ECMAScript 2015 brought us a lot of news, which resulted in a large number of improvements. Today we will take a closer look at two features that make life easier. Meet the rest paremeters and the spread syntax.
The rest syntax allows us to represent an indefinite number of arguments as an array. Take a look at a function that adds up all the arguments passed.
const sum = (...args) => args.reduce((prev, current) => prev + current);
console.log(sum(1, 2)); // 3
console.log(sum(1, 2, 3)); // 6
The spread operator allows us to expand iterable objects into individual elements. This functionality is opposite to what we achieved with the rest parameters. It can be applied to all iterables, such as arrays, objects, sets, maps, etc.
const sum = (x, y, z) => x + y + z;
const numbers = \[1, 2, 3\];
console.log(sum(...numbers)); // 6
The spread syntax effectively goes one level deeper while copying an array. One level means that the first level of references is copied.
const array0 = \[1, \[2, 3\]\];
const array1 = \[...array0\];
console.log(array1); // \[1, \[2, 3\]\]
Create the Set which stores only unique elements and convert its back to an array.
const array = \[1, 1, 2, 3\];
const uniqueElements = \[...new Set(array)\];
console.log(uniqueElements); // \[1, 2, 3\]
const array0 = \[1, 2\];
const array1 = \[3, 4\];
const concated = \[...array0, ...array1\];
console.log(concated); // \[1, 2, 3, 4\]
const \[firstElement, ...newArray\] = \[1, 2, 3, 4\];
console.log(firstElement); // 1
console.log(newArray); // \[2, 3, 4\]
We can also remove n first elements with comma.
const \[, , , ...newArray\] = \[1, 2, 3, 4\];
console.log(newArray); // \[4\]
const array0 = \[4, 5, 6\];
const array1 = \[1, 2, 3\];
const newArray = \[...array1, ...array0\];
console.log(newArray); // \[ 1, 2, 3, 4, 5, 6 \]
nconst array = \[...Array(10)\].map((\_, i) => i);
console.log(array); // \[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 \]
const { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }
let person = {
name: 'John',
age: 25,
wallet: {
sum: 500,
currency: 'USD'
}
};
let personCopy = { ...person };
console.log(personCopy);
// {
// name: 'John',
// age: 25,
// wallet: {
// sum: 500,
// currency: 'USD'
// }
// }
Note that the copy of the object that is created is a new object with all the original object’s properties but none of its prototypal information.
person.age = 20;
console.log(person.age); // 20
console.log(personCopy.age); // 25
Notice that spread syntax creates 'shallow' copy of the object, so 'wallet' property will be copied only as a reference to the original sub-object. For deep-cloning you can use JSON stringify/parse approach or 'cloneDeep' method provided by Lodash depending on your object's complexity. In some cases this method can be helpful as well:
let personCopy = { ...person, wallet = {...person.wallet}};
We can conditionally add properties to a new object that we are creating by making use of the spread operator along with short circuit evaluation.
const colors = {
red: '#ff0000',
green: '#00ff00',
blue: '#0000ff'
};
const black = {
black: '#000000'
};
let extraBlack = false;
let conditionalMerge = {
...colors,
...(extraBlack ? black : {})
};
console.log(conditionalMerge);
// {
// red: '#ff0000',
// green: '#00ff00',
// blue: '#0000ff'
// }
extraBlack = true;
conditionalMerge = {
...colors,
...(extraBlack ? black : {})
};
console.log(conditionalMerge);
// {
// red: '#ff0000',
// green: '#00ff00',
// blue: '#0000ff'
// black: '#000000'
// }
This is similar to calling the split method with an empty string as the parameter.
const split = \[...'qwerty'\];
console.log(split); // \[ 'q', 'w', 'e', 'r', 't', 'y' \]
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