Spread syntax

function sum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

console.log(sum(...numbers));
// Expected output: 6

console.log(sum.apply(null, numbers));
// Expected output: 6

function myFunction(x, y, z) {}
var args = [0, 1, 2];
myFunction.apply(null, args);

function myFunction(x, y, z) {}
var args = [0, 1, 2];
myFunction(...args);

function myFunction(v, w, x, y, z) {}
var args = [0, 1];
myFunction(-1, ...args, 2, ...[3]);

var dateFields = [1970, 0, 1]; // 1 Jan 1970
var d = new Date(...dateFields);

function applyAndNew(constructor, args) {
  function partial() {
    return constructor.apply(this, args);
  }
  if (typeof constructor.prototype === "object") {
    partial.prototype = Object.create(constructor.prototype);
  }
  return partial;
}

function myConstructor() {
  console.log("arguments.length: " + arguments.length);
  console.log(arguments);
  this.prop1 = "val1";
  this.prop2 = "val2";
}

var myArguments = ["hi", "how", "are", "you", "mr", null];
var myConstructorWithArguments = applyAndNew(myConstructor, myArguments);

console.log(new myConstructorWithArguments());
// (internal log of myConstructor):           arguments.length: 6
// (internal log of myConstructor):           ["hi", "how", "are", "you", "mr", null]
// (log of "new myConstructorWithArguments"): {prop1: "val1", prop2: "val2"}

var parts = ["shoulders", "knees"];
var lyrics = ["head", ...parts, "and", "toes"];
// ["head", "shoulders", "knees", "and", "toes"]

var arr = [1, 2, 3];
var arr2 = [...arr]; // like arr.slice()
arr2.push(4);

// arr2 becomes [1, 2, 3, 4]
// arr remains unaffected

const a = [[1], [2], [3]];
const b = [...a];
b.shift().shift(); // 1
//  .    ""  : 
//[[], [2], [3]]

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
// Append all items from arr2 onto arr1
arr1 = arr1.concat(arr2);

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
arr1 = [...arr1, ...arr2];

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
// Prepend all items from arr2 onto arr1
Array.prototype.unshift.apply(arr1, arr2); // arr1 is now [3, 4, 5, 0, 1, 2]

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
arr1 = [...arr2, ...arr1]; // arr1 is now [3, 4, 5, 0, 1, 2]

var obj1 = { foo: "bar", x: 42 };
var obj2 = { foo: "baz", y: 13 };

var clonedObj = { ...obj1 };
// Object { foo: "bar", x: 42 }

var mergedObj = { ...obj1, ...obj2 };
// Object { foo: "baz", x: 42, y: 13 }

var obj1 = { foo: "bar", x: 42 };
var obj2 = { foo: "baz", y: 13 };
const merge = (...objects) => ({ ...objects });

var mergedObj = merge(obj1, obj2);
// Object { 0: { foo: 'bar', x: 42 }, 1: { foo: 'baz', y: 13 } }

var mergedObj = merge({}, obj1, obj2);
// Object { 0: {}, 1: { foo: 'bar', x: 42 }, 2: { foo: 'baz', y: 13 } }

var obj = { key1: "value1" };
var array = [...obj]; // TypeError: obj is not iterable