JavaScript ECMAScript 6 / ES6

What is ECMAScript 6 (ES6)?

ECMAScript 6 is also known as ES6 and ECMAScript 2015.

Some people calls it JavaScript 6.

JavaScript let

The let statement allows you to declare a variable with block scope.

                      var x = 10;
                      // Here x is 10
                      { 
                        let x = 2;
                        // Here x is 2
                      }
                      // Here x is 10
                  

Note: You can find a detailed explanation about let in Let variable lesson

JavaScript const

The const statement allows you to declare a constant (a JavaScript variable with a constant value).

Constants are similar to let variables, except that the value cannot be changed.

                      var x = 10;
                      // Here x is 10
                      { 
                        const x = 2;
                        // Here x is 2
                      }
                      // Here x is 10
                  

Note: You can find a detailed explanation about const in Const variable lesson

Exponentiation Operator

The exponentiation operator (**) raises the first operand to the power of the second operand.

                      var x = 5;
                      var z = x ** 2;          // result is 25
                  

x ** y produces the same result as Math.pow(x,y):

                      var x = 5;
                      var z = Math.pow(x,2);   // result is 25
                  

Default Parameter Values

ES6 allows function parameters to have default values.

                      function myFunction(x, y = 10) {
                        // y is 10 if not passed or undefined
                        return x + y;
                      }
                      myFunction(5); // will return 15
                  

Array.find()

The find() method returns the value of the first array element that passes a test function.

This example finds (returns the value of ) the first element that is larger than 18:

                          var numbers = [4, 9, 16, 25, 29];
                          var first = numbers.find(myFunction);
                          
                          function myFunction(value, index, array) {
                            return value > 18;
                          }
                      

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

Array.findIndex()

The findIndex() method returns the index of the first array element that passes a test function.

This example finds the index of the first element that is larger than 18:

                    var numbers = [4, 9, 16, 25, 29];
                    var first = numbers.findIndex(myFunction);
                    
                    function myFunction(value, index, array) {
                      return value > 18;
                    }
                

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

The Number.isInteger() Method

The Number.isInteger() method returns true if the argument is an integer.

                        Number.isInteger(10);        // returns true
                        Number.isInteger(10.5);      // returns false
                    

The Number.isSafeInteger() Method

A safe integer is an integer that can be exactly represented as a double precision number.

The Number.isSafeInteger() method returns true if the argument is a safe integer.

                      Number.isSafeInteger(10);    // returns true
                      Number.isSafeInteger(12345678901234567890);  // returns false
                    

Safe integers are all integers from -(2⁵³ - 1) to +(2⁵³ - 1).
This is safe: 9007199254740991. This is not safe: 9007199254740992.

Arrow Functions

Arrow functions allows a short syntax for writing function expressions.

You don't need the function keyword, the return keyword, and the curly brackets.

                        // ES5
                        var x = function(x, y) {
                           return x * y;
                        }
                        
                        // ES6
                        const x = (x, y) => x * y;
                    

Arrow functions do not have their own this. They are not well suited for defining object methods.

Arrow functions are not hoisted. They must be defined before they are used.

Using const is safer than using var, because a function expression is always constant value.

You can only omit the return keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:

                          const x = (x, y) => { return x * y };