Javascript：在对象数组中创建新列

Question

I have an array of objects like so:

var arr1 = [
    {"Date": "2017-04-15", "Price":"300"},
    {"Date": "2017-04-16", "Price":"310"},
    {"Date": "2017-04-17", "Price":"320"},
]

I wish to add a new key( newDate )-value(JS Date object) pair in each row. That is, I want to end up with:

[
    {"newDate":2017-04-14T14:00:00.000Z, "Date": "2017-04-15", "Price":"300"},
    {"newDate":2017-04-15T14:00:00.000Z, "Date": "2017-04-16", "Price":"310"},
    {"newDate":2017-04-16T14:00:00.000Z, "Date": "2017-04-17", "Price":"320"},
]

My script so far, which I run in node :

function getDateObject(dateString) {
    // Splits a yyyy-mm-dd string, and returns a Date object
    var parts = dateString.split("-")
    var myDate = new Date(parts[0], parts[1]-1, parts[2])
    return myDate


for (var i = 0; i < arr1.length; i++) {
    console.log(arr1[i])
    arr1[i]["newDate"] = getDateObject(arr1[i]["Date"])
}

This gives an error:

TypeError: Cannot read property 'split' of undefined at getDateObject

That is, it's throwing at error at the line: var parts = dateString.split("-") . I'm puzzled by this syntax error because of course dateString is not defined; it's meant to be an input parameter, rather than a variable instantiated by var , let or const or something. Am I missing something?

Bonus: would be great if someone could suggest a way to do this with array.map .

Answer 1

Another solution using Array.map() and destructuring could be next one:

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"}, ]; console.log( arr1.map(o => ({newDate: new Date(o.Date), ...o})) );

 .as-console {background-color:black !important; color:lime;} .as-console-wrapper {max-height:100% !important; top:0;}

Or use Array.forEach() if you don't mind to change the original array :

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"}, ]; arr1.forEach(o => o.newDate = new Date(o.Date)); console.log(arr1);

 .as-console {background-color:black !important; color:lime;} .as-console-wrapper {max-height:100% !important; top:0;}

Answer 2

In your code arr[i] is has no key 0 . If you want to access the Date use Property Accessors arr[i].Date or you can use arr[i]['Date']
A better way would be to use map() and Object.assign()

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"}, ] arr1 = arr1.map(obj => (Object.assign(obj,{newDate:new Date(obj.Date)}))); console.log(arr1);

Answer 3

arr1 is an array of objects, not an array of arrays. Referencing arr1[i][0] only makes sense if you're trying to get something from the 0th property on the object being iterated over, but no such property exists, thus undefined . Pass arr[i].date instead:

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"} ] function getDateObject(dateString) { // Splits a yyyy-mm-dd string, and returns a Date object var parts = dateString.split("-") var myDate = new Date(parts[0], parts[1]-1, parts[2]) return myDate } for (var i = 0; i < arr1.length; i++) { arr1[i]["newDate"] = getDateObject(arr1[i].Date) } console.log(arr1);

Or, with .map :

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"} ]; const arr2 = arr1.map((obj) => { const [y, m, d] = obj.Date.split('-'); return { ...obj, newDate: new Date(y, m - 1, d) }; }); console.log(arr2);

Answer 4

I think you need to access "Date" instead of 0 in your for-loop

arr1[i]["newDate"] = getDateObject(arr1[i]["Date"])

// or

arr1[i].newDate = getDateObject(arr1[i].Date)

Answer 5

Since your Date property of the array object in valid JS date format you can simply use new Date() function of JS to create newDate property following sinpet is simplest pure JS code

 var arr1 = [ {"Date": "2017-04-15", "Price":"300"}, {"Date": "2017-04-16", "Price":"310"}, {"Date": "2017-04-17", "Price":"320"} ] //adding new date for(var i = 0; i < arr1.length; i++) arr1[i].newDate = new Date(arr1[i].Date); //test console.log(arr1)