Array.from()

The array. from method is used to convert two types of objects into true arrays: array-like objects and iterable objects (including the new ES6 data structures Set and Map)

let arr2 = Array.from(arrayLike);
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Array.of()

The getValue () method converts a set of values to an array

Array.of(3.11.8) / /,11,8 [3]
Array.of(3) / / [3]
Array.of(3).length / / 1
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Array. The find and findIndex

Find returns the number, and findIndex returns the corner index

[1.5.10.15].find(function(value, index, arr) {
  return value > 9;
}) 
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Array.fill()

The fill method fills an array with the given value.

['a'.'b'.'c'].fill(7)
/ / (7, 7, 7)

new Array(3).fill(7)
/ / (7, 7, 7)
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Array.includes()

[1.2.3].includes(2);     // true
[1.2.3].includes(4);     // false
[1.2.NaN].includes(NaN); // true
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Array.filter every some map foreach

Item, index, arr forEach: no return value, undefined if returned. Arr forEach: no return value, undefined if returned. There are three arguments item, index, and arr every: the return value is a Boolean value. True is returned only if all elements have a certain property. Return false if either item is not satisfied. Some: The return value is a Boolean true. As long as one of these values is true, the loop ends, and the following data does not continue to judge the value of the filter: filter array. The result of the condition is true.Copy the code
    let arr = ['c'.'d'.'e'];
    arr.forEach((item) = >{
        item = 'a';
    });
    console.log(arr);// The array does not change

    let arrMap = arr.map((item) = >{
        return item = 'a';
    });
    console.log(arrMap);// return ["a", "a", "a"]

    let flag = arr.every((item) = >{
        return item==='a'
    });
    console.log(flag);//false

    flag = arrMap.every((item) = >{
        return item==='a'
    });
    console.log(flag);//true

    flag = arr.some((item) = >{
        return item==='c'
    });
    console.log(flag);//true

    flag = arrMap.some((item) = >{
        return item==='a'
    });
    console.log(flag);//true
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About the vacancy of the array

ES5’s treatment of empty seats is already quite inconsistent, mostly ignoring empty seats.

  • forEach().filter().every() 和some()All jump over the empty space.
  • map()The void is skipped, but the value is preserved
  • join()andtoString()Will treat the empty space asundefinedAnd theundefinedandnullWill be processed as an empty string

The following

/ / the forEach method
[,'a'].forEach((x,i) = > console.log(i)); / / 1

/ / filter method
['a'.'b'].filter(x= > true) // ['a','b']

/ / every method
[,'a'].every(x= > x==='a') // true

/ / some methods
[,'a'].some(x= >x ! = ='a') // false

/ / the map method
[,'a'].map(x= > 1) // [,1]

/ / join method
[,'a'.undefined.null].join(The '#') // "#a##"

/ / the toString method
[,'a'.undefined.null].toString() // ",a,,"
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ES6 explicitly converts empty space to undefined

The array. from method converts the empty space of the Array to undefined, which means it doesn’t ignore the empty space.

Array.from(['a'.'b'])
// [ "a", undefined, "b" ]
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Extended operators (…) It also changes the empty space to undefined.

[[...'a'.'b']]// [ "a", undefined, "b" ]
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CopyWithin () copies the empty space.

[,'a'.'b',,].copyWithin(2.0) // [,"a",,"a"]
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Fill () treats the empty space as a normal array position.

new Array(3).fill('a') // ["a","a","a"]
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for… The of loop also iterates over the empty space.

let arr = [, ,];
for (let i of arr) {
  console.log(1);
}
/ / 1
/ / 1
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Entries (), keys(), values(), find() and findIndex() treat empty Spaces 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) / / 0
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