Reputation: 27306
This is a SSCCE.
I have a map container class where an internal Map
is created on demand with the first set
method that gets called:
// @flow
'use strict';
class MapContainer {
map: ?Map<any, any>;
constructor() {
this.map=null;
}
set(key: any, value: any): ?any {
if (this.map===null) {
this.map = new Map();
}
let prevValue: ?any;
if (this.map!=null) { // first check
prevValue = this.map.get(key);
}
if (this.map!=null) { // second check
this.map.set(key, value);
}
return prevValue;
}
}
exports.MapContainer = MapContainer;
The above code passes npm run flow
with no warnings.
However if I merge the two if (this.map!=null)
checks into one:
// @flow
'use strict';
class MapContainer {
map: ?Map<any, any>;
constructor() {
this.map=null;
}
set(key: any, value: any): ?any {
if (this.map===null) {
this.map = new Map();
}
let prevValue: ?any;
if (this.map!=null) { // merged check
prevValue = this.map.get(key);
this.map.set(key, value);
}
return prevValue;
}
}
exports.MapContainer = MapContainer;
… then running flow fails with the following message:
es6/map-container.js:19
19: this.map.set(key, value);
^^^^^^^^^^^^^^^^^^^^^^^^ call of method `set`. Method cannot be called on possibly null value
19: this.map.set(key, value);
^^^^^^^^ null
es6/map-container.js:19
19: this.map.set(key, value);
^^^^^^^^^^^^^^^^^^^^^^^^ call of method `set`. Method cannot be called on possibly undefined value
19: this.map.set(key, value);
^^^^^^^^ undefined
… which makes no sense at all as the access on line 19:
this.map.set(key,value)
… is still covered by the check:
if (this.map!=null)
What gives?
Upvotes: 2
Views: 217
Reputation: 13570
The problem is that call to get
method can invalidates refinement. What if get
sets this.map
to null
? Flow has no way of knowing, so it assumes the worst. Here is what you can do:
class MapContainer {
map: ?Map<any, any>;
constructor() {
this.map=null;
}
set(key: any, value: any): ?any {
if (!this.map) {
this.map = new Map();
}
const map = this.map;
let prevValue: ?any;
if (this.map!=null) {
prevValue = map.get(key);
map.set(key, value);
}
return prevValue;
}
}
Upvotes: 1