如何在Javascript / ES6中获得这样的输出

Question

Let's say you have an array like this:

//Input [1,2,3,4,5,6,7]

How to write the function which will get us the output of

//Output

 Array1 = [1]
 Array2 = [1,2]
 Array3 = [1,2,3]
 Array4 = [1,2,3,4]
 Array5 = [1,2,3,4,5]

And

//Output 1. [1,1,2,1,2,3,1,2,3,4...]

// tried this

for (i = 0; i < arr.length; i++) { 
    arr = new Array(arr[i]);
}

Answer 1

You can use functions like Array#map and Array#slice to easily create the first function, and use Array#concat with spread syntax to flatten a 2D array.

 function prefixes(array) { return array.map((_, index) => array.slice(0, index + 1)); } function flatten(array) { return [].concat(...array); } const output = prefixes([1,2,3,4,5,6,7]); console.log(output); console.log(flatten(output)); 

Answer 2

You could reduce the array by slicing the wanted part with spread syntax ... .

 var array = [1, 2, 3, 4, 5, 6, 7], result = array.reduce((r, _, i, a) => [...r, ...a.slice(0, i + 1)], []); console.log(result); 

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Answer 3

Use forEach and for loop

 var arr = [1, 2, 3, 4, 5, 6, 7]; var arrys = []; //lopping over the array, and creating the new array and putting values // equal to number of index arr.forEach(function(item, index) { var tempArray = []; for (var m = 0; m <= index; m++) { tempArray.push(arr[m]) } arrys.push(tempArray) }) console.log(arrys) // flat the previously created array var flatten = []; arrys.forEach(function(item) { item.forEach(function(item2) { flatten.push(item2) }) }) console.log(flatten) 

Answer 4

You can do this simply by using two nested for loops like this,

 let arr = [1, 2, 3, 4, 5, 6, 7]; function foo(arr) { let rarr = [];//result array for (let i = 0; i < arr.length; i++) { let a = [];//array[i] for (let j = 0; j <= i; j++) { a.push(arr[j]); } rarr.push(a); } return rarr; } console.log(foo(arr));