我怎样才能返回 JavaScript 数组中所有以前的项目而不是当前值？

Question

Let's say I have an array:

var myArr = new Array('alpha','beta','gamma','delta');

And that I want a function to return an array of all items before a given item:

function getAllBefore(current) {
    var myArr = new Array('alpha','beta','gamma','delta');
    var newArr = ???
    return newArr;
}

getAllBefore('beta'); // returns Array('alpha');
getAllBefore('delta'); // returns Array('alpha','beta','gamma');

What's the fastest way to get this? Can I split an array on a value? Do I have to loop each one and build a new array on the fly? What do you recommend?

What about if I wanted the opposite, ie getAllAfter() ?

Answer 1

function getAllBefore(current) {
    var myArr = new Array('alpha','beta','gamma','delta');
    var i = myArr.indexOf(current);
    return i > -1 ? myArr.slice(0, i) : [];
}

Get the index of the specified item. If found, .slice() from 0 to that index. If not found, return an empty array (or whatever other default value you like).

Note that .indexOf() is not supported (for arrays) in IE8 and older, but there is a shim you can use, or you could just use a simple for loop instead.

Answer 2

javascript slice array

// array.slice(start, end)
const FRUITS = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = FRUITS.slice(1, 3);
// citrus => [ 'Orange', 'Lemon' ]

// Negative values slice in the opposite direction
var fromTheEnd = FRUITS.slice(-3, -1);
// fromTheEnd => [ 'Lemon', 'Apple' ]

array cut only last 5 element

 arr.slice(Math.max(arr.length - 5, 0))

Answer 3

使用indexOf和slice ：

newArr = myArr.slice(0, myArr.indexOf(current));

Answer 4

Try something like this

var index = myArr.indexOf('beta');
var before = myArray.slice(0, index);

Answer 5

I recently had to do something like this for an array of objects. This is what I went with:

const myArr = [
    { controlId: 1, value: 'alpha'},
    { controlId: 2, value: 'beta' },
    { controlId: 3, value: 'gamma' },
    { controlId: 4, value: 'delta'}
];

function getAllBefore(id) {
    const index = myArr.findIndex( ({ controlId }) => controlId === id);
    return myArr.filter((_, i) => i < index);
}