Javascript how to initialize array using object
> a = ["key","value"]
["key", "value"]
> o = {a[0]:a[1]}
SyntaxError: Unexpected token [
But it normal
> o = {}
Object {}
> o[a[0]] = a[1];
"value"
> o
Object {key: "value"}
browser version: Chrome 37.0.2062.124 m
Why Syntax error? Does dose introduce new context here? I am not familiar with the ECMA specification.
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According to the ECMA Script 5.1 Specification , Object Literal is defined as follows
ObjectLiteral : { } { PropertyNameAndValueList } { PropertyNameAndValueList , } PropertyNameAndValueList : PropertyAssignment PropertyNameAndValueList , PropertyAssignment PropertyAssignment : PropertyName : AssignmentExpression get PropertyName ( ) { FunctionBody } set PropertyName ( PropertySetParameterList ) { FunctionBody } PropertyName : IdentifierName StringLiteral NumericLiteral PropertySetParameterList : Identifier
Since it is []
not allowed in any of the IdentifierName
, StringLiteral
and NumericLiteral
, the JavaScript engine cannot parse the code. This is why it gives a syntax error.
So, to actually create an object with keys and values from an array, you need to first construct the object and then assign the properties individually, for example
var newObject = {};
newObject[arr[0]] = arr[1];
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In object literature, property names must be identifier ( foo
), string literal ( "foo"
), or number literal ( 1
). a[0]
- none of this.
When you add a property to an existing object using the square bracket syntax, you are using an expression that can be evaluated for a string (which does a[0]
).
If you want to use an expression to set a property name, then you must first build an object and then add the property to another expression.
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In ES6, you have "computed property names" (see http://www.ecma-international.org/ecma-262/6.0/ ):
PropertyName : See 12.2.6
LiteralPropertyName
ComputedPropertyName
LiteralPropertyName : See 12.2.6
IdentifierName
StringLiteral
NumericLiteral
ComputedPropertyName : See 12.2.6
[ AssignmentExpression ]
This means that when written [Expression]
as a key, instead of StringLiteral
, NumberLiteral
or IdentifierName
. The expression is evaluated and used in place of the key.
Like this:
> a=["key", "value"]
["key", "value"]
> o={[a[0]]: a[1], [3*3]: "hello"}
Object {key: "value", 9: "hello"}
Please note that ES6 is still not supported everywhere. In case this feature is not supported, just assign values after object creation (like in your second example).
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