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Array object in JavaScript

The Array object in JavaScript is a global object used to construct arrays, which are high-level objects similar to lists.

Description

In JavaScript, arrays can typically be created using the Array constructor or literal notation.

console.log(Array(3)); // (3) [empty × 3]
console.log(new Array(3)); // (3) [empty × 3]
console.log([,,,]); // (3) [empty × 3]

In JavaScript, arrays exist in the form of sparse arrays, and can be thought of as special objects that can be sorted according to index values. Therefore, when there are no values at certain positions, it's necessary to use a certain value to fill them. Of course, sparse arrays are optimized in various browsers. For example, the V8 engine has operations to convert between fast arrays and slow arrays. In addition, in V8, the description of empty is a reference to an empty object. When creating arrays using the Array constructor in Js, the issue of existence of empty slots is not filled by default with undefined, but with empty as the value. It's important to note that an empty slot is not undefined. Undefined means it's not defined, but undefined itself is a primitive data type, a value, while empty means that there is no value at that location. Furthermore, the treatment of empty slots in ES6 and ES6 is different, so some methods will skip empty, while others will treat empty as undefined. Therefore, it's best to avoid empty slots.

console.log([,,,]); // (3) [empty × 3]
console.log(new Array(3)); // (3) [empty × 3]
console.log([undefined, undefined, undefined]); // (3) [undefined, undefined, undefined]
console.log(0 in [undefined, undefined, undefined]); // true
console.log(0 in [,,,]); // false // in checks the index, it means the position 0 has no value

Properties

Array.length: length is an instance property of Array, which returns or sets the number of elements in an array. The value is an unsigned 32-bit integer, ranging from 0 to 2^32-1, and is always greater than the highest index of the array item. By setting the value of the length property, it's possible to truncate any array. When extending an array by changing the value of the length property, the actual number of elements will increase. If the value passed in exceeds a valid value, a RangeError exception will be thrown. Additionally, if the index in the array is set to -1 or a string, the length of the array will not change, and these indices in the array will be treated as object properties. In fact, an array is a collection of data that can be sorted by index values, is a special type of object.
Array.prototype[@@unscopables]: The Symbol property @@unscopable contains all property names newly defined in ES2015 (ES6) and not included in earlier ECMAScript standards. These properties are excluded from the environment bound by the with statement, preventing certain array methods from being added to the scope of the with statement. Use Array.prototype[Symbol.unscopables] to see the array default properties not included in the with binding.

Methods

Array.from()

Array.from(arrayLike[, mapFn[, thisArg]])
arrayLike: The pseudo-array object or iterable object to be converted into an array.
mapFn: Optional. If specified, this callback function will be executed for each element in the new array.
thisArg: Optional. The this object when executing the callback function mapFn.
The Array.from() method creates a new array instance from an array-like or iterable object.

console.log(Array.from("123")); // (3) ["1", "2", "3"]
console.log(Array.from([,,,])); // (3) [undefined, undefined, undefined] // Treats empty as undefined
console.log(Array.from({length: 3})); // (3) [undefined, undefined, undefined]

Array.isArray()

Array.isArray(obj)
Array.isArray() is used to determine whether the value passed is an Array.

console.log(Array.isArray([])); // true
console.log(Array.isArray(Array())); // true
console.log(Array.isArray(new Array())); // true
console.log(Array.isArray(new Uint8Array(32))); // false
console.log(Array.isArray(Array.prototype)); // true // Array.prototype is an array
console.log(Array.isArray({ __proto__: Array.prototype })); // false // Does not check the prototype chain, different from instanceof

/**
Polyfill
  if (!Array.isArray) {
    Array.isArray = function(arg) {
      return Object.prototype.toString.call(arg) === '[object Array]';
    };
  }
*/

Array.of()

Array.of(element0[, element1[, ...[, elementN]]])
elementN: Any number of arguments that will become elements in the returned array. The Array.of() method creates a new array instance with a variable number of arguments, regardless of the quantity or type of the arguments. The difference between Array.of() and the Array constructor lies in the handling of integer parameters. For example, Array.of(7) creates an array with a single element 7, while Array(7) creates an empty array with a length of 7.

console.log(Array.of(1));         // [1]
console.log(Array.of(1, 2, 3));   // (3) [1, 2, 3]
console.log(Array.of(undefined)); // [undefined]

Array.prototype.concat()

var new_array = old_array.concat(value1[, value2[, ...[, valueN]]])
The valueN is optional and is used to connect arrays and/or values into a new array. If the valueN parameters is omitted, concat will return a shallow copy of the existing array it was called on. The concat() method is used to merge two or more arrays, and does not change the existing arrays, but returns a new array.

var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];
var arr3 = arr1.concat(arr2);

console.log(arr3); // [1, 2, 3, 4, 5, 6]
console.log(arr1); // [1, 2, 3] // Does not change the original array

Array.prototype.copyWithin()

arr.copyWithin(target[, start[, end]])
target is the index to copy the sequence to, using 0 as the base. If it is negative, target will be counted from the end. If target is greater than or equal to the length of arr, no copying will take place. If target is after start, the sequence will be modified to fit arr.length.
start is the index to start copying elements from, using 0 as the base. If it is negative, start will be counted from the end, and if omitted, copyWithin will start from the beginning (0).
end is the index to end copying elements to, excluding the element at end. If it is negative, end will be counted from the end. If omitted, copyWithin will copy until the end of the array, defaulting to arr.length.
The copyWithin() method shallow copies a part of an array to another location in the same array and returns it, without changing the length of the original array.

var arr = [1, 2, 3, 4, 5, 6];
console.log(arr.copyWithin(0, 2, 5)); // [3, 4, 5, 4, 5, 6] // Shallow copies values from index 2 to 5 to the position with index 0
console.log(arr); // [3, 4, 5, 4, 5, 6] // Modifies the original array without changing its length

Array.prototype.entries()

arr.entries()
The entries() method returns a new Array Iterator object containing key/value pairs for each index in the array. The prototype __proto__:Array Iterator of the Array Iterator object has a next method that can be used to iterate and retrieve the [key, value] of the original array.

var arr = ['a', 'b', 'c'];
console.log(arr.entries().next().value); // [0, "a"]
console.log(arr); // ["a", "b", "c"] // Does not change the original array

Array.prototype.every()

arr.every(callback[, thisArg])
The callback is a function to test for each element in the array, which takes one to three arguments.
currentValue is the current element being processed in the array.
index is optional, the index of the current element being processed in the array.
array is optional, the array every was called upon.
thisArg is optional and is used as this when executing the callback function. If an arrow function expression is used to pass in callback, the thisArg parameter will be ignored, as arrow functions are lexically bound to their this value.
The every() method tests whether all elements in the array pass the test implemented by the provided function, and it returns a Boolean value.

var arr = [1, 2, 3, 4, 5];
console.log(arr.every( currentValue => currentValue > 1)) // false
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.fill()

arr.fill(value[, start[, end]])
value is the value to fill the elements of the array with.
start is optional, the index to start filling the array with, defaulting to 0.
end is optional, the index to end filling the array, defaulting to this.length.
The fill() method fills all the elements of an array from a start index to an end index with a static value, excluding the end index.

var arr = [1, 2, 3, 4, 5];
console.log(arr.fill(0, 0, 5)); // [0, 0, 0, 0, 0]
console.log(arr); // [0, 0, 0, 0, 0] // Modifies the original array

Array.prototype.filter()

arr.filter(callback(currentValue [, index [, array]])[, thisArg])
The callback is a function to test for each element in the array, which takes one to three arguments.
currentValue is the current element being processed in the array.
index is optional, the index of the current element being processed in the array.
array is optional, the array filter was called upon.
thisArg is optional and is used as this when executing the callback function. If an arrow function expression is used to pass in callback, the thisArg parameter will be ignored, as arrow functions are lexically bound to their this value.
The filter() method creates a new array with all elements that pass the test implemented by the provided function.

var arr = [1, 2, 3, 4, 5];
console.log(arr.filter( currentValue => currentValue > 2 )); // [3, 4, 5]
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.find()

arr.find(callback(currentValue [, index [, array]])[, thisArg])
The callback is a function executed for each element in the array, receiving one to three arguments.
currentValue is the current element being processed in the array.
index is an optional parameter representing the index of the current element being processed in the array.
array is an optional parameter representing the array being operated on.
thisArg is an optional parameter used as the this value when executing the callback function. It's worth noting that if an arrow function expression is used to pass in the callback, the thisArg parameter will be ignored, as arrow functions lexically bind the this value.
The find() method returns the value of the first element in the array that satisfies the provided testing function, otherwise it returns undefined.

var arr = [1, 2, 3, 4, 5];
console.log(arr.find( currentValue => currentValue > 2 )); // 3
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.findIndex()

arr.findIndex(callback(currentValue [, index [, array]])[, thisArg])
The callback is a function executed for each element in the array, receiving one to three arguments.
currentValue is the current element being processed in the array.
index is an optional parameter representing the index of the current element being processed in the array.
array is an optional parameter representing the array being operated on.
thisArg is an optional parameter used as the this value when executing the callback function. It's worth noting that if an arrow function expression is used to pass in the callback, the thisArg parameter will be ignored, as arrow functions lexically bind the this value.
The findIndex() method returns the index of the first element in the array that satisfies the provided testing function, otherwise it returns -1.

var arr = [1, 2, 3, 4, 5];
console.log(arr.findIndex( currentValue => currentValue > 2 )); // 2
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.flat()

var newArray = arr.flat([depth])
depth is an optional parameter that specifies the depth of nesting to flatten. The default value is 1.
The flat() method recursively flattens the array to the specified depth and returns a new array with all elements and elements of sub-arrays merged into it.

var arr = [1, 2, [3, 4, [5]]];
console.log(arr.flat(2)); // [1, 2, 3, 4, 5]
console.log(arr); // [1, 2, [3, 4, [5]]] // Does not change the original array

Array.prototype.flatMap()

arr.flatMap(callback(currentValue [, index [, array]])[, thisArg])
The callback is a function executed for each element in the array, receiving one to three arguments.
currentValue is the current element being processed in the array.
index is an optional parameter representing the index of the current element being processed in the array.
array is an optional parameter representing the array being called upon by map.
thisArg is an optional parameter used as the this value when executing the callback function. It's worth noting that if an arrow function expression is used to pass in the callback, the thisArg parameter will be ignored, as arrow functions lexically bind the this value.
The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It's almost identical to map followed by a depth of 1 flat, but flatMap is generally slightly more efficient in some cases when combining into one method.

var arr = [[1], [2], [3], [4], [5]];
console.log(arr.flatMap( currentValue => currentValue *2 )); // [2, 4, 6, 8, 10] // First calls the mapping function, then flattens into a new array
console.log(arr); // [[1], [2], [3], [4], [5]] // Does not change the original array

Array.prototype.forEach()

arr.forEach(callback(currentValue [, index [, array]])[, thisArg])
The callback is a function executed for each element in the array, receiving one to three arguments.
currentValue is the current element being processed in the array.
index is an optional parameter representing the index of the current element being processed in the array.
array is an optional parameter representing the array being operated on.
thisArg is an optional parameter used as the this value when executing the callback function. It's worth noting that if an arrow function expression is used to pass in the callback, the thisArg parameter will be ignored, as arrow functions lexically bind the this value.
The forEach() method executes the given function once for each element in the array.
Note that if you want to end the iteration before it's completed, forEach is not the best choice as compared to map.

var arr = [1, 2, 3, 4, 5];
var obj = { a: 1 }; // Defined obj for demonstration of 'this'
arr.forEach( function(currentValue,index,array) {
    console.log("Current value",currentValue);
    console.log("Current value index",index);
    console.log("Current processing array",array);
    console.log("Current this reference",this);
    console.log("End of one callback, no need to return a value");
    console.log("");
},obj);
/*
    Current value 1
    Current value index 0
    Current processing array (5)[1, 2, 3, 4, 5]
    Current this reference {a: 1}
    End of one callback, no need to return a value
    
    Current value 2
    Current value index 1
    Current processing array (5)[1, 2, 3, 4, 5]
    Current this reference {a: 1}
    End of one callback, no need to return a value
    
    ...........
*/
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.includes()

arr.includes(valueToFind[, fromIndex])
valueToFind - The value to search for.
fromIndex - Optional. The position in the array at which to begin searching for valueToFind. If negative, it's an offset from the end of array.length + fromIndex, even if fromIndex is greater than array.length. If omitted, fromIndex is set to 0.
The includes() method determines whether an array includes a certain value among its entries, returning true or false as appropriate.

var arr = [1, 2, 3, 4, 5];
console.log(arr.includes(2)); // true
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.indexOf()

arr.indexOf(searchElement[, fromIndex])
searchElement - The element to search for within the array.
fromIndex - Optional. The starting index at which to begin the search for valueToFind. If the index is negative, it is taken as the offset from the end of the array array.length + fromIndex. The default is 0.
The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

var arr = [1, 2, 3, 4, 5];
console.log(arr.indexOf(2)); // 1
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.join()

arr.join([separator])
separator - Optional. A string used to separate one element of the array from the next in the resulting string. If omitted, the array elements are separated with a comma.
The join() method joins all the elements of an array (or an array-like object) into a string and returns the string. If the array has only one item, then that item will be returned without using the separator.

var arr = ['a', 'b', 'c'];
console.log(arr.join('&')); // a&b&c
console.log(arr); // ["a", "b", "c"] // Original array remains unchanged

Array.prototype.keys()

arr.keys()
The keys() method returns an array iterator object with the keys of the array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.keys().next().value); // 0
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.lastIndexOf()

arr.lastIndexOf(searchElement[, fromIndex])
searchElement - The element to locate in the array.
fromIndex - Optional. The index at which to start the search in reverse. Defaults to arr.length - 1. If the index is greater than or equal to the array's length, -1 is returned, and the array is not searched. If negative, it is taken as the offset from the end of the array. If the absolute value of fromIndex is greater than the length of the array, the entire array will be searched.
The lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.

var arr = [1, 2, 3, 2, 1];
console.log(arr.lastIndexOf(2)); // 3
console.log(arr); // [1, 2, 3, 2, 1] // Original array remains unchanged

Array.prototype.map()

arr.map(callback(currentValue [, index [, array]])[, thisArg])
callback - Function that produces an element of the new Array, taking three arguments:
currentValue - The current element being processed in the array.
index - Optional. The index of the current element being processed in the array.
array - Optional. The array map was called upon.
thisArg - Optional. Value to use as this when executing callback, if a function is used. Notice that if an arrow function is used as callback, thisArg is ignored, as arrow functions do not have their own bindings of this.
The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

var arr = [1, 2, 3, 4, 5];
var obj = { a: 1 }; // Define obj for demonstrating the use of this
var newArr = arr.map( function(currentValue,index,array) {
    console.log("Current value",currentValue);
    console.log("Current value index",index);
    console.log("Current array being processed",array);
    console.log("Current this pointer",this);
    console.log("");
    return currentValue + 10; // Return the arr value plus 10 to form a new array
},obj);
console.log(newArr); // [11, 12, 13, 14, 15]
/*
    Current value 1
    Current value index 0
    Current array being processed [1, 2, 3, 4, 5]
    Current this pointer {a: 1}
    
    Current value 2
    Current value index 1
    Current array being processed [1, 2, 3, 4, 5]
    Current this pointer {a: 1}
    
    ...........
*/
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.pop()

arr.pop()
The pop() method removes the last element from an array and returns its value. When the array is empty, it returns undefined. This method changes the length of the array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.pop()); // 5
console.log(arr); // [1, 2, 3, 4] // Modifies the original array

Array.prototype.push()

arr.push(element1[, ..., elementN])
elementN - The elements to add to the end of the array.
The push() method adds one or more elements to the end of an array and returns the new length of the array.

var arr = ['a', 'b', 'c', 'd', 'e'];
console.log(arr.push('f','g')); // 7
console.log(arr); // ["a", "b", "c", "d", "e", "f", "g"] // Modifies the original array

Array.prototype.reduce()

arr.reduce(callback(accumulator, currentValue[, index[, array]])[, initialValue])
callback - A function that executes on each value in the array (excluding the first value if no initialValue is provided), taking between two and four arguments.
accumulator - The accumulated value of the callback's return values. It is the accumulated value returned by the last call to the callback, or initialValue.
currentValue - The current element being processed in the array.
index - Optional. The index of the current element being processed in the array. If an initialValue is provided, the index starts at 0; otherwise, it starts at 1.
array - Optional. The array reduce() was called upon.
initialValue - Optional. The initial value to use as the first argument when calling the callback function. If no initial value is provided, the first element in the array will be used. Calling reduce on an empty array without an initial value will result in an error.
The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

var arr = [1, 2, 3, 4, 5];
console.log(arr.reduce((accumulator,currentValue) => accumulator+currentValue , 5)); // 20 // 5 + 1 + 2 + 3 + 4 + 5
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.reduceRight()

arr.reduceRight(callback(accumulator, currentValue[, index[, array]])[, initialValue])
callback - A function that executes on each value in the array, taking between two and four arguments.
accumulator - The accumulated value of the callback's return values. It is the accumulated value returned by the last call to the callback, or initialValue.
currentValue - The current element being processed in the array.
index - Optional. The index of the current element being processed in the array. If an initialValue is provided, the index starts at 0; otherwise, it starts at 1.
array - Optional. The array reduce() was called upon.
initialValue - Optional. The initial value of the accumulator accumulator when the callback function is called the first time. If not provided, the last element in the array will be used, skipping that element. Calling reduce on an empty array without an initial value will result in an error.
The reduceRight() method reduces the array to a single value from right to left using a function as an accumulator.

var arr = [1, 2, 3, 4, 5];
console.log(arr.reduceRight((accumulator,currentValue) => accumulator+currentValue , 5)); // 20 // 5 + 5 + 4 + 3 + 2 + 1
console.log(arr); // [1, 2, 3, 4, 5] // Does not change the original array

Array.prototype.reverse()

arr.reverse()
The reverse() method reverses the order of the elements in an array and returns the array. This method changes the original array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.reverse()); // [5, 4, 3, 2, 1]
console.log(arr); // [5, 4, 3, 2, 1] // Modifies the original array

Array.prototype.shift()

arr.shift()
The shift() method removes the first element from an array and returns that element. This method changes the original array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.shift()); // 1
console.log(arr); // [2, 3, 4, 5] // Original array altered

Array.prototype.slice()

arr.slice([begin[, end]])
begin Optional The starting index to extract, from where the original array elements begin to be extracted. If this parameter is negative, it represents extracting from the last element of the original array. If begin is omitted, slice begins from index 0. If begin is greater than the length of the original array, an empty array will be returned.
end Optional The ending index to extract, where the extraction of original array elements ends. slice extracts all elements from the index begin to end, including begin, but not including end. If end is omitted, slice continues to extract to the end of the original array. If end is greater than the length of the array, slice continues to extract to the end of the original array.
The slice() method returns a new array object, which is a shallow copy of the original array determined by begin and end, includes begin but not includes end, the original array remains unchanged.

var arr = [1, 2, 3, 4, 5];
console.log(arr.slice(1, -2)); // [2, 3]
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.some()

arr.some(callback(currentValue [, index [, array]])[, thisArg])
callback A function executed for each element in the array, this function takes one to three parameters.
currentValue The current element being processed in the array.
index Optional The index of the current element being processed in the array.
array Optional The array being operated on.
thisArg Optional The value used as this when executing the callback function, note that if an arrow function expression is used to pass in callback, the thisArg parameter is ignored because arrow functions bind the this value lexically.
The some() method tests whether at least one element in the array passes the test provided by the function, it returns a boolean value.

var arr = [1, 2, 3, 4, 5];
console.log(arr.some( currentValue => currentValue > 1)) // true
console.log(arr); // [1, 2, 3, 4, 5] // Original array remains unchanged

Array.prototype.sort()

arr.sort([compareFunction])
compareFunction Optional A function that specifies the order in which to sort. If omitted, elements are sorted according to the Unicode code points of their string representations.
firstEl The first element for comparison.
secondEl The second element for comparison.
The sort() method sorts the elements of an array in place and returns the array. The default sort order is built upon converting the elements into strings, then comparing their sequence of UTF-16 code unit values.

var arr = [1, 2, 3, 4, 5];
console.log(arr.sort( (a,b) =>  b - a)) // [5, 4, 3, 2, 1]
console.log(arr); // [5, 4, 3, 2, 1] // Original array altered

Array.prototype.splice()

array.splice(start[, deleteCount[, item1[, item2[, ...]]]])
start Specifies the starting point of the modification, if it exceeds the length of the array, contents will be added from the end of the array; if it is negative, it indicates the position from the end of the array (counting from -1, which means -n is the nth element from the end and equivalent to array.length-n); if the absolute value of the negative number is greater than the length of the array, the starting position is the 0th position.
deleteCount Optional An integer that specifies the number of array elements to remove. If deleteCount is greater than the total number of elements after start, all elements after start (including the element at start) will be deleted. If deleteCount is omitted or its value is greater than or equal to array.length - start (i.e., if it is greater than or equal to the number of elements after start), all elements after start in the array will be deleted.
item1, item2, ... Optional Elements to be added to the array, starting from the start position. If not specified, splice() will only delete array elements.
The splice() method modifies the array by deleting or replacing existing elements or adding new elements in place, and returns the modified content as an array. This method will change the original array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.splice(1, 1)); // [2]
console.log(arr); // [1, 3, 4, 5] // Original array altered

Array.prototype.toLocaleString()

arr.toLocaleString([locales[,options]])
locales Optional A string or an array of strings with BCP 47 language tags.
options An object with properties that configure its behavior. toLocaleString() returns a string representing the elements of the array. The elements in the array are converted to strings using their own toLocaleString method, and these strings are separated by a locale-specific string.

var arr = [1, 'a', new Date('21 Dec 1997 14:12:00 UTC')];
console.log(arr.toLocaleString('en', {timeZone: "UTC"})); // 1,a,12/21/1997, 2:12:00 PM
console.log(arr); // [1, "a", Sun Dec 21 1997 22:12:00 GMT+0800 (中国标准时间)] // Original array remains unchanged

Array.prototype.toString()

arr.toString()
toString() returns a string that represents the specified array and its elements.

var arr = [1, 'a', new Date('21 Dec 1997 14:12:00 UTC')];
console.log(arr.toString()); // 1,a,Sun Dec 21 1997 22:12:00 GMT+0800 (中国标准时间)
console.log(arr); // [1, "a", Sun Dec 21 1997 22:12:00 GMT+0800 (中国标准时间)] // Doesn't change the original array

Array.prototype.unshift()

arr.unshift(element1[, ..., elementN])
elementN is the element or elements to add to the beginning of the array.
The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array, modifying the original array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.unshift(-1, 0)); // 7
console.log(arr); // [-1, 0, 1, 2, 3, 4, 5] // Modifies the original array

Array.prototype.values()

arr.values()
The values() method returns a new Array Iterator object that contains the values for each index in the array.

var arr = [1, 2, 3, 4, 5];
console.log(arr.values().next().value); // 1
console.log(arr); // [1, 2, 3, 4, 5] // Doesn't change the original array

Array.prototype[@@iterator]()

arr[Symbol.iterator]()
The @@iterator property's initial value of arr is the same function object as the Array.prototype.values() property, by default it returns the same value as values(), and arr[Symbol.iterator] will return the values() function.

var arr = [1, 2, 3, 4, 5];
console.log(...arr[Symbol.iterator]()); // 1 2 3 4 5
console.log(arr); // [1, 2, 3, 4, 5] // Doesn't change the original array

Array[@@species]

Array[Symbol.species]
The Array[@@species] accessor property returns the constructor of Array.

console.log(Array[Symbol.species] === Array); // true

Daily Question

https://github.com/WindrunnerMax/EveryDay

Reference

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array

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Array object in JavaScript#

Description#

Properties#

Methods#

Array.from()#

Array.isArray()#

Array.of()#

Array.prototype.concat()#

Array.prototype.copyWithin()#

Array.prototype.entries()#

Array.prototype.every()#

Array.prototype.fill()#

Array.prototype.filter()#

Array.prototype.find()#

Array.prototype.findIndex()#

Array.prototype.flat()#

Array.prototype.flatMap()#

Array.prototype.forEach()#

Array.prototype.includes()#

Array.prototype.indexOf()#

Array.prototype.join()#

Array.prototype.keys()#

Array.prototype.lastIndexOf()#

Array.prototype.map()#

Array.prototype.pop()#

Array.prototype.push()#

Array.prototype.reduce()#

Array.prototype.reduceRight()#

Array.prototype.reverse()#

Array.prototype.shift()#

Array.prototype.slice()#

Array.prototype.some()#

Array.prototype.sort()#

Array.prototype.splice()#

Array.prototype.toLocaleString()#

Array.prototype.toString()#

Array.prototype.unshift()#

Array.prototype.values()#

Array.prototype[@@iterator]()#

Array[@@species]#

Daily Question#

Reference#