ECMAScript 6 - ECMAScript 2015
What is ECMAScript 6?
ECMAScript 6 is also known as ES6 and ECMAScript 2015
Some people like to call it JavaScript 6.
This chapter will introduce some of the new features in ES6.
- JavaScript let
- JavaScript const
- JavaScript default parameter values
- Array.find()
- Array.findIndex()
JavaScript let
The let statement allows you to declare a variable with block scope.
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.
Example
var x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
Try it Yourself »
Read more about let and const in JS Let / Const.
Default Parameter Values
ES6 allows function parameters to have default values.
Example
function myFunction(x, y = 10) {
// y is 10 if not passed or undefined
return x + y;
}
myFunction(5); // will return 15
Try it Yourself »
Array.find()
The find() method returns the value of the first array element that passes a test function.
This example finds (returns the falue of ) the first element that is larger than 18:
Example
var numbers = [4, 9, 16, 25, 29];
var first =
numbers.find(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
Try it Yourself »
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:
Example
var numbers = [4, 9, 16, 25, 29];
var first =
numbers.findIndex(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
Try it Yourself »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
New Number Properties
ES6 added the following properties to the Number object:
- EPSILON
- MIN_SAFE_INTEGER
- MAX_SAFE_INTEGER
New Number Methods
ES6 added 2 new methods to the Number object:
- Number.isInteger()
- Number.isSafeInteger()
The Number.isInteger() Method
The Number.isInteger() method returns true if the argument is an integer.
Example
Number.isInteger(10); // returns true
Number.isInteger(10.5); // returns false
Try it Yourself »
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.
Example
Number.isSafeInteger(10); // returns true
Number.isSafeInteger(12345678901234567890); // returns false
Try it Yourself »
Safe integers are all integers from -(253 - 1) to +(253 - 1).
This is safe: 9007199254740991. This is not safe: 9007199254740992.
New Global Methods
ES6 also added 2 new global number methods:
- isFinite()
- isNan
The isFinite() Method
The global isFinite() method returns false if the argument is Infinity or NaN.
Otherwise it returns true:
The isNaN() Method
The global isNan() method returns true if the argument is NaN. Otherwise it returns false:
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.
Example
// ES5
var x = function(x, y) {
return x * y;
}
// ES6
const x = (x, y) => x * y;
Try it Yourself »
Arrow functions do not have their own this. They are not well suited for defining object methods.
Arrow functions must be defined before they are used. Using const is safer than using var, because a function expression is a constant value.
You can only omit the return keyword and the curly brackets if the function is a single statement.
It might be a good habit to keep them:
