const
betterCoefficients
=
new
Proxy
(
coefficients
, {
get
(
target
,
key
) {
return
target
[
key
]
||
0
;
},
});
As I write this, proxies are not supported in Babel. However, they
are supported in the current build of Firefox, and these code sam‐
ples can be tested there.
The first argument to the
Proxy
constructor is the target, or object that’s being prox‐
ied. The second argument is the handler, which specifies the actions to be intercepted.
In this case, we’re only intercepting property access, denoted by the
get
function (this
is distinct from a
get
property accessor: this will work for regular properties and get
accessors). The
get
function takes three arguments (we’re only using the first two):
the target, the property key (either a string or a symbol), and the receiver (the proxy
itself, or something that derives from it).
In this example, we simply check to see if the key is set on the target; if it’s not, we
return the value
0
. Go ahead and try it:
betterCoefficients
.
a
;
// 1
betterCoefficients
.
b
;
// 0
betterCoefficients
.
c
;
// 3
betterCoefficients
.
d
;
// 0
betterCoefficients
.
anything
;
// 0;
We’ve essentially created a proxy for our
coefficients
object that appears to have an
infinite number of properties (all set to
0
, except the ones we define)!
We could further modify our proxy to only proxy single lowercase letters:
const
betterCoefficients
=
new
Proxy
(
coefficients
, {
get
(
target
,
key
) {
if
(
!
/^[a-z]$/
.
test
(
key
))
return
target
[
key
];
return
target
[
key
]
||
0
;
},
});
Instead of doing our easy check to see if
target[key]
is truthy, we could return
0
if
it’s anything but a number…I’ll leave that as a reader’s exercise.
Similarly, we can intercept properties (or accessors) being set with the
set
handler.
Let’s consider an example where we have dangerous properties on an object. We want
to prevent these properties from being set, and the methods from being called,
without an extra step. The extra step we’ll use is a setter called
allowDangerousOpera
tions
, which you have to set to
true
before accessing dangerous functionality:
Proxies | 309
