How to use a variable for a key in a JavaScript object literal?

问题

Why does the following work?

<something>.stop().animate(
    { \'top\' : 10 }, 10
);

Whereas this doesn\'t work:

var thetop = \'top\';
<something>.stop().animate(
    { thetop : 10 }, 10
);

To make it even clearer: At the moment I\'m not able to pass a CSS property to the animate function as a variable.

回答1:

{ thetop : 10 } is a valid object literal. The code will create an object with a property named thetop that has a value of 10. Both the following are the same:

obj = { thetop : 10 };
obj = { "thetop" : 10 };

In ES5 and earlier, you cannot use a variable as a property name inside an object literal. Your only option is to do the following:

var thetop = "top";

// create the object literal
var aniArgs = {};

// Assign the variable property name with a value of 10
aniArgs[thetop] = 10; 

// Pass the resulting object to the animate method
<something>.stop().animate(
    aniArgs, 10  
);

ES6 defines ComputedPropertyName as part of the grammar for object literals, which allows you to write the code like this:

var thetop = "top",
    obj = { [thetop]: 10 };

console.log(obj.top); // -> 10

You can use this new syntax in the latest versions of each mainstream browser.

回答2:

With ECMAScript 2015 you are now able to do it directly in object declaration with the brackets notation:

var obj = {
  [key]: value
}

Where key can be any sort of expression (e.g. a variable) returning a value.

So here your code would look like:

<something>.stop().animate({
  [thetop]: 10
}, 10)

Where thetop will be evaluated before being used as key.

回答3:

ES5 quote that says it should not work

Note: rules have changed for ES6: https://stackoverflow.com/a/2274327/895245

Spec: http://www.ecma-international.org/ecma-262/5.1/#sec-11.1.5

PropertyName :

IdentifierName

StringLiteral

NumericLiteral

[...]

The production PropertyName : IdentifierName is evaluated as follows:

Return the String value containing the same sequence of characters as the IdentifierName.

The production PropertyName : StringLiteral is evaluated as follows:

Return the SV [String value] of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:

Let nbr be the result of forming the value of the NumericLiteral.

Return ToString(nbr).

This means that:

{ theTop : 10 } is the exact same as { 'theTop' : 10 }

The PropertyName theTop is an IdentifierName, so it gets converted to the 'theTop' string value, which is the string value of 'theTop'.
It is not possible to write object initializers (literals) with variable keys.

The only three options are IdentifierName (expands to string literal), StringLiteral, and NumericLiteral (also expands to a string).

回答4:

I have used the following to add a property with a "dynamic" name to an object:

var key = 'top';
$('#myElement').animate(
   (function(o) { o[key]=10; return o;})({left: 20, width: 100}),
   10
);

key is the name of the new property.

The object of properties passed to animate will be {left: 20, width: 100, top: 10}

This is just using the required [] notation as recommended by the other answers, but with fewer lines of code!

回答5:

Adding square bracket around the variable works good for me. Try this

var thetop = 'top';
<something>.stop().animate(
    { [thetop] : 10 }, 10
);

回答6:

I couldn't find a simple example about the differences between ES6 and ES5, so I made one. Both code samples create exactly the same object. But the ES5 example also works in older browsers (like IE11), wheres the ES6 example doesn't.

ES6

var matrix = {};
var a = 'one';
var b = 'two';
var c = 'three';
var d = 'four';

matrix[a] = {[b]: {[c]: d}};

ES5

var matrix = {};
var a = 'one';
var b = 'two';
var c = 'three';
var d = 'four';

function addObj(obj, key, value) {
  obj[key] = value;
  return obj;
}

matrix[a] = addObj({}, b, addObj({}, c, d));

回答7:

Given code:

var thetop = 'top';
<something>.stop().animate(
    { thetop : 10 }, 10
);

Translation:

var thetop = 'top';
var config = { thetop : 10 }; // config.thetop = 10
<something>.stop().animate(config, 10);

As you can see, the { thetop : 10 } declaration doesn't make use of the variable thetop. Instead it creates an object with a key named thetop. If you want the key to be the value of the variable thetop, then you will have to use square brackets around thetop:

var thetop = 'top';
var config = { [thetop] : 10 }; // config.top = 10
<something>.stop().animate(config, 10);

The square bracket syntax has been introduced with ES6. In earlier versions of JavaScript, you would have to do the following:

var thetop = 'top';
var config = (
  obj = {},
  obj['' + thetop] = 10,
  obj
); // config.top = 10
<something>.stop().animate(config, 10);

回答8:

This way also you can achieve desired output

var jsonobj={};
var count=0;
$(document).on('click','#btnadd', function() {
    jsonobj[count]=new Array({ "1"  : $("#txtone").val()},{ "2"  : $("#txttwo").val()});
    count++;
    console.clear();
    console.log(jsonobj);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<span>value 1</span><input id="txtone" type="text"/>
<span>value 2</span><input id="txttwo" type="text"/>
<button id="btnadd">Add</button>

回答9:

ES5 implementation to assign keys is below:

var obj = Object.create(null),
    objArgs = (
      (objArgs = {}),
      (objArgs.someKey = {
        value: 'someValue'
      }), objArgs);

Object.defineProperties(obj, objArgs);

I've attached a snippet I used to convert to bare object.

var obj = {
  'key1': 'value1',
  'key2': 'value2',
  'key3': [
    'value3',
    'value4',
  ],
  'key4': {
    'key5': 'value5'
  }
}

var bareObj = function(obj) {

  var objArgs,
    bareObj = Object.create(null);

  Object.entries(obj).forEach(function([key, value]) {

    var objArgs = (
      (objArgs = {}),
      (objArgs[key] = {
        value: value
      }), objArgs);

    Object.defineProperties(bareObj, objArgs);

  });

  return {
    input: obj,
    output: bareObj
  };

}(obj);

if (!Object.entries) {
  Object.entries = function(obj){
    var arr = [];
    Object.keys(obj).forEach(function(key){
      arr.push([key, obj[key]]);
    });
    return arr;
  }
}

console(bareObj);

回答10:

You could do the following for ES5:

var theTop = 'top'
<something>.stop().animate(
  JSON.parse('{"' + theTop + '":' + JSON.stringify(10) + '}'), 10
)

Or extract to a function:

function newObj (key, value) {
  return JSON.parse('{"' + key + '":' + JSON.stringify(value) + '}')
}

var theTop = 'top'
<something>.stop().animate(
  newObj(theTop, 10), 10
)

回答11:

If you want object key to be same as variable name, there's a short hand in ES 2015. New notations in ECMAScript 2015

var thetop = 10;
var obj = { thetop };
console.log(obj.thetop); // print 10

回答12:

You can do it this way:

var thetop = 'top';
<something>.stop().animate(
    new function() {this[thetop] = 10;}, 10
);

来源：https://stackoverflow.com/questions/2274242/how-to-use-a-variable-for-a-key-in-a-javascript-object-literal

标签

javascript

jquery

variables

properties

object-literal