如何获取数组的第一个元素？

Question

How do you get the first element from an array like this:

var ary = ['first', 'second', 'third', 'fourth', 'fifth'];

I tried this:

alert($(ary).first());

But it would return [object Object] . So I need to get the first element from the array which should be the element 'first' .

Answer 1

像这样

alert(ary[0])

Answer 2

Why are you jQuery-ifying a vanilla JavaScript array? Use standard JavaScript!

var ary = ['first', 'second', 'third', 'fourth', 'fifth'];
alert(ary[0]);

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

Also,

Source , courtesy of bobince

Answer 3

Some of ways below for different circumstances.

In most normal cases, the simplest way to access the first element is by

yourArray[0]

but this requires you to check if [0] actually exists.

Anther commonly used method is shift() but you should avoid using this for the purpose of accessing the first element.

Well, this method modifies the original array (removes the first item and returns it) but re-indexes what is left in the array to make it start from 0 (shifts everything down). Therefore the length of an array is reduced by one. There are good cases where you may need this, for example, to take the first customer waiting in the queue, but it is very inefficient to use this for the purpose of accessing the first element.

In addition, this requires a perfect array with [0] index pointer intact, exactly as using [0];

yourArray.shift()

The important thing to know is that the two above are only an option if your array starts with a [0] index.

There are cases where the first element has been deleted, example with, delete yourArray[0] leaving your array with "holes". Now the element at [0] is simply undefined, but you want to get the first "existing" element. I have seen many real world cases of this.

So, assuming we have no knowledge of the array and the first key (or we know there are holes), we can still get the first element.

You can use find() to get the first element.

The advantage of find() is its efficiency as it exits the loop when the first value satisfying the condition is reached (more about this below). (You can customize the condition to exclude null or other empty values too)

var firstItem = yourArray.find(x=>x!==undefined);

I'd also like to include filter() here as an option to first "fix" the array in the copy and then get the first element while keeping the the original array intact (unmodified).

Another reason to include filter() here is that it existed before find() and many programmers have already been using it (it is ES5 against find() being ES6).

var firstItem = yourArray.filter(x => typeof x!==undefined).shift();

Warning that filter() is not really an efficient way (filter() runs through all elements) and creates another array. It is fine to use on small arrays as performance impact would be marginal, closer to using forEach, for example.

Another one I have seen in some projects is splice() to get the first item in an array and then get it by index:

var firstItem = yourArray.splice(0, 1)[0];

I am not sure why you would do that. This method won't solve the problem with holes in array (sparse array). It is costly as it will re-index the array, and it returns an array and you have to access again to get the value. For example, if you delete first couple of elements and splice will return undefined instead of the first defined value from array.

(I see some people suggest using for...in loop to get the first element, but I would recommend against this method for...in should not be used to iterate over an Array where the index order is important because it doesn't guarantee the order although you can argue browsers mostly respect the order.By the way, forEach doesn't solve the issue as many suggest because you cant break it and it will run through all elements. You would be better off using a simple for loop and by checking key/value

Both find() and filter() guarantee the order of elements, so are safe to use as above.

Answer 4

Using ES6 destructuring

let [first] = [1,2,3];

Which is the same as

let first = [1,2,3][0];

Answer 5

You can just use find() :

const first = array.find(Boolean)

Or if you want the first element even if it is falsy :

const first = array.find(() => true)

Or if you want the first element even if it is falsy but not if it is null or undefined ( more information ):

const first = array.find(e => typeof e !== 'undefined')

---

Going the extra mile:

If you care about readability but don't want to rely on numeric incidences you could add a first() -function to Array.prototype by defining it with Object​.define​Property() which mitigates the pitfalls of modifying the built-in Array object prototype directly ( explained here ).

Performance is pretty good ( find() stops after the first element) but it isn't perfect or universally accessible (ES6 only). For more background read @Selays answer .

Object.defineProperty(Array.prototype, 'first', {
  value() {
    return this.find(e => true)     // or this.find(Boolean)
  }
})

To retrieve the first element you are now able to do this:

const array = ['a', 'b', 'c']
array.first()

> 'a'

Snippet to see it in action:

 Object.defineProperty(Array.prototype, 'first', { value() { return this.find(Boolean) } }) console.log( ['a', 'b', 'c'].first() )

Answer 6

Element of index 0 may not exist if the first element has been deleted:

 let a = ['a', 'b', 'c']; delete a[0]; for (let i in a) { console.log(i + ' ' + a[i]); }

Better way to get the first element without jQuery:

 function first(p) { for (let i in p) return p[i]; } console.log( first(['a', 'b', 'c']) );

Answer 7

If you want to preserve the readibility you could always add a first function to the Array.protoype :

Array.prototype.first = function () {
    return this[0];
};

A then you could easily retrieve the first element:

[1, 2, 3].first();
> 1

Answer 8

If your array is not guaranteed to be populated from index zero, you can use Array.prototype.find() :

var elements = []
elements[1] = 'foo'
elements[2] = 'bar'

var first = function(element) { return !!element }    
var gotcha = elements.find(first)

console.log(a[0]) // undefined
console.log(gotcha) // 'foo'

Answer 9

array.find(e => !!e);  // return the first element 

since "find" return the first element that matches the filter && !!e match any element.

Note This works only when the first element is not a "Falsy" : null , false , NaN , "" , 0 , undefined

Answer 10

In ES2015 and above, using array destructuring:

 const arr = [42, 555, 666, 777] const [first] = arr console.log(first)

Answer 11

只有在您使用 underscore.js ( http://underscorejs.org/ ) 的情况下，您才能执行以下操作：

_.first(your_array);

Answer 12

I know that people which come from other languages to JavaScript, looking for something like head() or first() to get the first element of an array, but how you can do that?

Imagine you have the array below:

const arr = [1, 2, 3, 4, 5];

In JavaScript, you can simply do:

const first = arr[0];

or a neater, newer way is:

const [first] = arr;

But you can also simply write a function like...

function first(arr) {
   if(!Array.isArray(arr)) return;
   return arr[0];
}

If using underscore, there are list of functions doing the same thing you looking for:

_.first 

_.head

_.take

Answer 13

Try alert(ary[0]); .

Answer 14

ES6 Spread operator + .shift() solution

Using myArray.shift() you can get the 1st element of the array, but .shift() will modify the original array, so to avoid this, first you can create a copy of the array with [...myArray] and then apply the .shift() to this copy:

 var myArray = ['first', 'second', 'third', 'fourth', 'fifth']; var first = [...myArray].shift(); console.log(first);

Answer 15

Method that works with arrays , and it works with objects too (beware, objects don't have a guaranteed order!).

I prefer this method the most, because original array is not modified.

// In case of array
var arr = [];
arr[3] = 'first';
arr[7] = 'last';
var firstElement;
for(var i in arr){
    firstElement = arr[i];
    break;
}
console.log(firstElement);  // "first"

// In case of object
var obj = {
    first: 'first',
    last: 'last',
};

var firstElement;

for(var i in obj){
    firstElement = obj[i];
    break;
}

console.log(firstElement) // First;

Answer 16

I prefer to use Array Destructuring

const [first, second, third] = ["Laide", "Gabriel", "Jets"];
console.log(first);  // Output: Laide
console.log(second); // Output: Gabriel
console.log(third);  // Output: Jets

Answer 17

Find the first element in an array using a filter:

In typescript:

function first<T>(arr: T[], filter: (v: T) => boolean): T {
    let result: T;
    return arr.some(v => { result = v; return filter(v); }) ? result : undefined;
}

In plain javascript:

function first(arr, filter) {
    var result;
    return arr.some(function (v) { result = v; return filter(v); }) ? result : undefined;
}

And similarly, indexOf:

In typescript:

function indexOf<T>(arr: T[], filter: (v: T) => boolean): number {
    let result: number;
    return arr.some((v, i) => { result = i; return filter(v); }) ? result : undefined;
}

In plain javascript:

function indexOf(arr, filter) {
    var result;
    return arr.some(function (v, i) { result = i; return filter(v); }) ? result : undefined;
}

Answer 18

另一个只关注真实元素的人

ary.find(Boolean);

Answer 19

Just use ary.slice(0,1).pop();

In

 var ary = ['first', 'second', 'third', 'fourth', 'fifth']; console.log("1º "+ary.slice(0,1).pop()); console.log("2º "+ary.slice(0,2).pop()); console.log("3º "+ary.slice(0,3).pop()); console.log("4º "+ary.slice(0,4).pop()); console.log("5º "+ary.slice(0,5).pop()); console.log("Last "+ary.slice(-1).pop());

array.slice(START,END).pop();

Answer 20

Why not account for times your array might be empty?

var ary = ['first', 'second', 'third', 'fourth', 'fifth'];
first = (array) => array.length ? array[0] : 'no items';
first(ary)
// output: first

var ary = [];
first(ary)
// output: no items

Answer 21

When there are multiple matches, JQuery's .first() is used for fetching the first DOM element that matched the css selector given to jquery.

You don't need jQuery to manipulate javascript arrays.

Answer 22

You could also use .get(0):

alert($(ary).first().get(0));

To get the first element of the array.

Answer 23

Declare a prototype to get first array element as:

Array.prototype.first = function () {
   return this[0];
};

Then use it as:

var array = [0, 1, 2, 3];
var first = array.first();
var _first = [0, 1, 2, 3].first();

Or simply (:

first = array[0];

Answer 24

The previous examples work well when the array index begins at zero. thomax's answer did not rely on the index starting at zero, but relied on Array.prototype.find to which I did not have access. The following solution using jQuery $.each worked well in my case.

let haystack = {100: 'first', 150: 'second'},
    found = null;

$.each(haystack, function( index, value ) {
    found = value;  // Save the first array element for later.
    return false;  // Immediately stop the $.each loop after the first array element.
});

console.log(found); // Prints 'first'.

Answer 25

 var ary = ['first', 'second', 'third', 'fourth', 'fifth']; alert(Object.values(ary)[0]);

Answer 26

If you're chaining a view functions to the array eg

array.map(i => i+1).filter(i => i > 3)

And want the first element after these functions you can simply add a .shift() it doesn't modify the original array , its a nicer way then array.map(i => i+1).filter(=> i > 3)[0]

If you want the first element of an array without modifying the original you can use array[0] or array.map(n=>n).shift() (without the map you will modify the original. In this case btw i would suggest the ..[0] version.

Answer 27

You can do it by lodash _.head so easily.

 var arr = ['first', 'second', 'third', 'fourth', 'fifth']; console.log(_.head(arr));

 <script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

Answer 28

try

var array= ['first', 'second', 'third', 'fourth', 'fifth'];
firstElement = array[array.length - array.length];

https://playcode.io/908187

Answer 29

@NicoLwk You should remove elements with splice, that will shift your array back. So:

var a=['a','b','c'];
a.splice(0,1);
for(var i in a){console.log(i+' '+a[i]);}

Answer 30

Use this to split character in javascript.

var str = "boy, girl, dog, cat";
var arr = str.split(",");
var fst = arr.splice(0,1).join("");
var rest = arr.join(",");

Answer 31

var ary = ['first', 'second', 'third', 'fourth', 'fifth'];

console.log(Object.keys(ary)[0]);

Make any Object array ( req ), then simply do Object.keys(req)[0] to pick the first key in the Object array.

Answer 32

Using ES6.

let arr = [22,1,4,55,7,8,9,3,2,4];

let {0 : first ,[arr.length - 1] : last} = arr;
console.log(first, last);

or

let {0 : first ,length : l, [l - 1] : last} = [22,1,4,55,7,8,9,3,2,4];
console.log(first, last);

Answer 33

var ary = ["first", "second", "third", "fourth", "fifth"];
console.log(ary.shift());
//first

Answer 34

@thomax 's answer is pretty good, but will fail if the first element in the array is false or false-y (0, empty string, etc.). Better to just return true for anything other than undefined:

const arr = [];
arr[1] = '';
arr[2] = 'foo';

const first = arr.find((v) => { return (typeof v !== 'undefined'); });
console.log(first); // ''

Answer 35

ES6 easy:

let a = []
a[7] = 'A'
a[10] = 'B'

let firstValue = a.find(v => v)
let firstIndex = a.findIndex(v => v)