Javascript如何为未定义的对象键赋值？

Question

I would like to programatically assign values to 'not yet' initialized key in object, like below:

const a = {};
// year, month, date is a variable
a[year][month][date] = some data; 

I know above wouldn't work unless I assign year and month before assigning to date key,

but I am assigning this programatically so I can't do something like

a:{
  2020:{
    10:{
      3: some data
    }
  }
}

manually like this.

What would be the best way to assign value to undefined key?

Answer 1

There is no simple way to do that.

You would need to so something like:

a[year] = {}
a[year][month] = {}
a[year][month][date] = 'some data';

You could use some function like this:

function setByPath(obj, path, value) {
    var parts = path.split('.');
    var o = obj;
    if (parts.length > 1) {
      for (var i = 0; i < parts.length - 1; i++) {
          if (!o[parts[i]])
              o[parts[i]] = {};
          o = o[parts[i]];
      }
    }

    o[parts[parts.length - 1]] = value;
}

Usage:

setByPath(a, 'year.month.date', 'some data');

Please note that this function is only as an example, and it can cause problems in certain cases. It is to show the idea.

Answer 2

You could iterate the keys without the last one for assigning the value.

 const assign = (object, [...keys], value) => { const last = keys.pop(); keys.reduce((o, k) => o[k] ??= {}, object)[last] = value }, a = {}; assign(a, [2020, 10, 3], 42); console.log(a);