Extension of arrays
Extension operator
meaning
Extension operator （spread） Three points （...）. It's like rest Inverse operation of parameter , Convert an array to a comma separated sequence of parameters .
console.log(...[1, 2, 3])
// 1 2 3 console.log(1, ...[2, 3, 4], 5)
// 1 2 3 4 5 [...document.querySelectorAll('div')]
// [<div>, <div>, <div>]
This operator is mainly used for function calls .
function push(array, ...items) {
array.push(...items);
}
function add(x, y) {
return x + y;
}
const numbers = [4, 38];
add(...numbers) //
In the above code ,array.push(...items) and add(...numbers) These two lines , All are function calls , They all use extension operators . This operator takes an array , Change to parameter sequence .
Extension operators can be used in combination with normal function parameters , Very flexible .
function f(v, w, x, y, z) { }
const args = [0, 1];
f(-1, ...args, 2, ...[3]);
You can also place expressions after extension operators .
const arr = [
...(x > 0 ? ['a'] : []),
'b',
];
If the extension operator is followed by an empty array , It doesn't have any effect .
[...[], 1]
// [1]
Substitution function apply Method
Because extension operators can expand arrays , So you don't need it anymore apply Method , Turn the array into a function parameter .
// ES5 Writing
function f(x, y, z) {
// ...
}
var args = [0, 1, 2];
f.apply(null, args); // ES6 Writing
function f(x, y, z) {
// ...
}
let args = [0, 1, 2];
f(...args);
Here's the extension operator replacement apply A practical example of the method , application Math.max Method , Simplify the way to find the largest element of an array .
// ES5 Writing
Math.max.apply(null, [14, 3, 77]) // ES6 Writing
Math.max(...[14, 3, 77]) // Equate to
Math.max(14, 3, 77);
In the above code , because JavaScript Not to mention the function of the largest element of an array , So we can only apply Math.max function , Turn the array into a sequence of parameters , Then find the maximum . With the extension operator , You can use it directly Math.max 了 .
Another example is through push function , Add one array to the end of another .
// ES5 Of How to write it
var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
Array.prototype.push.apply(arr1, arr2); // ES6 Writing
let arr1 = [0, 1, 2];
let arr2 = [3, 4, 5];
arr1.push(...arr2);
Code above ES5 In writing ,push The argument to a method cannot be an array , So we have to go through apply The method can be used flexibly push Method . With extension operators , You can pass the array directly into push Method .
Here's another example .
// ES5
new (Date.bind.apply(Date, [null, 2015, 1, 1]))
// ES6
new Date(...[2015, 1, 1]);
Application of extension operators
（1） Copy the array
Arrays are composite data types , If you copy directly , Just copied the pointer to the underlying data structure , Instead of cloning a whole new array .
const a1 = [1, 2];
const a2 = a1; a2[0] = 2;
a1 // [2, 2]
In the above code ,a2 Not at all a1 The clone , It's another pointer to the same data . modify a2, Will lead directly to a1 The change of .
ES5 You can only copy arrays in a flexible way .
const a1 = [1, 2];
const a2 = a1.concat(); a2[0] = 2;
a1 // [1, 2]
In the above code ,a1 Will return a clone of the original array , Revise a2 It won't be right a1 An impact .
Extended operators provide a simple way to copy arrays .
const a1 = [1, 2];
// Writing a
const a2 = [...a1];
// Write two
const [...a2] = a1;
The above two ways of writing ,a2 All are a1 The clone .
（2） Merge array
The extension operator provides a new way to write array merging .
const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e']; // ES5 The merge array of
arr1.concat(arr2, arr3);
// [ 'a', 'b', 'c', 'd', 'e' ] // ES6 The merge array of
[...arr1, ...arr2, ...arr3]
// [ 'a', 'b', 'c', 'd', 'e' ]
however , Both methods are shallow copies , You need to pay attention to .
const a1 = [{ foo: 1 }];
const a2 = [{ bar: 2 }];
const a3 = a1.concat(a2);
const a4 = [...a1, ...a2];
a3[0] === a1[0] // true
a4[0] === a1[0] // true
In the above code ,a3 and a4 It's a new array formed by combining two different methods , But their members are all references to the original array members , This is a shallow copy . If you modify the members of the original array , It will be synchronized to the new array .
（3） Combined with deconstruction assignment
Extension operators can be combined with deconstruction assignments , Used to generate arrays .
// ES5
a = list[0], rest = list.slice(1)
// ES6
[a, ...rest] = list
Here are some other examples .
const [first, ...rest] = [1, 2, 3, 4, 5];
first //
rest // [2, 3, 4, 5] const [first, ...rest] = [];
first // undefined
rest // [] const [first, ...rest] = ["foo"];
first // "foo"
rest // []
If the extended operator is used for array assignment , It can only be placed in the last bit of the parameter , Otherwise, an error will be reported .
const [...butLast, last] = [1, 2, 3, 4, 5];
// Report errors const [first, ...middle, last] = [1, 2, 3, 4, 5];
// Report errors
（4） character string
Extension operators can also turn strings into real arrays .
[...'hello']
// [ "h", "e", "l", "l", "o" ]
The way it's written is , There is an important benefit , That is to be able to correctly identify four bytes Unicode character .
'x\uD83D\uDE80y'.length //
[...'x\uD83D\uDE80y'].length //
The first way to write the above code ,JavaScript Four bytes of Unicode character , Identified as 2 Characters , There is no such problem with extension operators . therefore , Functions that correctly return the length of a string , You can write like this .
function length(str) {
return [...str].length;
}
length('x\uD83D\uDE80y') //
Anything that involves manipulating four bytes Unicode Character functions , All have this problem . therefore , It's best to rewrite them all with extension operators .
let str = 'x\uD83D\uDE80y';
str.split('').reverse().join('')
// 'y\uDE80\uD83Dx'
[...str].reverse().join('')
// 'y\uD83D\uDE80x'
In the above code , If you don't use extension operators , A string of reverse The operation is not correct .
（5） Realized Iterator Object of the interface
whatever Iterator Object of the interface （ Refer to the Iterator chapter ）, Can be converted to a real array with the extension operator .
let nodeList = document.querySelectorAll('div');
let array = [...nodeList];
In the above code ,querySelectorAll Method returns a nodeList object . It's not an array , It's an array like object . At this time , Extension operators can turn it into a real array , And the reason is that NodeList Object implementation Iterator .
For those who are not deployed Iterator Interface is an array like object , The extension operator can't turn it into a real array .
let arrayLike = {
'0': 'a',
'1': 'b',
'2': 'c',
length: 3
};
// TypeError: Cannot spread non-iterable object.
let arr = [...arrayLike];
In the above code ,arrayLike Is an array-like object , But there's no deployment Iterator Interface , The extension operator will report an error . At this time , You can use Array.from Methods will arrayLike Turns into a real array .
（6）Map and Set structure ,Generator function
The extension operator internally calls the data structure Iterator Interface , So as long as you have Iterator Object of the interface , You can use extension operators , such as Map structure .
let map = new Map([
[1, 'one'],
[2, 'two'],
[3, 'three'],
]); let arr = [...map.keys()]; // [1, 2, 3]
Generator After the function runs , Returns a traverser object , So you can also use extension operators .
const go = function*(){
yield 1;
yield 2;
yield 3;
};
[...go()] // [1, 2, 3]
In the above code , Variable go It's a Generator function , After execution, a traverser object is returned , Execute the extension operator on this traverser object , The value obtained by internal traversal will be , Into an array .
If not Iterator Object of the interface , Use the extension operator , Will be an error .
const obj = {a: 1, b: 2};
let arr = [...obj]; // TypeError: Cannot spread non-iterable object
Array.from()
Array.from Method to convert two types of objects into real arrays ： Array like objects （array-like object） Ergodic （iterable） The object of （ Include ES6 New data structure Set and Map）.
Here is an array like object ,Array.from Turn it into a real array .
let arrayLike = {
'0': 'a',
'1': 'b',
'2': 'c',
length: 3
};
// ES5 Writing
var arr1 = [].slice.call(arrayLike); // ['a', 'b', 'c']
// ES6 Writing
let arr2 = Array.from(arrayLike); // ['a', 'b', 'c']
Practical application , Common array like objects are DOM Operation returned NodeList aggregate , And inside the function arguments object .Array.from You can turn them into real arrays .
// NodeList object
let ps = document.querySelectorAll('p');
Array.from(ps).filter(p => {
return p.textContent.length > 100;
}); // arguments object
function foo() {
var args = Array.from(arguments);
// ...
}
In the above code ,querySelectorAll Method returns an array like object , You can turn this object into a real array , Reuse filter Method .
As long as it's deployed Iterator Data structure of the interface ,Array.from Can be converted to an array .
Array.from('hello')
// ['h', 'e', 'l', 'l', 'o']
let namesSet = new Set(['a', 'b'])
Array.from(namesSet) // ['a', 'b']
In the above code , String and Set Structure has Iterator Interface , So it can be Array.from Turns into a real array .
If the parameter is a real array ,Array.from A new array as like as two peas .
Array.from([1, 2, 3])
// [1, 2, 3]
It's worth reminding , Extension operator （...） You can also convert some data structures to arrays .
// arguments object
function foo() {
const args = [...arguments];
} // NodeList object
[...document.querySelectorAll('div')]
The traversal interface is invoked behind the extension operator （Symbol.iterator）, If an object does not deploy this interface , Can't convert .Array.from Method also supports array like objects . So called array like objects , There is only one essential feature , That is, there must be length attribute . therefore , Any length Object of property , Both can pass Array.from Method to array , At this point, extension operators cannot be converted .
Array.from({ length: 3 });
// [ undefined, undefined, undefined ]
In the above code ,Array.from Returns an array with three members , The value of each position is undefined. Extension operators can't convert this object .
For browsers that have not yet deployed this method , It can be used Array.prototype.slice Method substitution .
const toArray = (() =>
Array.from ? Array.from : obj => [].slice.call(obj)
)();
Array.from You can also accept the second parameter , Act like an array map Method , Used to process each element , Put the processed value into the returned array .
Array.from(arrayLike, x => x * x);
// Equate to
Array.from(arrayLike).map(x => x * x); Array.from([1, 2, 3], (x) => x * x)
// [1, 4, 9]
Here's an example of taking out a set of DOM The text content of the node .
let spans = document.querySelectorAll('span.name');
// map()
let names1 = Array.prototype.map.call(spans, s => s.textContent);
// Array.from()
let names2 = Array.from(spans, s => s.textContent)
In the following example, the Boolean value in the array is false Members of 0.
Array.from([1, , 2, , 3], (n) => n || 0)
// [1, 0, 2, 0, 3]
Another example is to return various types of data .
function typesOf () {
return Array.from(arguments, value => typeof value)
}
typesOf(null, [], NaN)
// ['object', 'object', 'number']
If map The function uses this keyword , You can also pass in Array.from The third parameter of , Used to bind this.
Array.from() You can turn various values into real arrays , And it also provides map function . This actually means , As long as there is an original data structure , You can process its value first , Then it's converted to a canonical array structure , Then you can use a large number of array methods .
Array.from({ length: 2 }, () => 'jack')
// ['jack', 'jack']
In the above code ,Array.from The first parameter of specifies the number of times the second parameter runs . This feature can make the usage of this method very flexible .
Array.from() Another application of , Convert string to array , Then return the length of the string . Because it can handle all kinds of Unicode character , You can avoid JavaScript Will be bigger than the \uFFFF Of Unicode character , Count as two characters bug.
function countSymbols(string) {
return Array.from(string).length;
}
Array.of()
Array.of Method to use a set of values , Convert to array .
Array.of(3, 11, 8) // [3,11,8]
Array.of(3) // [3]
Array.of(3).length //
The main purpose of this method , Is a complement to array constructors Array() Deficiency . Because the number of parameters is different , It can lead to Array() Different behaviors .
Array() // []
Array(3) // [, , ,]
Array(3, 11, 8) // [3, 11, 8]
In the above code ,Array Method has no parameters 、 One parameter 、 When there are three parameters , The results are not the same . Only when the number of parameters is not less than 2 Time ,Array() Will return a new array of parameters . When there is only one parameter , It's actually specifying the length of the array .
Array.of Basically, it can be used as an alternative Array() or new Array(), And there is no overload due to different parameters . Its behavior is very uniform .
Array.of() // []
Array.of(undefined) // [undefined]
Array.of(1) // [1]
Array.of(1, 2) // [1, 2]
Array.of Always return an array of parameter values . If there are no parameters , It returns an empty array .
Array.of Method can be simulated with the following code .
function ArrayOf(){
return [].slice.call(arguments);
}
Of the array instance copyWithin()
Of the array instance copyWithin Method , Inside the current array , Copies a member of the specified location to another location （ Will overwrite the original members ）, Then return the current array . in other words , Using this method , The current array will be modified .
Array.prototype.copyWithin(target, start = 0, end = this.length)
It takes three parameters .
- target（ It's necessary ）： Replace data from this location . If it's negative , Said the bottom .
- start（ Optional ）： The data is read from this location , The default is 0. If it's negative , Said the bottom .
- end（ Optional ）： Stop reading data before reaching this location , The default is equal to the array length . If it's negative , Said the bottom .
All three parameters should be numerical , If not , It will automatically convert to a value .
[1, 2, 3, 4, 5].copyWithin(0, 3)
// [4, 5, 3, 4, 5]
The above code indicates that it will be from 3 A member that is signed until the end of an array （4 and 5）, Copy to from 0 The starting position of the sign , The result covers the original 1 and 2.
Here are more examples .
// take 3 The number is copied to 0 No. A
[1, 2, 3, 4, 5].copyWithin(0, 3, 4)
// [4, 2, 3, 4, 5] // -2 amount to 3 No. A ,-1 amount to 4 No. A
[1, 2, 3, 4, 5].copyWithin(0, -2, -1)
// [4, 2, 3, 4, 5] // take 3 The number is copied to 0 No. A
[].copyWithin.call({length: 5, 3: 1}, 0, 3)
// {0: 1, 3: 1, length: 5} // take 2 The number goes to the end of the array , Copied to the 0 No. A
let i32a = new Int32Array([1, 2, 3, 4, 5]);
i32a.copyWithin(0, 2);
// Int32Array [3, 4, 5, 4, 5] // For no deployment TypedArray Of copyWithin The platform of the method
// It needs to be written in the following way
[].copyWithin.call(new Int32Array([1, 2, 3, 4, 5]), 0, 3, 4);
// Int32Array [4, 2, 3, 4, 5]
Of the array instance find() and findIndex()
Of the array instance find Method , To find the first qualified member of the array . Its argument is a callback function , All array members execute the callback function in turn , Until you find out that the first return value is true Members of , Then return to the member . If there are no eligible members , Then return to undefined.
[1, 4, -5, 10].find((n) => n < 0)
// -5
The above code finds the first one in the array that is less than 0 Members of .
Copy to clipboardErrorCopied
In the above code ,find The callback function of method can take three parameters , In order is the current value 、 Current position and original array .
Of the array instance findIndex Method usage and find The method is very similar , Returns the location of the first qualified array member , If all the members are not eligible , Then return to -1.
[1, 5, 10, 15].findIndex(function(value, index, arr) {
return value > 9;
}) //
Both methods can take the second parameter , Used to bind callback functions this object .
function f(v){
return v > this.age;
}
let person = {name: 'John', age: 20};
[10, 12, 26, 15].find(f, person); //
In the above code ,find Function takes the second parameter person object , In callback function this The object points to person object .
in addition , Both of these methods can be found NaN, Make up for the indexOf The inadequacy of the method .
[NaN].indexOf(NaN)
// -1 [NaN].findIndex(y => Object.is(NaN, y))
//
In the above code ,indexOf Method does not recognize the NaN member , however findIndex We can use Object.is Method to achieve .
Of the array instance fill()
fill Method uses the given value , Fill an array .
['a', 'b', 'c'].fill(7)
// [7, 7, 7] new Array(3).fill(7)
// [7, 7, 7]
The above code indicates ,fill Method is very convenient for initialization of empty arrays . Elements already in the array , It'll be erased .
fill Methods can also take second and third arguments , Used to specify the start and end positions of the fill .
['a', 'b', 'c'].fill(7, 1, 2)
// ['a', 7, 'c']
The code above indicates ,fill Methods from 1 The number position starts , Fill the original array with 7, To 2 It's over before the number .
Be careful , If the type of fill is object , Then the assigned object is the object with the same memory address , Instead of deep copying objects .
let arr = new Array(3).fill({name: "Mike"});
arr[0].name = "Ben";
arr
// [{name: "Ben"}, {name: "Ben"}, {name: "Ben"}]
let arr = new Array(3).fill([]);
arr[0].push(5);
arr
// [[5], [5], [5]]
Of the array instance entries(),keys() and values()
ES6 There are three new ways ——entries(),keys() and values()—— For traversing arrays . They all return a traverser object （ See 《Iterator》 chapter ）, It can be used for...of Loop through , The only difference is keys() Is the traversal of key names 、values() Is the traversal of the key value ,entries() Is the traversal of key value pairs .
for (let index of ['a', 'b'].keys()) {
console.log(index);
}
//
//
for (let elem of ['a', 'b'].values()) {
console.log(elem);
}
// 'a'
// 'b'
for (let [index, elem] of ['a', 'b'].entries()) {
console.log(index, elem);
}
// 0 "a"
// 1 "b"
If not used for...of loop , The traverser object can be called manually next Method , Traversal .
let letter = ['a', 'b', 'c'];
let entries = letter.entries();
console.log(entries.next().value); // [0, 'a']
console.log(entries.next().value); // [1, 'b']
console.log(entries.next().value); // [2, 'c']
Of the array instance includes()
Array.prototype.includes Method returns a Boolean value , Indicates whether an array contains a given value , With string includes The method is similar to .ES2016 This method is introduced .
[1, 2, 3].includes(2) // true
[1, 2, 3].includes(4) // false
[1, 2, NaN].includes(NaN) // true
The second parameter of the method represents the starting position of the search , The default is 0. If the second parameter is negative , It means the position of the reciprocal , If it is greater than the array length at this time （ For example, the second parameter is -4, But the length of the array is 3）, Will be reset to from 0 Start .
[1, 2, 3].includes(3, 3); // false
[1, 2, 3].includes(3, -1); // true
Without this method , We usually use arrays of indexOf Method , Check to see if there is a value .
if (arr.indexOf(el) !== -1) {
// ...
}
indexOf There are two drawbacks to this approach , One is not semantic enough , It means to find the first occurrence of the parameter value , So to compare is not equal to -1, It's not intuitive enough . Two is , It uses strict equality operators inside （===） Judge , This will lead to the right NaN Miscarriage of Justice .
[NaN].indexOf(NaN)
// -1
includes Using a different algorithm , There is no such problem .
[NaN].includes(NaN)
// true
The following code is used to check whether the current environment supports this method , If not , Deploy a simple alternative version .
const contains = (() =>
Array.prototype.includes
? (arr, value) => arr.includes(value)
: (arr, value) => arr.some(el => el === value)
)();
contains(['foo', 'bar'], 'baz'); // => false
in addition ,Map and Set The data structure has a has Method , We need to pay attention to includes distinguish .
- Map Structural
hasMethod , It's used to find the key name , such asMap.prototype.has(key)、WeakMap.prototype.has(key)、Reflect.has(target, propertyKey). - Set Structural
hasMethod , Is used to find values , such asSet.prototype.has(value)、WeakSet.prototype.has(value).
Array vacancy
Empty bits in an array refer to , There is no value in one position of the array . such as ,Array The array returned by the constructor is empty .
Array(3) // [, , ,]
In the above code ,Array(3) Return a with 3 An array of empty bits .
Be careful , No vacancy undefined, The value of a position is equal to undefined, It's still worth it . The vacancy has no value ,in Operators illustrate this .
0 in [undefined, undefined, undefined] // true
0 in [, , ,] // false
The above code shows , The first array of 0 The number position is valuable , Of the second array 0 There is no value for position number .
ES5 Handling of vacancies , It's very inconsistent , Empty spaces are ignored in most cases .
forEach(),filter(),reduce(),every()andsome()Will skip the empty space .map()Will skip empty space , But this value will be retainedjoin()andtoString()Will treat empty spaces asundefined, andundefinedandnullWill be processed to an empty string .
// forEach Method
[,'a'].forEach((x,i) => console.log(i)); // // filter Method
['a',,'b'].filter(x => true) // ['a','b'] // every Method
[,'a'].every(x => x==='a') // true // reduce Method
[1,,2].reduce((x,y) => return x+y) // // some Method
[,'a'].some(x => x !== 'a') // false // map Method
[,'a'].map(x => 1) // [,1] // join Method
[,'a',undefined,null].join('#') // "#a##" // toString Method
[,'a',undefined,null].toString() // ",a,,"
ES6 It is clear that the vacancy will be changed to undefined.
Array.from Method will empty the array , To undefined, in other words , This method doesn't ignore vacancies .
Array.from(['a',,'b'])
// [ "a", undefined, "b" ]
Extension operator （...） It will also change the vacancy to undefined.
[...['a',,'b']]
// [ "a", undefined, "b" ]
copyWithin() It will be copied together with the empty space .
[,'a','b',,].copyWithin(2,0) // [,"a",,"a"]
fill() Treat empty bits as normal array positions .
new Array(3).fill('a') // ["a","a","a"]
for...of The loop will also traverse the vacancy .
let arr = [, ,];
for (let i of arr) {
console.log(1);
}
//
//
In the above code , Array arr There are two vacancies ,for...of They are not ignored . If change to map Methods through , Empty seats will be skipped .
entries()、keys()、values()、find() and findIndex() The empty space will be treated as undefined.
// entries()
[...[,'a'].entries()] // [[0,undefined], [1,"a"]] // keys()
[...[,'a'].keys()] // [0,1] // values()
[...[,'a'].values()] // [undefined,"a"] // find()
[,'a'].find(x => true) // undefined // findIndex()
[,'a'].findIndex(x => true) //
Because the processing rules of empty space are very inconsistent , Therefore, it is recommended to avoid empty space .
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