale = 5;

    // Build empty 2D array to hold graph
    const graphHeight = 7;
    const graphWidth = 22;
    const graphYZero = Math.floor(graphHeight / 2);
    const graphOrigin = [graphYZero, 0];
    const graphArray = Array(graphHeight).fill('').map(() => Array(graphWidth).fill(' '));

    // Add y and x-axes using box-drawing characters
    graphArray.forEach((horRow => (horRow[0] = '│'))); // eslint-disable-line no-param-reassign
    graphArray[graphYZero] = graphArray[graphYZero].map(() => '─');
    graphArray[graphOrigin[0]][graphOrigin[1]] = '├';

    // Convert input into string representation, repeat until long enough, split to array
    const inputString = `${input}`;
    const longerInputString = inputString.repeat(Math.ceil(graphWidth / inputString.length));
    const inputArray = longerInputString.split('');

    // Get the sine values for each input element
    const inputSinValues = inputArray.map((_, index) =>
      Math.round(sinScaled(index, xScale, yScale)) * -1,
    );

    // Graph values
    inputSinValues.slice(0, graphWidth).forEach((val, index) => {
      graphArray[val + graphYZero][index] = inputArray[index];
    });

    // Join up the graphArray into a string
    return graphArray.map(horRow => horRow.join('')).join('\n');
  };

  // Replace Math.sin
  Math.sin = (input) => {
    if (input === undefined) {
      return sinUndefined(input);
    }
    return originalSin(input);
  };

  // Test function
  const x = undefined;
  return Math.sin(x);

  /**
   * │
   * │ de        ef
   * │n  f      d  i      e
   * u────i────n────n────d─
   * │     n  u      e  n
   * │      ed        du
   * │
   */
}());