A simple JavaScript (ES6) function to flatten a nested integer array.

FlattenIntArray.js

/**
 * Flattens an integer array. The function should handle deeply nested arrays
 * without issue using an ES6 interpretter which supports Tail Call
 * Optimisation.
 *  
 * @returns {Array} a flattened Integer array 
 */
const flattenIntArray = array => array
    .filter(item => {
        if (Number.isInteger(item) || Array.isArray(item)) {
            return true
        } else {
            throw new TypeError("Invalid type in the array. Expected an Integer or an Array.");
        }
    })
    .reduce(
        (flattenedArray, item) => flattenedArray.concat(
            Array.isArray(item) ? flattenIntArray(item) : item
        ), []
    );

describe("Flatten Array", function(){
  it("should flatten nested arrays", function(){
    const test = [[[[[1]]]]];
    
    expect(flattenIntArray(test)).toEqual([1]);
  });
  
  it("should handle an empty array", function(){
    const test = [];
    
    expect(flattenIntArray(test)).toEqual([]);
  });
  
  it("should handle nested empty arrays", function(){
    const test = [[[],[]]];
    
    expect(flattenIntArray(test)).toEqual([]);
  });
  
  it("should handle mixed arrays", function(){
    const test = [[1,2,[3]],4];
    
    expect(flattenIntArray(test)).toEqual([1,2,3,4]);
  });
  
  it("should handle a flat array", function(){
    const test = [1,2,3,4];
    
    expect(flattenIntArray(test)).toEqual([1,2,3,4]);
  });
  
  it("should throw a TypeError for nulls in the array", function(){    
    const testCall = function() {
      const test = [1,2,3,4,null];
      flattenIntArray(test)
    }
    expect(testCall).toThrowError(TypeError);
  });

});