如何从数组中获取最后n个元素

Question

I'm working with angular4 and I've been having a lot of trouble trying to display some elements of an array in my view. I got this

the number of elements can change according to the current month so if it was February it would be only M01 and M02 in the array. I've been trying to loop it in for loop but I can't seem to get it right. any ideas on how to do it?

I'd like to get something like this, all values that start with "M" in a single property

[0:  M: [...]
     cod_item:...
     cod_condominio:...
...
]

it doesn't matter if it is an array or object as long as i can loop that element to show it in my view.

Answer 1

You can use nested loops to check is the current property begins with "M" , set the properties, values to a an object, push the object to an array

 const arr = [{M01:1, M02:2, M03:3, A:4, B:5}, {M04:6, M05:7}]; let res = []; let match = "M"; for (let o of Object.values(arr)) { const curr = {}; for (let [key, prop, [k] = key] of Object.entries(o)) { if (k === match) { curr[key] = prop; } } res.push(curr); } console.log(res); 

Answer 2

If you want just the properties starting with "M", you can use reduce to loop over all the objects and collect just those elements:

 // data var arr = [{}, { M0:'foo', M1:'bar', M2:'fum', blah:'blah' }, { M0:'foo', blah:'blah', gee:'gee' } ]; // Build an array of objects only containing // properties starting with "M" followed by digits var result = arr.reduce(function(acc, obj) { var o = {}; // Loop over object keys Object.keys(obj).forEach(function(key){ // Collect those matching the required pattern if (/^M\\d+$/.test(key)) { o[key] = obj[key]; } }); // If found some properties, push into accumulator if (Object.keys(o).length) acc.push(o); return acc; }, []); // Show results console.log(result) // Concise var r = arr.reduce((acc, obj) => { var o = {}; Object.keys(obj).forEach(key => { if (/^M\\d+$/.test(key)) o[key] = obj[key]; }); if (Object.keys(o).length) acc.push(o); return acc; }, []); console.log(r) 

Answer 3

This below solution is rookie way of doing it. But it would be technical debt.

The below snippet will give you array of elements based on current month.

 var d = new Date(); var n = d.getMonth(); a =[] for(i=1;i<=n;i++){ a.push('M'+('0'+(i).toString()).slice(-2)) } console.log(a) 

in angular template while looping over the object check if the key exist in the array

Answer 4

If you have access to backend, the best option is to change the response from your backend, it's not a good idea to refactor the response in front-end. So if you have access just change the response to get your needed items in a separated list.

But if you using a third party and have no access to backend code, you can use something like this to get only values of items which start with "M" :

var a = [
 {
   "M01": 1,
   "M02": 2,
   "code": 44
 },
 {
   "M01": 13,
   "M02": 4,
   "code": 3
 },{
   "M01": 4,
   "M02": 21,
   "code": 11
 }
]

 var t = [];

 for(var i in a){
    for(var j in a[i]){
         if (j[0]=="M") t.push(a[i][j])
     }  
 console.log(t);// it will only have values of items which start with "M"
}

https://jsfiddle.net/f772o781/1/

Answer 5

 const matchStr = (str) => { const matchArr = str.match(/^M(\\d\\d)/i); if (matchArr) return +matchArr[1]; return 0; } const extractor = (month) => (dataArr) => { return dataArr.map((data) => { const tempObj = {}; const keys = Object.keys(data); keys.forEach((key) => { if (matchStr(key) && matchStr(key) > month) return; tempObj[key] = data[key]; }); return tempObj }); } const yourData = [{ M01: '35238', M02: '123121', M03: '132222', cod_item: '1', filtro: 'saldo' }, { M01: '35238', M02: '123121', M03: '132222', cod_item: '1', filtro: 'saldo' }, { M01: '35238', M02: '123121', M03: '132222', cod_item: '1', filtro: 'saldo' } ]; console.log(extractor(2)(yourData));