Can anyone explain how the why/how the below method of assigning keys in JavaScript works?
a = "b"
c = {[a]: "d"}
return:
Object {b: "d"}
It's the new ES2015 (the EcmaScript spec formally known as ES6) computed property name syntax . It's a shorthand for the someObject[someKey]
assignment that you know from ES3/5:
var a = "b"
var c = {[a]: "d"}
is syntactic sugar for:
var a = "b"
var c = {}
c[a] = "d"
Really the use of []
gives an excellent way to use actual value of variable as key /
property while creating JavaScript objects .
I'm pretty much statisfied with the above answer and I appreciate it as it allowed me to write this with a little example.
I've executed the code line by line on Node REPL (Node shell).
> var key = "fullName"; // Assignment
undefined
>
> var obj = {key: "Rishikesh Agrawani"} // Here key's value will not be used
undefined
> obj // Inappropriate, which we don't want
{ key: 'Rishikesh Agrawani' }
>
> // Let's fix
undefined
> var obj2 = {[key]: "Rishikesh Agrawani"}
undefined
> obj2
{ fullName: 'Rishikesh Agrawani' }
>
const animalSounds = {cat: 'meow', dog: 'bark'};
const animal = 'lion';
const sound = 'roar';
{...animalSounds, [animal]: sound};
The result will be
{cat: 'meow', dog: 'bark', lion: 'roar'};
此外,只有在评估或运行时我们还不知道它将是什么时,才使用[]
符号来访问或分配对象中的内容。
I want to make an object but I don't know the name of the key
until runtime.
Back in the ES5 days:
var myObject = {};
myObject[key] = "bar";
Writing two lines of code is so painful... Ah, ES6 just came along:
var myObject = {[key]:"bar"};
If the value of key
equals foo
, then both approaches result in:
{foo : "bar"}
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